patch, 6 partsMake sure fsck_commit_buffer() does not run out of the buffer

object.c, object.h

diff --git a/object.c b/object.c
index a16b9f9..5eee592 100644
--- a/object.c
+++ b/object.c
@@ -33,13 +33,24 @@ const char *typename(unsigned int type)
 	return object_type_strings[type];
 }
 
-int type_from_string(const char *str)
+int type_from_string_gently(const char *str)
 {
 	int i;
 
 	for (i = 1; i < ARRAY_SIZE(object_type_strings); i++)
 		if (!strcmp(str, object_type_strings[i]))
 			return i;
+
+	return -1;
+}
+
+int type_from_string(const char *str)
+{
+	int i = type_from_string_gently(str);
+
+	if (i >= 0)
+		return i;
+
 	die("invalid object type \"%s\"", str);
 }
 
diff --git a/object.h b/object.h
index 5e8d8ee..5c5d22f 100644
--- a/object.h
+++ b/object.h
@@ -54,6 +54,7 @@ struct object {
 
 extern const char *typename(unsigned int type);
 extern int type_from_string(const char *str);
+extern int type_from_string_gently(const char *str);
 
 /*
  * Return the current number of buckets in the object hashmap.

builtin/fsck.c, builtin/index-pack.c, builtin/unpack-objects.c, fsck.c, fsck.h

diff --git a/builtin/fsck.c b/builtin/fsck.c
index d42a27d..d9f4e6e 100644
--- a/builtin/fsck.c
+++ b/builtin/fsck.c
@@ -298,7 +298,7 @@ static int fsck_obj(struct object *obj)
 
 	if (fsck_walk(obj, mark_used, NULL))
 		objerror(obj, "broken links");
-	if (fsck_object(obj, check_strict, fsck_error_func))
+	if (fsck_object(obj, NULL, 0, check_strict, fsck_error_func))
 		return -1;
 
 	if (obj->type == OBJ_TREE) {
diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 5568a5b..f2465ff 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -773,7 +773,8 @@ static void sha1_object(const void *data, struct object_entry *obj_entry,
 			if (!obj)
 				die(_("invalid %s"), typename(type));
 			if (do_fsck_object &&
-			    fsck_object(obj, 1, fsck_error_function))
+			    fsck_object(obj, buf, size, 1,
+				    fsck_error_function))
 				die(_("Error in object"));
 			if (fsck_walk(obj, mark_link, NULL))
 				die(_("Not all child objects of %s are reachable"), sha1_to_hex(obj->sha1));
diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index 99cde45..855d94b 100644
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c
@@ -164,10 +164,10 @@ static unsigned nr_objects;
  * Called only from check_object() after it verified this object
  * is Ok.
  */
-static void write_cached_object(struct object *obj)
+static void write_cached_object(struct object *obj, struct obj_buffer *obj_buf)
 {
 	unsigned char sha1[20];
-	struct obj_buffer *obj_buf = lookup_object_buffer(obj);
+
 	if (write_sha1_file(obj_buf->buffer, obj_buf->size, typename(obj->type), sha1) < 0)
 		die("failed to write object %s", sha1_to_hex(obj->sha1));
 	obj->flags |= FLAG_WRITTEN;
@@ -180,6 +180,8 @@ static void write_cached_object(struct object *obj)
  */
 static int check_object(struct object *obj, int type, void *data)
 {
+	struct obj_buffer *obj_buf;
+
 	if (!obj)
 		return 1;
 
@@ -198,11 +200,15 @@ static int check_object(struct object *obj, int type, void *data)
 		return 0;
 	}
 
-	if (fsck_object(obj, 1, fsck_error_function))
+	obj_buf = lookup_object_buffer(obj);
+	if (!obj_buf)
+		die("Whoops! Cannot find object '%s'", sha1_to_hex(obj->sha1));
+	if (fsck_object(obj, obj_buf->buffer, obj_buf->size, 1,
+			fsck_error_function))
 		die("Error in object");
 	if (fsck_walk(obj, check_object, NULL))
 		die("Error on reachable objects of %s", sha1_to_hex(obj->sha1));
-	write_cached_object(obj);
+	write_cached_object(obj, obj_buf);
 	return 0;
 }
 
diff --git a/fsck.c b/fsck.c
index 56156ff..dd77628 100644
--- a/fsck.c
+++ b/fsck.c
@@ -277,7 +277,7 @@ static int fsck_ident(const char **ident, struct object *obj, fsck_error error_f
 }
 
 static int fsck_commit_buffer(struct commit *commit, const char *buffer,
-			      fsck_error error_func)
+	unsigned long size, fsck_error error_func)
 {
 	unsigned char tree_sha1[20], sha1[20];
 	struct commit_graft *graft;
@@ -322,15 +322,18 @@ static int fsck_commit_buffer(struct commit *commit, const char *buffer,
 	return 0;
 }
 
-static int fsck_commit(struct commit *commit, fsck_error error_func)
+static int fsck_commit(struct commit *commit, const char *data,
+	unsigned long size, fsck_error error_func)
 {
-	const char *buffer = get_commit_buffer(commit, NULL);
-	int ret = fsck_commit_buffer(commit, buffer, error_func);
-	unuse_commit_buffer(commit, buffer);
+	const char *buffer = data ?  data : get_commit_buffer(commit, &size);
+	int ret = fsck_commit_buffer(commit, buffer, size, error_func);
+	if (!data)
+		unuse_commit_buffer(commit, buffer);
 	return ret;
 }
 
-static int fsck_tag(struct tag *tag, fsck_error error_func)
+static int fsck_tag(struct tag *tag, const char *data,
+	unsigned long size, fsck_error error_func)
 {
 	struct object *tagged = tag->tagged;
 
@@ -339,7 +342,8 @@ static int fsck_tag(struct tag *tag, fsck_error error_func)
 	return 0;
 }
 
-int fsck_object(struct object *obj, int strict, fsck_error error_func)
+int fsck_object(struct object *obj, void *data, unsigned long size,
+	int strict, fsck_error error_func)
 {
 	if (!obj)
 		return error_func(obj, FSCK_ERROR, "no valid object to fsck");
@@ -349,9 +353,11 @@ int fsck_object(struct object *obj, int strict, fsck_error error_func)
 	if (obj->type == OBJ_TREE)
 		return fsck_tree((struct tree *) obj, strict, error_func);
 	if (obj->type == OBJ_COMMIT)
-		return fsck_commit((struct commit *) obj, error_func);
+		return fsck_commit((struct commit *) obj, (const char *) data,
+			size, error_func);
 	if (obj->type == OBJ_TAG)
-		return fsck_tag((struct tag *) obj, error_func);
+		return fsck_tag((struct tag *) obj, (const char *) data,
+			size, error_func);
 
 	return error_func(obj, FSCK_ERROR, "unknown type '%d' (internal fsck error)",
 			  obj->type);
diff --git a/fsck.h b/fsck.h
index 1e4f527..d1e6387 100644
--- a/fsck.h
+++ b/fsck.h
@@ -28,6 +28,8 @@ int fsck_error_function(struct object *obj, int type, const char *fmt, ...);
  *    0		everything OK
  */
 int fsck_walk(struct object *obj, fsck_walk_func walk, void *data);
-int fsck_object(struct object *obj, int strict, fsck_error error_func);
+/* If NULL is passed for data, we assume the object is local and read it. */
+int fsck_object(struct object *obj, void *data, unsigned long size,
+	int strict, fsck_error error_func);
 
 #endif

diff --git a/fsck.c b/fsck.c
index dd77628..db6aaa4 100644
--- a/fsck.c
+++ b/fsck.c
@@ -237,6 +237,26 @@ static int fsck_tree(struct tree *item, int strict, fsck_error error_func)
 	return retval;
 }
 
+static int must_have_empty_line(const void *data, unsigned long size,
+	struct object *obj, fsck_error error_func)
+{
+	const char *buffer = (const char *)data;
+	int i;
+
+	for (i = 0; i < size; i++) {
+		switch (buffer[i]) {
+		case '\0':
+			return error_func(obj, FSCK_ERROR,
+				"invalid message: NUL at offset %d", i);
+		case '\n':
+			if (i + 1 < size && buffer[i + 1] == '\n')
+				return 0;
+		}
+	}
+
+	return error_func(obj, FSCK_ERROR, "invalid buffer: missing empty line");
+}
+
 static int fsck_ident(const char **ident, struct object *obj, fsck_error error_func)
 {
 	char *end;
@@ -284,6 +304,9 @@ static int fsck_commit_buffer(struct commit *commit, const char *buffer,
 	unsigned parent_count, parent_line_count = 0;
 	int err;
 
+	if (must_have_empty_line(buffer, size, &commit->object, error_func))
+		return -1;
+
 	if (!skip_prefix(buffer, "tree ", &buffer))
 		return error_func(&commit->object, FSCK_ERROR, "invalid format - expected 'tree' line");
 	if (get_sha1_hex(buffer, tree_sha1) || buffer[40] != '\n')

diff --git a/fsck.c b/fsck.c
index db6aaa4..d30946b 100644
--- a/fsck.c
+++ b/fsck.c
@@ -6,6 +6,7 @@
 #include "commit.h"
 #include "tag.h"
 #include "fsck.h"
+#include "refs.h"
 
 static int fsck_walk_tree(struct tree *tree, fsck_walk_func walk, void *data)
 {
@@ -355,6 +356,90 @@ static int fsck_commit(struct commit *commit, const char *data,
 	return ret;
 }
 
+static int fsck_tag_buffer(struct tag *tag, const char *data,
+	unsigned long size, fsck_error error_func)
+{
+	unsigned char commit_sha1[20];
+	int ret = 0;
+	const char *buffer;
+	char *tmp = NULL, *eol;
+
+	if (data)
+		buffer = data;
+	else {
+		enum object_type type;
+
+		buffer = tmp = read_sha1_file(tag->object.sha1, &type, &size);
+		if (!buffer)
+			return error_func(&tag->object, FSCK_ERROR,
+				"cannot read tag object");
+
+		if (type != OBJ_TAG) {
+			ret = error_func(&tag->object, FSCK_ERROR,
+				"expected tag got %s",
+			    typename(type));
+			goto done;
+		}
+	}
+
+	if (must_have_empty_line(buffer, size, &tag->object, error_func))
+		goto done;
+
+	if (!skip_prefix(buffer, "object ", &buffer)) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - expected 'object' line");
+		goto done;
+	}
+	if (get_sha1_hex(buffer, commit_sha1) || buffer[40] != '\n') {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid 'object' line format - bad sha1");
+		goto done;
+	}
+	buffer += 41;
+
+	if (!skip_prefix(buffer, "type ", &buffer)) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - expected 'type' line");
+		goto done;
+	}
+	eol = strchr(buffer, '\n');
+	if (!eol) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - unexpected end after 'type' line");
+		goto done;
+	}
+	*eol = '\0';
+	if (type_from_string_gently(buffer) < 0)
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid 'type' value");
+	*eol = '\n';
+	if (ret)
+		goto done;
+	buffer = eol + 1;
+
+	if (!skip_prefix(buffer, "tag ", &buffer)) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - expected 'tag' line");
+		goto done;
+	}
+	eol = strchr(buffer, '\n');
+	if (!eol) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - unexpected end after 'type' line");
+		goto done;
+	}
+	*eol = '\0';
+	if (check_refname_format(buffer, REFNAME_ALLOW_ONELEVEL))
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid 'tag' name: %s", buffer);
+	*eol = '\n';
+	if (ret)
+		goto done;
+	buffer = eol + 1;
+
+	if (!skip_prefix(buffer, "tagger ", &buffer)) {
+		/* early tags do not contain 'tagger' lines; warn only */
+		error_func(&tag->object, FSCK_WARN, "invalid format - expected 'tagger' line");
+	}
+	ret = fsck_ident(&buffer, &tag->object, error_func);
+
+done:
+	free(tmp);
+	return ret;
+}
+
 static int fsck_tag(struct tag *tag, const char *data,
 	unsigned long size, fsck_error error_func)
 {
@@ -362,7 +447,8 @@ static int fsck_tag(struct tag *tag, const char *data,
 
 	if (!tagged)
 		return error_func(&tag->object, FSCK_ERROR, "could not load tagged object");
-	return 0;
+
+	return fsck_tag_buffer(tag, data, size, error_func);
 }
 
 int fsck_object(struct object *obj, void *data, unsigned long size,

diff --git a/t/t1450-fsck.sh b/t/t1450-fsck.sh
index 8c739c9..16b3e4a 100755
--- a/t/t1450-fsck.sh
+++ b/t/t1450-fsck.sh
@@ -194,6 +194,45 @@ test_expect_success 'tag pointing to something else than its type' '
 	test_must_fail git fsck --tags
 '
 
+test_expect_success 'tag with incorrect tag name' '
+	sha=$(git rev-parse HEAD) &&
+	cat >wrong-tag <<-EOF &&
+	object $sha
+	type commit
+	tag wrong name format
+	tagger T A Gger <tagger@example.com> 1234567890 -0000
+
+	This is an invalid tag.
+	EOF
+
+	tag=$(git hash-object -t tag -w --stdin <wrong-tag) &&
+	test_when_finished "remove_object $tag" &&
+	echo $tag >.git/refs/tags/wrong &&
+	test_when_finished "git update-ref -d refs/tags/wrong" &&
+	git fsck --tags 2>out &&
+	cat out &&
+	grep "invalid .tag. name" out
+'
+
+test_expect_success 'tag with missing tagger' '
+	sha=$(git rev-parse HEAD) &&
+	cat >wrong-tag <<-EOF &&
+	object $sha
+	type commit
+	tag gutentag
+
+	This is an invalid tag.
+	EOF
+
+	tag=$(git hash-object -t tag -w --stdin <wrong-tag) &&
+	test_when_finished "remove_object $tag" &&
+	echo $tag >.git/refs/tags/wrong &&
+	test_when_finished "git update-ref -d refs/tags/wrong" &&
+	git fsck --tags 2>out &&
+	cat out &&
+	grep "expected .tagger. line" out
+'
+
 test_expect_success 'cleaned up' '
 	git fsck >actual 2>&1 &&
 	test_cmp empty actual

diff --git a/t/t5302-pack-index.sh b/t/t5302-pack-index.sh
index 4bbb718..80bd3ee 100755
--- a/t/t5302-pack-index.sh
+++ b/t/t5302-pack-index.sh
@@ -243,4 +243,23 @@ test_expect_success 'running index-pack in the object store' '
     test -f .git/objects/pack/pack-${pack1}.idx
 '
 
+test_expect_success 'index-pack --strict fails upon invalid tag' '
+    sha=$(git rev-parse HEAD) &&
+    cat >wrong-tag <<EOF &&
+object $sha
+type commit
+tag guten tag
+
+This is an invalid tag.
+EOF
+
+    tag=$(git hash-object -t tag -w --stdin <wrong-tag) &&
+    pack1=$(echo $tag | git pack-objects tag-test) &&
+    echo remove tag object &&
+    thirtyeight=${tag#??} &&
+    rm -f .git/objects/${tag%$thirtyeight}/$thirtyeight &&
+    test_must_fail git index-pack --strict tag-test-${pack1}.pack 2> err &&
+    grep "invalid .tag. name" err
+'
+
 test_done

diff --git a/fsck.c b/fsck.c
index 8341a30..6e6ea25 100644
--- a/fsck.c
+++ b/fsck.c
@@ -248,14 +248,14 @@ static int require_end_of_header(const void *data, unsigned long size,
 		switch (buffer[i]) {
 		case '\0':
 			return error_func(obj, FSCK_ERROR,
-				"invalid message: NUL at offset %d", i);
+				"invalid header: NUL at offset %d", i);
 		case '\n':
 			if (i + 1 < size && buffer[i + 1] == '\n')
 				return 0;
 		}
 	}
 
-	return error_func(obj, FSCK_ERROR, "invalid buffer: missing empty line");
+	return error_func(obj, FSCK_ERROR, "invalid buffer: missing end of header");
 }
 
 static int fsck_ident(const char **ident, struct object *obj, fsck_error error_func)
@@ -426,14 +426,16 @@ static int fsck_tag_buffer(struct tag *tag, const char *data,
 		error_func(&tag->object, FSCK_WARN, "invalid 'tag' name: %s", buffer);
 	buffer = eol + 1;
 
-	if (!skip_prefix(buffer, "tagger ", &buffer)) {
+	if (!skip_prefix(buffer, "tagger ", &buffer))
 		/* early tags do not contain 'tagger' lines; warn only */
-		error_func(&tag->object, FSCK_WARN, "invalid format - expected 'tagger' line");
-	}
-	ret = fsck_ident(&buffer, &tag->object, error_func);
+		error_func(&tag->object, FSCK_WARN,
+			   "invalid format - expected 'tagger' line");
+	else
+		ret = fsck_ident(&buffer, &tag->object, error_func);
 
 done:
 	free(to_free);
+	strbuf_release(&sb);
 	return ret;
 }
 

object.c, object.h

diff --git a/object.c b/object.c
index a16b9f9..aedac24 100644
--- a/object.c
+++ b/object.c
@@ -33,13 +33,20 @@ const char *typename(unsigned int type)
 	return object_type_strings[type];
 }
 
-int type_from_string(const char *str)
+int type_from_string_gently(const char *str, ssize_t len, int gentle)
 {
 	int i;
 
+	if (len < 0)
+		len = strlen(str);
+
 	for (i = 1; i < ARRAY_SIZE(object_type_strings); i++)
-		if (!strcmp(str, object_type_strings[i]))
+		if (!strncmp(str, object_type_strings[i], len))
 			return i;
+
+	if (gentle)
+		return -1;
+
 	die("invalid object type \"%s\"", str);
 }
 
diff --git a/object.h b/object.h
index 5e8d8ee..e028ced 100644
--- a/object.h
+++ b/object.h
@@ -53,7 +53,8 @@ struct object {
 };
 
 extern const char *typename(unsigned int type);
-extern int type_from_string(const char *str);
+extern int type_from_string_gently(const char *str, ssize_t, int gentle);
+#define type_from_string(str) type_from_string_gently(str, -1, 0)
 
 /*
  * Return the current number of buckets in the object hashmap.

builtin/fsck.c, builtin/index-pack.c, builtin/unpack-objects.c, fsck.c, fsck.h

diff --git a/builtin/fsck.c b/builtin/fsck.c
index d42a27d..d9f4e6e 100644
--- a/builtin/fsck.c
+++ b/builtin/fsck.c
@@ -298,7 +298,7 @@ static int fsck_obj(struct object *obj)
 
 	if (fsck_walk(obj, mark_used, NULL))
 		objerror(obj, "broken links");
-	if (fsck_object(obj, check_strict, fsck_error_func))
+	if (fsck_object(obj, NULL, 0, check_strict, fsck_error_func))
 		return -1;
 
 	if (obj->type == OBJ_TREE) {
diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 5568a5b..f2465ff 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -773,7 +773,8 @@ static void sha1_object(const void *data, struct object_entry *obj_entry,
 			if (!obj)
 				die(_("invalid %s"), typename(type));
 			if (do_fsck_object &&
-			    fsck_object(obj, 1, fsck_error_function))
+			    fsck_object(obj, buf, size, 1,
+				    fsck_error_function))
 				die(_("Error in object"));
 			if (fsck_walk(obj, mark_link, NULL))
 				die(_("Not all child objects of %s are reachable"), sha1_to_hex(obj->sha1));
diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index 99cde45..855d94b 100644
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c
@@ -164,10 +164,10 @@ static unsigned nr_objects;
  * Called only from check_object() after it verified this object
  * is Ok.
  */
-static void write_cached_object(struct object *obj)
+static void write_cached_object(struct object *obj, struct obj_buffer *obj_buf)
 {
 	unsigned char sha1[20];
-	struct obj_buffer *obj_buf = lookup_object_buffer(obj);
+
 	if (write_sha1_file(obj_buf->buffer, obj_buf->size, typename(obj->type), sha1) < 0)
 		die("failed to write object %s", sha1_to_hex(obj->sha1));
 	obj->flags |= FLAG_WRITTEN;
@@ -180,6 +180,8 @@ static void write_cached_object(struct object *obj)
  */
 static int check_object(struct object *obj, int type, void *data)
 {
+	struct obj_buffer *obj_buf;
+
 	if (!obj)
 		return 1;
 
@@ -198,11 +200,15 @@ static int check_object(struct object *obj, int type, void *data)
 		return 0;
 	}
 
-	if (fsck_object(obj, 1, fsck_error_function))
+	obj_buf = lookup_object_buffer(obj);
+	if (!obj_buf)
+		die("Whoops! Cannot find object '%s'", sha1_to_hex(obj->sha1));
+	if (fsck_object(obj, obj_buf->buffer, obj_buf->size, 1,
+			fsck_error_function))
 		die("Error in object");
 	if (fsck_walk(obj, check_object, NULL))
 		die("Error on reachable objects of %s", sha1_to_hex(obj->sha1));
-	write_cached_object(obj);
+	write_cached_object(obj, obj_buf);
 	return 0;
 }
 
diff --git a/fsck.c b/fsck.c
index 56156ff..dd77628 100644
--- a/fsck.c
+++ b/fsck.c
@@ -277,7 +277,7 @@ static int fsck_ident(const char **ident, struct object *obj, fsck_error error_f
 }
 
 static int fsck_commit_buffer(struct commit *commit, const char *buffer,
-			      fsck_error error_func)
+	unsigned long size, fsck_error error_func)
 {
 	unsigned char tree_sha1[20], sha1[20];
 	struct commit_graft *graft;
@@ -322,15 +322,18 @@ static int fsck_commit_buffer(struct commit *commit, const char *buffer,
 	return 0;
 }
 
-static int fsck_commit(struct commit *commit, fsck_error error_func)
+static int fsck_commit(struct commit *commit, const char *data,
+	unsigned long size, fsck_error error_func)
 {
-	const char *buffer = get_commit_buffer(commit, NULL);
-	int ret = fsck_commit_buffer(commit, buffer, error_func);
-	unuse_commit_buffer(commit, buffer);
+	const char *buffer = data ?  data : get_commit_buffer(commit, &size);
+	int ret = fsck_commit_buffer(commit, buffer, size, error_func);
+	if (!data)
+		unuse_commit_buffer(commit, buffer);
 	return ret;
 }
 
-static int fsck_tag(struct tag *tag, fsck_error error_func)
+static int fsck_tag(struct tag *tag, const char *data,
+	unsigned long size, fsck_error error_func)
 {
 	struct object *tagged = tag->tagged;
 
@@ -339,7 +342,8 @@ static int fsck_tag(struct tag *tag, fsck_error error_func)
 	return 0;
 }
 
-int fsck_object(struct object *obj, int strict, fsck_error error_func)
+int fsck_object(struct object *obj, void *data, unsigned long size,
+	int strict, fsck_error error_func)
 {
 	if (!obj)
 		return error_func(obj, FSCK_ERROR, "no valid object to fsck");
@@ -349,9 +353,11 @@ int fsck_object(struct object *obj, int strict, fsck_error error_func)
 	if (obj->type == OBJ_TREE)
 		return fsck_tree((struct tree *) obj, strict, error_func);
 	if (obj->type == OBJ_COMMIT)
-		return fsck_commit((struct commit *) obj, error_func);
+		return fsck_commit((struct commit *) obj, (const char *) data,
+			size, error_func);
 	if (obj->type == OBJ_TAG)
-		return fsck_tag((struct tag *) obj, error_func);
+		return fsck_tag((struct tag *) obj, (const char *) data,
+			size, error_func);
 
 	return error_func(obj, FSCK_ERROR, "unknown type '%d' (internal fsck error)",
 			  obj->type);
diff --git a/fsck.h b/fsck.h
index 1e4f527..d1e6387 100644
--- a/fsck.h
+++ b/fsck.h
@@ -28,6 +28,8 @@ int fsck_error_function(struct object *obj, int type, const char *fmt, ...);
  *    0		everything OK
  */
 int fsck_walk(struct object *obj, fsck_walk_func walk, void *data);
-int fsck_object(struct object *obj, int strict, fsck_error error_func);
+/* If NULL is passed for data, we assume the object is local and read it. */
+int fsck_object(struct object *obj, void *data, unsigned long size,
+	int strict, fsck_error error_func);
 
 #endif

diff --git a/fsck.c b/fsck.c
index dd77628..73da6f8 100644
--- a/fsck.c
+++ b/fsck.c
@@ -237,6 +237,26 @@ static int fsck_tree(struct tree *item, int strict, fsck_error error_func)
 	return retval;
 }
 
+static int require_end_of_header(const void *data, unsigned long size,
+	struct object *obj, fsck_error error_func)
+{
+	const char *buffer = (const char *)data;
+	unsigned long i;
+
+	for (i = 0; i < size; i++) {
+		switch (buffer[i]) {
+		case '\0':
+			return error_func(obj, FSCK_ERROR,
+				"unterminated header: NUL at offset %d", i);
+		case '\n':
+			if (i + 1 < size && buffer[i + 1] == '\n')
+				return 0;
+		}
+	}
+
+	return error_func(obj, FSCK_ERROR, "unterminated header");
+}
+
 static int fsck_ident(const char **ident, struct object *obj, fsck_error error_func)
 {
 	char *end;
@@ -284,6 +304,9 @@ static int fsck_commit_buffer(struct commit *commit, const char *buffer,
 	unsigned parent_count, parent_line_count = 0;
 	int err;
 
+	if (require_end_of_header(buffer, size, &commit->object, error_func))
+		return -1;
+
 	if (!skip_prefix(buffer, "tree ", &buffer))
 		return error_func(&commit->object, FSCK_ERROR, "invalid format - expected 'tree' line");
 	if (get_sha1_hex(buffer, tree_sha1) || buffer[40] != '\n')

diff --git a/fsck.c b/fsck.c
index 73da6f8..2fffa43 100644
--- a/fsck.c
+++ b/fsck.c
@@ -6,6 +6,7 @@
 #include "commit.h"
 #include "tag.h"
 #include "fsck.h"
+#include "refs.h"
 
 static int fsck_walk_tree(struct tree *tree, fsck_walk_func walk, void *data)
 {
@@ -355,6 +356,88 @@ static int fsck_commit(struct commit *commit, const char *data,
 	return ret;
 }
 
+static int fsck_tag_buffer(struct tag *tag, const char *data,
+	unsigned long size, fsck_error error_func)
+{
+	unsigned char sha1[20];
+	int ret = 0;
+	const char *buffer;
+	char *to_free = NULL, *eol;
+	struct strbuf sb = STRBUF_INIT;
+
+	if (data)
+		buffer = data;
+	else {
+		enum object_type type;
+
+		buffer = to_free =
+			read_sha1_file(tag->object.sha1, &type, &size);
+		if (!buffer)
+			return error_func(&tag->object, FSCK_ERROR,
+				"cannot read tag object");
+
+		if (type != OBJ_TAG) {
+			ret = error_func(&tag->object, FSCK_ERROR,
+				"expected tag got %s",
+			    typename(type));
+			goto done;
+		}
+	}
+
+	if (require_end_of_header(buffer, size, &tag->object, error_func))
+		goto done;
+
+	if (!skip_prefix(buffer, "object ", &buffer)) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - expected 'object' line");
+		goto done;
+	}
+	if (get_sha1_hex(buffer, sha1) || buffer[40] != '\n') {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid 'object' line format - bad sha1");
+		goto done;
+	}
+	buffer += 41;
+
+	if (!skip_prefix(buffer, "type ", &buffer)) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - expected 'type' line");
+		goto done;
+	}
+	eol = strchr(buffer, '\n');
+	if (!eol) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - unexpected end after 'type' line");
+		goto done;
+	}
+	if (type_from_string_gently(buffer, eol - buffer, 1) < 0)
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid 'type' value");
+	if (ret)
+		goto done;
+	buffer = eol + 1;
+
+	if (!skip_prefix(buffer, "tag ", &buffer)) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - expected 'tag' line");
+		goto done;
+	}
+	eol = strchr(buffer, '\n');
+	if (!eol) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - unexpected end after 'type' line");
+		goto done;
+	}
+	strbuf_addf(&sb, "refs/tags/%.*s", (int)(eol - buffer), buffer);
+	if (check_refname_format(sb.buf, 0))
+		error_func(&tag->object, FSCK_WARN, "invalid 'tag' name: %s", buffer);
+	buffer = eol + 1;
+
+	if (!skip_prefix(buffer, "tagger ", &buffer))
+		/* early tags do not contain 'tagger' lines; warn only */
+		error_func(&tag->object, FSCK_WARN, "invalid format - expected 'tagger' line");
+	else
+		ret = fsck_ident(&buffer, &tag->object, error_func);
+
+done:
+	strbuf_release(&sb);
+	free(to_free);
+	return ret;
+}
+
 static int fsck_tag(struct tag *tag, const char *data,
 	unsigned long size, fsck_error error_func)
 {
@@ -362,7 +445,8 @@ static int fsck_tag(struct tag *tag, const char *data,
 
 	if (!tagged)
 		return error_func(&tag->object, FSCK_ERROR, "could not load tagged object");
-	return 0;
+
+	return fsck_tag_buffer(tag, data, size, error_func);
 }
 
 int fsck_object(struct object *obj, void *data, unsigned long size,

diff --git a/t/t1450-fsck.sh b/t/t1450-fsck.sh
index 8c739c9..2b6a6f2 100755
--- a/t/t1450-fsck.sh
+++ b/t/t1450-fsck.sh
@@ -194,6 +194,26 @@ test_expect_success 'tag pointing to something else than its type' '
 	test_must_fail git fsck --tags
 '
 
+test_expect_success 'tag with incorrect tag name & missing tagger' '
+	sha=$(git rev-parse HEAD) &&
+	cat >wrong-tag <<-EOF &&
+	object $sha
+	type commit
+	tag wrong name format
+
+	This is an invalid tag.
+	EOF
+
+	tag=$(git hash-object -t tag -w --stdin <wrong-tag) &&
+	test_when_finished "remove_object $tag" &&
+	echo $tag >.git/refs/tags/wrong &&
+	test_when_finished "git update-ref -d refs/tags/wrong" &&
+	git fsck --tags 2>out &&
+	cat out &&
+	grep "invalid .tag. name" out &&
+	grep "expected .tagger. line" out
+'
+
 test_expect_success 'cleaned up' '
 	git fsck >actual 2>&1 &&
 	test_cmp empty actual

diff --git a/t/t5302-pack-index.sh b/t/t5302-pack-index.sh
index 4bbb718..4d033df 100755
--- a/t/t5302-pack-index.sh
+++ b/t/t5302-pack-index.sh
@@ -243,4 +243,23 @@ test_expect_success 'running index-pack in the object store' '
     test -f .git/objects/pack/pack-${pack1}.idx
 '
 
+test_expect_success 'index-pack --strict warns upon missing tagger in tag' '
+    sha=$(git rev-parse HEAD) &&
+    cat >wrong-tag <<EOF &&
+object $sha
+type commit
+tag guten tag
+
+This is an invalid tag.
+EOF
+
+    tag=$(git hash-object -t tag -w --stdin <wrong-tag) &&
+    pack1=$(echo $tag $sha | git pack-objects tag-test) &&
+    echo remove tag object &&
+    thirtyeight=${tag#??} &&
+    rm -f .git/objects/${tag%$thirtyeight}/$thirtyeight &&
+    git index-pack --strict tag-test-${pack1}.pack 2> err &&
+    grep "^error:.* expected .tagger. line" err
+'
+
 test_done

diff --git a/t/t1450-fsck.sh b/t/t1450-fsck.sh
index 9118253..6ecb844 100755
--- a/t/t1450-fsck.sh
+++ b/t/t1450-fsck.sh
@@ -234,6 +234,25 @@ test_expect_success 'tag with incorrect tag name & missing tagger' '
 	grep "expected .tagger. line" out
 '
 
+test_expect_success 'tag with bad tagger' '
+	sha=$(git rev-parse HEAD) &&
+	cat >wrong-tag <<-EOF &&
+	object $sha
+	type commit
+	tag not-quite-wrong
+	tagger Bad Tagger Name
+
+	This is an invalid tag.
+	EOF
+
+	tag=$(git hash-object --literally -t tag -w --stdin <wrong-tag) &&
+	test_when_finished "remove_object $tag" &&
+	echo $tag >.git/refs/tags/wrong &&
+	test_when_finished "git update-ref -d refs/tags/wrong" &&
+	test_must_fail git fsck --tags 2>out &&
+	grep "error in tag .*: invalid author/committer" out
+'
+
 test_expect_success 'cleaned up' '
 	git fsck >actual 2>&1 &&
 	test_cmp empty actual

diff --git a/builtin/hash-object.c b/builtin/hash-object.c
index d7fcf4c..40008e2 100644
--- a/builtin/hash-object.c
+++ b/builtin/hash-object.c
@@ -36,9 +36,7 @@ static void hash_object(const char *path, const char *type, int write_object,
 	hash_fd(fd, type, write_object, vpath);
 }
 
-static int no_filters;
-
-static void hash_stdin_paths(const char *type, int write_objects)
+static void hash_stdin_paths(const char *type, int write_objects, int no_filters)
 {
 	struct strbuf buf = STRBUF_INIT, nbuf = STRBUF_INIT;
 
@@ -50,42 +48,38 @@ static void hash_stdin_paths(const char *type, int write_objects)
 			strbuf_swap(&buf, &nbuf);
 		}
 		hash_object(buf.buf, type, write_objects,
-		    no_filters ? NULL : buf.buf);
+			    no_filters ? NULL : buf.buf);
 	}
 	strbuf_release(&buf);
 	strbuf_release(&nbuf);
 }
 
-static const char * const hash_object_usage[] = {
-	N_("git hash-object [-t <type>] [-w] [--path=<file>|--no-filters] [--stdin] [--] <file>..."),
-	N_("git hash-object  --stdin-paths < <list-of-paths>"),
-	NULL
-};
-
-static const char *type;
-static int write_object;
-static int hashstdin;
-static int stdin_paths;
-static const char *vpath;
-
-static const struct option hash_object_options[] = {
-	OPT_STRING('t', NULL, &type, N_("type"), N_("object type")),
-	OPT_BOOL('w', NULL, &write_object, N_("write the object into the object database")),
-	OPT_COUNTUP( 0 , "stdin", &hashstdin, N_("read the object from stdin")),
-	OPT_BOOL( 0 , "stdin-paths", &stdin_paths, N_("read file names from stdin")),
-	OPT_BOOL( 0 , "no-filters", &no_filters, N_("store file as is without filters")),
-	OPT_STRING( 0 , "path", &vpath, N_("file"), N_("process file as it were from this path")),
-	OPT_END()
-};
-
 int cmd_hash_object(int argc, const char **argv, const char *prefix)
 {
+	static const char * const hash_object_usage[] = {
+		N_("git hash-object [-t <type>] [-w] [--path=<file>|--no-filters] [--stdin] [--] <file>..."),
+		N_("git hash-object  --stdin-paths < <list-of-paths>"),
+		NULL
+	};
+	const char *type = blob_type;
+	int hashstdin = 0;
+	int stdin_paths = 0;
+	int write_object = 0;
+	int no_filters = 0;
+	const char *vpath = NULL;
+	const struct option hash_object_options[] = {
+		OPT_STRING('t', NULL, &type, N_("type"), N_("object type")),
+		OPT_BOOL('w', NULL, &write_object, N_("write the object into the object database")),
+		OPT_COUNTUP( 0 , "stdin", &hashstdin, N_("read the object from stdin")),
+		OPT_BOOL( 0 , "stdin-paths", &stdin_paths, N_("read file names from stdin")),
+		OPT_BOOL( 0 , "no-filters", &no_filters, N_("store file as is without filters")),
+		OPT_STRING( 0 , "path", &vpath, N_("file"), N_("process file as it were from this path")),
+		OPT_END()
+	};
 	int i;
 	int prefix_length = -1;
 	const char *errstr = NULL;
 
-	type = blob_type;
-
 	argc = parse_options(argc, argv, NULL, hash_object_options,
 			     hash_object_usage, 0);
 
@@ -131,7 +125,7 @@ int cmd_hash_object(int argc, const char **argv, const char *prefix)
 	}
 
 	if (stdin_paths)
-		hash_stdin_paths(type, write_object);
+		hash_stdin_paths(type, write_object, no_filters);
 
 	return 0;
 }

diff --git a/builtin/hash-object.c b/builtin/hash-object.c
index 40008e2..1fb07ee 100644
--- a/builtin/hash-object.c
+++ b/builtin/hash-object.c
@@ -10,33 +10,31 @@
 #include "parse-options.h"
 #include "exec_cmd.h"
 
-static void hash_fd(int fd, const char *type, int write_object, const char *path)
+static void hash_fd(int fd, const char *type, const char *path, unsigned flags)
 {
 	struct stat st;
 	unsigned char sha1[20];
-	unsigned flags = (HASH_FORMAT_CHECK |
-			  (write_object ? HASH_WRITE_OBJECT : 0));
 
 	if (fstat(fd, &st) < 0 ||
 	    index_fd(sha1, fd, &st, type_from_string(type), path, flags))
-		die(write_object
+		die((flags & HASH_WRITE_OBJECT)
 		    ? "Unable to add %s to database"
 		    : "Unable to hash %s", path);
 	printf("%s\n", sha1_to_hex(sha1));
 	maybe_flush_or_die(stdout, "hash to stdout");
 }
 
-static void hash_object(const char *path, const char *type, int write_object,
-			const char *vpath)
+static void hash_object(const char *path, const char *type, const char *vpath,
+			unsigned flags)
 {
 	int fd;
 	fd = open(path, O_RDONLY);
 	if (fd < 0)
 		die_errno("Cannot open '%s'", path);
-	hash_fd(fd, type, write_object, vpath);
+	hash_fd(fd, type, vpath, flags);
 }
 
-static void hash_stdin_paths(const char *type, int write_objects, int no_filters)
+static void hash_stdin_paths(const char *type, int no_filters, unsigned flags)
 {
 	struct strbuf buf = STRBUF_INIT, nbuf = STRBUF_INIT;
 
@@ -47,8 +45,7 @@ static void hash_stdin_paths(const char *type, int write_objects, int no_filters
 				die("line is badly quoted");
 			strbuf_swap(&buf, &nbuf);
 		}
-		hash_object(buf.buf, type, write_objects,
-			    no_filters ? NULL : buf.buf);
+		hash_object(buf.buf, type, no_filters ? NULL : buf.buf, flags);
 	}
 	strbuf_release(&buf);
 	strbuf_release(&nbuf);
@@ -64,12 +61,13 @@ int cmd_hash_object(int argc, const char **argv, const char *prefix)
 	const char *type = blob_type;
 	int hashstdin = 0;
 	int stdin_paths = 0;
-	int write_object = 0;
 	int no_filters = 0;
+	unsigned flags = HASH_FORMAT_CHECK;
 	const char *vpath = NULL;
 	const struct option hash_object_options[] = {
 		OPT_STRING('t', NULL, &type, N_("type"), N_("object type")),
-		OPT_BOOL('w', NULL, &write_object, N_("write the object into the object database")),
+		OPT_BIT('w', NULL, &flags, N_("write the object into the object database"),
+			HASH_WRITE_OBJECT),
 		OPT_COUNTUP( 0 , "stdin", &hashstdin, N_("read the object from stdin")),
 		OPT_BOOL( 0 , "stdin-paths", &stdin_paths, N_("read file names from stdin")),
 		OPT_BOOL( 0 , "no-filters", &no_filters, N_("store file as is without filters")),
@@ -83,7 +81,7 @@ int cmd_hash_object(int argc, const char **argv, const char *prefix)
 	argc = parse_options(argc, argv, NULL, hash_object_options,
 			     hash_object_usage, 0);
 
-	if (write_object) {
+	if (flags & HASH_WRITE_OBJECT) {
 		prefix = setup_git_directory();
 		prefix_length = prefix ? strlen(prefix) : 0;
 		if (vpath && prefix)
@@ -113,19 +111,19 @@ int cmd_hash_object(int argc, const char **argv, const char *prefix)
 	}
 
 	if (hashstdin)
-		hash_fd(0, type, write_object, vpath);
+		hash_fd(0, type, vpath, flags);
 
 	for (i = 0 ; i < argc; i++) {
 		const char *arg = argv[i];
 
 		if (0 <= prefix_length)
 			arg = prefix_filename(prefix, prefix_length, arg);
-		hash_object(arg, type, write_object,
-			    no_filters ? NULL : vpath ? vpath : arg);
+		hash_object(arg, type, no_filters ? NULL : vpath ? vpath : arg,
+			    flags);
 	}
 
 	if (stdin_paths)
-		hash_stdin_paths(type, write_object, no_filters);
+		hash_stdin_paths(type, no_filters, flags);
 
 	return 0;
 }

diff --git a/builtin/hash-object.c b/builtin/hash-object.c
index 1fb07ee..6158363 100644
--- a/builtin/hash-object.c
+++ b/builtin/hash-object.c
@@ -10,13 +10,36 @@
 #include "parse-options.h"
 #include "exec_cmd.h"
 
-static void hash_fd(int fd, const char *type, const char *path, unsigned flags)
+/*
+ * This is to create corrupt objects for debugging and as such it
+ * needs to bypass the data conversion performed by, and the type
+ * limitation imposed by, index_fd() and its callees.
+ */
+static int hash_literally(unsigned char *sha1, int fd, const char *type, unsigned flags)
+{
+	struct strbuf buf = STRBUF_INIT;
+	int ret;
+
+	if (strbuf_read(&buf, fd, 4096) < 0)
+		ret = -1;
+	else if (flags & HASH_WRITE_OBJECT)
+		ret = write_sha1_file(buf.buf, buf.len, type, sha1);
+	else
+		ret = hash_sha1_file(buf.buf, buf.len, type, sha1);
+	strbuf_release(&buf);
+	return ret;
+}
+
+static void hash_fd(int fd, const char *type, const char *path, unsigned flags,
+		    int literally)
 {
 	struct stat st;
 	unsigned char sha1[20];
 
 	if (fstat(fd, &st) < 0 ||
-	    index_fd(sha1, fd, &st, type_from_string(type), path, flags))
+	    (literally
+	     ? hash_literally(sha1, fd, type, flags)
+	     : index_fd(sha1, fd, &st, type_from_string(type), path, flags)))
 		die((flags & HASH_WRITE_OBJECT)
 		    ? "Unable to add %s to database"
 		    : "Unable to hash %s", path);
@@ -25,16 +48,17 @@ static void hash_fd(int fd, const char *type, const char *path, unsigned flags)
 }
 
 static void hash_object(const char *path, const char *type, const char *vpath,
-			unsigned flags)
+			unsigned flags, int literally)
 {
 	int fd;
 	fd = open(path, O_RDONLY);
 	if (fd < 0)
 		die_errno("Cannot open '%s'", path);
-	hash_fd(fd, type, vpath, flags);
+	hash_fd(fd, type, vpath, flags, literally);
 }
 
-static void hash_stdin_paths(const char *type, int no_filters, unsigned flags)
+static void hash_stdin_paths(const char *type, int no_filters, unsigned flags,
+			     int literally)
 {
 	struct strbuf buf = STRBUF_INIT, nbuf = STRBUF_INIT;
 
@@ -45,7 +69,8 @@ static void hash_stdin_paths(const char *type, int no_filters, unsigned flags)
 				die("line is badly quoted");
 			strbuf_swap(&buf, &nbuf);
 		}
-		hash_object(buf.buf, type, no_filters ? NULL : buf.buf, flags);
+		hash_object(buf.buf, type, no_filters ? NULL : buf.buf, flags,
+			    literally);
 	}
 	strbuf_release(&buf);
 	strbuf_release(&nbuf);
@@ -62,6 +87,7 @@ int cmd_hash_object(int argc, const char **argv, const char *prefix)
 	int hashstdin = 0;
 	int stdin_paths = 0;
 	int no_filters = 0;
+	int literally = 0;
 	unsigned flags = HASH_FORMAT_CHECK;
 	const char *vpath = NULL;
 	const struct option hash_object_options[] = {
@@ -71,6 +97,7 @@ int cmd_hash_object(int argc, const char **argv, const char *prefix)
 		OPT_COUNTUP( 0 , "stdin", &hashstdin, N_("read the object from stdin")),
 		OPT_BOOL( 0 , "stdin-paths", &stdin_paths, N_("read file names from stdin")),
 		OPT_BOOL( 0 , "no-filters", &no_filters, N_("store file as is without filters")),
+		OPT_BOOL( 0, "literally", &literally, N_("just hash any random garbage to create corrupt objects for debugging Git")),
 		OPT_STRING( 0 , "path", &vpath, N_("file"), N_("process file as it were from this path")),
 		OPT_END()
 	};
@@ -111,7 +138,7 @@ int cmd_hash_object(int argc, const char **argv, const char *prefix)
 	}
 
 	if (hashstdin)
-		hash_fd(0, type, vpath, flags);
+		hash_fd(0, type, vpath, flags, literally);
 
 	for (i = 0 ; i < argc; i++) {
 		const char *arg = argv[i];
@@ -119,11 +146,11 @@ int cmd_hash_object(int argc, const char **argv, const char *prefix)
 		if (0 <= prefix_length)
 			arg = prefix_filename(prefix, prefix_length, arg);
 		hash_object(arg, type, no_filters ? NULL : vpath ? vpath : arg,
-			    flags);
+			    flags, literally);
 	}
 
 	if (stdin_paths)
-		hash_stdin_paths(type, no_filters, flags);
+		hash_stdin_paths(type, no_filters, flags, literally);
 
 	return 0;
 }

object.c, object.h

diff --git a/object.c b/object.c
index a16b9f9..aedac24 100644
--- a/object.c
+++ b/object.c
@@ -33,13 +33,20 @@ const char *typename(unsigned int type)
 	return object_type_strings[type];
 }
 
-int type_from_string(const char *str)
+int type_from_string_gently(const char *str, ssize_t len, int gentle)
 {
 	int i;
 
+	if (len < 0)
+		len = strlen(str);
+
 	for (i = 1; i < ARRAY_SIZE(object_type_strings); i++)
-		if (!strcmp(str, object_type_strings[i]))
+		if (!strncmp(str, object_type_strings[i], len))
 			return i;
+
+	if (gentle)
+		return -1;
+
 	die("invalid object type \"%s\"", str);
 }
 
diff --git a/object.h b/object.h
index 5e8d8ee..e028ced 100644
--- a/object.h
+++ b/object.h
@@ -53,7 +53,8 @@ struct object {
 };
 
 extern const char *typename(unsigned int type);
-extern int type_from_string(const char *str);
+extern int type_from_string_gently(const char *str, ssize_t, int gentle);
+#define type_from_string(str) type_from_string_gently(str, -1, 0)
 
 /*
  * Return the current number of buckets in the object hashmap.

builtin/fsck.c, builtin/index-pack.c, builtin/unpack-objects.c, fsck.c, fsck.h

diff --git a/builtin/fsck.c b/builtin/fsck.c
index d42a27d..d9f4e6e 100644
--- a/builtin/fsck.c
+++ b/builtin/fsck.c
@@ -298,7 +298,7 @@ static int fsck_obj(struct object *obj)
 
 	if (fsck_walk(obj, mark_used, NULL))
 		objerror(obj, "broken links");
-	if (fsck_object(obj, check_strict, fsck_error_func))
+	if (fsck_object(obj, NULL, 0, check_strict, fsck_error_func))
 		return -1;
 
 	if (obj->type == OBJ_TREE) {
diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 5568a5b..f2465ff 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -773,7 +773,8 @@ static void sha1_object(const void *data, struct object_entry *obj_entry,
 			if (!obj)
 				die(_("invalid %s"), typename(type));
 			if (do_fsck_object &&
-			    fsck_object(obj, 1, fsck_error_function))
+			    fsck_object(obj, buf, size, 1,
+				    fsck_error_function))
 				die(_("Error in object"));
 			if (fsck_walk(obj, mark_link, NULL))
 				die(_("Not all child objects of %s are reachable"), sha1_to_hex(obj->sha1));
diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index 99cde45..855d94b 100644
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c
@@ -164,10 +164,10 @@ static unsigned nr_objects;
  * Called only from check_object() after it verified this object
  * is Ok.
  */
-static void write_cached_object(struct object *obj)
+static void write_cached_object(struct object *obj, struct obj_buffer *obj_buf)
 {
 	unsigned char sha1[20];
-	struct obj_buffer *obj_buf = lookup_object_buffer(obj);
+
 	if (write_sha1_file(obj_buf->buffer, obj_buf->size, typename(obj->type), sha1) < 0)
 		die("failed to write object %s", sha1_to_hex(obj->sha1));
 	obj->flags |= FLAG_WRITTEN;
@@ -180,6 +180,8 @@ static void write_cached_object(struct object *obj)
  */
 static int check_object(struct object *obj, int type, void *data)
 {
+	struct obj_buffer *obj_buf;
+
 	if (!obj)
 		return 1;
 
@@ -198,11 +200,15 @@ static int check_object(struct object *obj, int type, void *data)
 		return 0;
 	}
 
-	if (fsck_object(obj, 1, fsck_error_function))
+	obj_buf = lookup_object_buffer(obj);
+	if (!obj_buf)
+		die("Whoops! Cannot find object '%s'", sha1_to_hex(obj->sha1));
+	if (fsck_object(obj, obj_buf->buffer, obj_buf->size, 1,
+			fsck_error_function))
 		die("Error in object");
 	if (fsck_walk(obj, check_object, NULL))
 		die("Error on reachable objects of %s", sha1_to_hex(obj->sha1));
-	write_cached_object(obj);
+	write_cached_object(obj, obj_buf);
 	return 0;
 }
 
diff --git a/fsck.c b/fsck.c
index 56156ff..dd77628 100644
--- a/fsck.c
+++ b/fsck.c
@@ -277,7 +277,7 @@ static int fsck_ident(const char **ident, struct object *obj, fsck_error error_f
 }
 
 static int fsck_commit_buffer(struct commit *commit, const char *buffer,
-			      fsck_error error_func)
+	unsigned long size, fsck_error error_func)
 {
 	unsigned char tree_sha1[20], sha1[20];
 	struct commit_graft *graft;
@@ -322,15 +322,18 @@ static int fsck_commit_buffer(struct commit *commit, const char *buffer,
 	return 0;
 }
 
-static int fsck_commit(struct commit *commit, fsck_error error_func)
+static int fsck_commit(struct commit *commit, const char *data,
+	unsigned long size, fsck_error error_func)
 {
-	const char *buffer = get_commit_buffer(commit, NULL);
-	int ret = fsck_commit_buffer(commit, buffer, error_func);
-	unuse_commit_buffer(commit, buffer);
+	const char *buffer = data ?  data : get_commit_buffer(commit, &size);
+	int ret = fsck_commit_buffer(commit, buffer, size, error_func);
+	if (!data)
+		unuse_commit_buffer(commit, buffer);
 	return ret;
 }
 
-static int fsck_tag(struct tag *tag, fsck_error error_func)
+static int fsck_tag(struct tag *tag, const char *data,
+	unsigned long size, fsck_error error_func)
 {
 	struct object *tagged = tag->tagged;
 
@@ -339,7 +342,8 @@ static int fsck_tag(struct tag *tag, fsck_error error_func)
 	return 0;
 }
 
-int fsck_object(struct object *obj, int strict, fsck_error error_func)
+int fsck_object(struct object *obj, void *data, unsigned long size,
+	int strict, fsck_error error_func)
 {
 	if (!obj)
 		return error_func(obj, FSCK_ERROR, "no valid object to fsck");
@@ -349,9 +353,11 @@ int fsck_object(struct object *obj, int strict, fsck_error error_func)
 	if (obj->type == OBJ_TREE)
 		return fsck_tree((struct tree *) obj, strict, error_func);
 	if (obj->type == OBJ_COMMIT)
-		return fsck_commit((struct commit *) obj, error_func);
+		return fsck_commit((struct commit *) obj, (const char *) data,
+			size, error_func);
 	if (obj->type == OBJ_TAG)
-		return fsck_tag((struct tag *) obj, error_func);
+		return fsck_tag((struct tag *) obj, (const char *) data,
+			size, error_func);
 
 	return error_func(obj, FSCK_ERROR, "unknown type '%d' (internal fsck error)",
 			  obj->type);
diff --git a/fsck.h b/fsck.h
index 1e4f527..d1e6387 100644
--- a/fsck.h
+++ b/fsck.h
@@ -28,6 +28,8 @@ int fsck_error_function(struct object *obj, int type, const char *fmt, ...);
  *    0		everything OK
  */
 int fsck_walk(struct object *obj, fsck_walk_func walk, void *data);
-int fsck_object(struct object *obj, int strict, fsck_error error_func);
+/* If NULL is passed for data, we assume the object is local and read it. */
+int fsck_object(struct object *obj, void *data, unsigned long size,
+	int strict, fsck_error error_func);
 
 #endif

diff --git a/fsck.c b/fsck.c
index dd77628..73da6f8 100644
--- a/fsck.c
+++ b/fsck.c
@@ -237,6 +237,26 @@ static int fsck_tree(struct tree *item, int strict, fsck_error error_func)
 	return retval;
 }
 
+static int require_end_of_header(const void *data, unsigned long size,
+	struct object *obj, fsck_error error_func)
+{
+	const char *buffer = (const char *)data;
+	unsigned long i;
+
+	for (i = 0; i < size; i++) {
+		switch (buffer[i]) {
+		case '\0':
+			return error_func(obj, FSCK_ERROR,
+				"unterminated header: NUL at offset %d", i);
+		case '\n':
+			if (i + 1 < size && buffer[i + 1] == '\n')
+				return 0;
+		}
+	}
+
+	return error_func(obj, FSCK_ERROR, "unterminated header");
+}
+
 static int fsck_ident(const char **ident, struct object *obj, fsck_error error_func)
 {
 	char *end;
@@ -284,6 +304,9 @@ static int fsck_commit_buffer(struct commit *commit, const char *buffer,
 	unsigned parent_count, parent_line_count = 0;
 	int err;
 
+	if (require_end_of_header(buffer, size, &commit->object, error_func))
+		return -1;
+
 	if (!skip_prefix(buffer, "tree ", &buffer))
 		return error_func(&commit->object, FSCK_ERROR, "invalid format - expected 'tree' line");
 	if (get_sha1_hex(buffer, tree_sha1) || buffer[40] != '\n')

diff --git a/fsck.c b/fsck.c
index 73da6f8..2fffa43 100644
--- a/fsck.c
+++ b/fsck.c
@@ -6,6 +6,7 @@
 #include "commit.h"
 #include "tag.h"
 #include "fsck.h"
+#include "refs.h"
 
 static int fsck_walk_tree(struct tree *tree, fsck_walk_func walk, void *data)
 {
@@ -355,6 +356,88 @@ static int fsck_commit(struct commit *commit, const char *data,
 	return ret;
 }
 
+static int fsck_tag_buffer(struct tag *tag, const char *data,
+	unsigned long size, fsck_error error_func)
+{
+	unsigned char sha1[20];
+	int ret = 0;
+	const char *buffer;
+	char *to_free = NULL, *eol;
+	struct strbuf sb = STRBUF_INIT;
+
+	if (data)
+		buffer = data;
+	else {
+		enum object_type type;
+
+		buffer = to_free =
+			read_sha1_file(tag->object.sha1, &type, &size);
+		if (!buffer)
+			return error_func(&tag->object, FSCK_ERROR,
+				"cannot read tag object");
+
+		if (type != OBJ_TAG) {
+			ret = error_func(&tag->object, FSCK_ERROR,
+				"expected tag got %s",
+			    typename(type));
+			goto done;
+		}
+	}
+
+	if (require_end_of_header(buffer, size, &tag->object, error_func))
+		goto done;
+
+	if (!skip_prefix(buffer, "object ", &buffer)) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - expected 'object' line");
+		goto done;
+	}
+	if (get_sha1_hex(buffer, sha1) || buffer[40] != '\n') {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid 'object' line format - bad sha1");
+		goto done;
+	}
+	buffer += 41;
+
+	if (!skip_prefix(buffer, "type ", &buffer)) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - expected 'type' line");
+		goto done;
+	}
+	eol = strchr(buffer, '\n');
+	if (!eol) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - unexpected end after 'type' line");
+		goto done;
+	}
+	if (type_from_string_gently(buffer, eol - buffer, 1) < 0)
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid 'type' value");
+	if (ret)
+		goto done;
+	buffer = eol + 1;
+
+	if (!skip_prefix(buffer, "tag ", &buffer)) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - expected 'tag' line");
+		goto done;
+	}
+	eol = strchr(buffer, '\n');
+	if (!eol) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - unexpected end after 'type' line");
+		goto done;
+	}
+	strbuf_addf(&sb, "refs/tags/%.*s", (int)(eol - buffer), buffer);
+	if (check_refname_format(sb.buf, 0))
+		error_func(&tag->object, FSCK_WARN, "invalid 'tag' name: %s", buffer);
+	buffer = eol + 1;
+
+	if (!skip_prefix(buffer, "tagger ", &buffer))
+		/* early tags do not contain 'tagger' lines; warn only */
+		error_func(&tag->object, FSCK_WARN, "invalid format - expected 'tagger' line");
+	else
+		ret = fsck_ident(&buffer, &tag->object, error_func);
+
+done:
+	strbuf_release(&sb);
+	free(to_free);
+	return ret;
+}
+
 static int fsck_tag(struct tag *tag, const char *data,
 	unsigned long size, fsck_error error_func)
 {
@@ -362,7 +445,8 @@ static int fsck_tag(struct tag *tag, const char *data,
 
 	if (!tagged)
 		return error_func(&tag->object, FSCK_ERROR, "could not load tagged object");
-	return 0;
+
+	return fsck_tag_buffer(tag, data, size, error_func);
 }
 
 int fsck_object(struct object *obj, void *data, unsigned long size,

diff --git a/t/t1450-fsck.sh b/t/t1450-fsck.sh
index 8c739c9..2b6a6f2 100755
--- a/t/t1450-fsck.sh
+++ b/t/t1450-fsck.sh
@@ -194,6 +194,26 @@ test_expect_success 'tag pointing to something else than its type' '
 	test_must_fail git fsck --tags
 '
 
+test_expect_success 'tag with incorrect tag name & missing tagger' '
+	sha=$(git rev-parse HEAD) &&
+	cat >wrong-tag <<-EOF &&
+	object $sha
+	type commit
+	tag wrong name format
+
+	This is an invalid tag.
+	EOF
+
+	tag=$(git hash-object -t tag -w --stdin <wrong-tag) &&
+	test_when_finished "remove_object $tag" &&
+	echo $tag >.git/refs/tags/wrong &&
+	test_when_finished "git update-ref -d refs/tags/wrong" &&
+	git fsck --tags 2>out &&
+	cat out &&
+	grep "invalid .tag. name" out &&
+	grep "expected .tagger. line" out
+'
+
 test_expect_success 'cleaned up' '
 	git fsck >actual 2>&1 &&
 	test_cmp empty actual

diff --git a/t/t5302-pack-index.sh b/t/t5302-pack-index.sh
index 4bbb718..61bc8da 100755
--- a/t/t5302-pack-index.sh
+++ b/t/t5302-pack-index.sh
@@ -243,4 +243,23 @@ test_expect_success 'running index-pack in the object store' '
     test -f .git/objects/pack/pack-${pack1}.idx
 '
 
+test_expect_success 'index-pack --strict warns upon missing tagger in tag' '
+    sha=$(git rev-parse HEAD) &&
+    cat >wrong-tag <<EOF &&
+object $sha
+type commit
+tag guten tag
+
+This is an invalid tag.
+EOF
+
+    tag=$(git hash-object -t tag -w --stdin <wrong-tag) &&
+    pack1=$(echo $tag $sha | git pack-objects tag-test) &&
+    echo remove tag object &&
+    thirtyeight=${tag#??} &&
+    rm -f .git/objects/${tag%$thirtyeight}/$thirtyeight &&
+    git index-pack --strict tag-test-${pack1}.pack 2>err &&
+    grep "^error:.* expected .tagger. line" err
+'
+
 test_done

object.c, object.h

diff --git a/object.c b/object.c
index a16b9f9..aedac24 100644
--- a/object.c
+++ b/object.c
@@ -33,13 +33,20 @@ const char *typename(unsigned int type)
 	return object_type_strings[type];
 }
 
-int type_from_string(const char *str)
+int type_from_string_gently(const char *str, ssize_t len, int gentle)
 {
 	int i;
 
+	if (len < 0)
+		len = strlen(str);
+
 	for (i = 1; i < ARRAY_SIZE(object_type_strings); i++)
-		if (!strcmp(str, object_type_strings[i]))
+		if (!strncmp(str, object_type_strings[i], len))
 			return i;
+
+	if (gentle)
+		return -1;
+
 	die("invalid object type \"%s\"", str);
 }
 
diff --git a/object.h b/object.h
index 5e8d8ee..e028ced 100644
--- a/object.h
+++ b/object.h
@@ -53,7 +53,8 @@ struct object {
 };
 
 extern const char *typename(unsigned int type);
-extern int type_from_string(const char *str);
+extern int type_from_string_gently(const char *str, ssize_t, int gentle);
+#define type_from_string(str) type_from_string_gently(str, -1, 0)
 
 /*
  * Return the current number of buckets in the object hashmap.

builtin/fsck.c, builtin/index-pack.c, builtin/unpack-objects.c, fsck.c, fsck.h

diff --git a/builtin/fsck.c b/builtin/fsck.c
index d42a27d..d9f4e6e 100644
--- a/builtin/fsck.c
+++ b/builtin/fsck.c
@@ -298,7 +298,7 @@ static int fsck_obj(struct object *obj)
 
 	if (fsck_walk(obj, mark_used, NULL))
 		objerror(obj, "broken links");
-	if (fsck_object(obj, check_strict, fsck_error_func))
+	if (fsck_object(obj, NULL, 0, check_strict, fsck_error_func))
 		return -1;
 
 	if (obj->type == OBJ_TREE) {
diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 5568a5b..f2465ff 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -773,7 +773,8 @@ static void sha1_object(const void *data, struct object_entry *obj_entry,
 			if (!obj)
 				die(_("invalid %s"), typename(type));
 			if (do_fsck_object &&
-			    fsck_object(obj, 1, fsck_error_function))
+			    fsck_object(obj, buf, size, 1,
+				    fsck_error_function))
 				die(_("Error in object"));
 			if (fsck_walk(obj, mark_link, NULL))
 				die(_("Not all child objects of %s are reachable"), sha1_to_hex(obj->sha1));
diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index 99cde45..855d94b 100644
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c
@@ -164,10 +164,10 @@ static unsigned nr_objects;
  * Called only from check_object() after it verified this object
  * is Ok.
  */
-static void write_cached_object(struct object *obj)
+static void write_cached_object(struct object *obj, struct obj_buffer *obj_buf)
 {
 	unsigned char sha1[20];
-	struct obj_buffer *obj_buf = lookup_object_buffer(obj);
+
 	if (write_sha1_file(obj_buf->buffer, obj_buf->size, typename(obj->type), sha1) < 0)
 		die("failed to write object %s", sha1_to_hex(obj->sha1));
 	obj->flags |= FLAG_WRITTEN;
@@ -180,6 +180,8 @@ static void write_cached_object(struct object *obj)
  */
 static int check_object(struct object *obj, int type, void *data)
 {
+	struct obj_buffer *obj_buf;
+
 	if (!obj)
 		return 1;
 
@@ -198,11 +200,15 @@ static int check_object(struct object *obj, int type, void *data)
 		return 0;
 	}
 
-	if (fsck_object(obj, 1, fsck_error_function))
+	obj_buf = lookup_object_buffer(obj);
+	if (!obj_buf)
+		die("Whoops! Cannot find object '%s'", sha1_to_hex(obj->sha1));
+	if (fsck_object(obj, obj_buf->buffer, obj_buf->size, 1,
+			fsck_error_function))
 		die("Error in object");
 	if (fsck_walk(obj, check_object, NULL))
 		die("Error on reachable objects of %s", sha1_to_hex(obj->sha1));
-	write_cached_object(obj);
+	write_cached_object(obj, obj_buf);
 	return 0;
 }
 
diff --git a/fsck.c b/fsck.c
index 56156ff..dd77628 100644
--- a/fsck.c
+++ b/fsck.c
@@ -277,7 +277,7 @@ static int fsck_ident(const char **ident, struct object *obj, fsck_error error_f
 }
 
 static int fsck_commit_buffer(struct commit *commit, const char *buffer,
-			      fsck_error error_func)
+	unsigned long size, fsck_error error_func)
 {
 	unsigned char tree_sha1[20], sha1[20];
 	struct commit_graft *graft;
@@ -322,15 +322,18 @@ static int fsck_commit_buffer(struct commit *commit, const char *buffer,
 	return 0;
 }
 
-static int fsck_commit(struct commit *commit, fsck_error error_func)
+static int fsck_commit(struct commit *commit, const char *data,
+	unsigned long size, fsck_error error_func)
 {
-	const char *buffer = get_commit_buffer(commit, NULL);
-	int ret = fsck_commit_buffer(commit, buffer, error_func);
-	unuse_commit_buffer(commit, buffer);
+	const char *buffer = data ?  data : get_commit_buffer(commit, &size);
+	int ret = fsck_commit_buffer(commit, buffer, size, error_func);
+	if (!data)
+		unuse_commit_buffer(commit, buffer);
 	return ret;
 }
 
-static int fsck_tag(struct tag *tag, fsck_error error_func)
+static int fsck_tag(struct tag *tag, const char *data,
+	unsigned long size, fsck_error error_func)
 {
 	struct object *tagged = tag->tagged;
 
@@ -339,7 +342,8 @@ static int fsck_tag(struct tag *tag, fsck_error error_func)
 	return 0;
 }
 
-int fsck_object(struct object *obj, int strict, fsck_error error_func)
+int fsck_object(struct object *obj, void *data, unsigned long size,
+	int strict, fsck_error error_func)
 {
 	if (!obj)
 		return error_func(obj, FSCK_ERROR, "no valid object to fsck");
@@ -349,9 +353,11 @@ int fsck_object(struct object *obj, int strict, fsck_error error_func)
 	if (obj->type == OBJ_TREE)
 		return fsck_tree((struct tree *) obj, strict, error_func);
 	if (obj->type == OBJ_COMMIT)
-		return fsck_commit((struct commit *) obj, error_func);
+		return fsck_commit((struct commit *) obj, (const char *) data,
+			size, error_func);
 	if (obj->type == OBJ_TAG)
-		return fsck_tag((struct tag *) obj, error_func);
+		return fsck_tag((struct tag *) obj, (const char *) data,
+			size, error_func);
 
 	return error_func(obj, FSCK_ERROR, "unknown type '%d' (internal fsck error)",
 			  obj->type);
diff --git a/fsck.h b/fsck.h
index 1e4f527..d1e6387 100644
--- a/fsck.h
+++ b/fsck.h
@@ -28,6 +28,8 @@ int fsck_error_function(struct object *obj, int type, const char *fmt, ...);
  *    0		everything OK
  */
 int fsck_walk(struct object *obj, fsck_walk_func walk, void *data);
-int fsck_object(struct object *obj, int strict, fsck_error error_func);
+/* If NULL is passed for data, we assume the object is local and read it. */
+int fsck_object(struct object *obj, void *data, unsigned long size,
+	int strict, fsck_error error_func);
 
 #endif

diff --git a/fsck.c b/fsck.c
index dd77628..9dd7d12 100644
--- a/fsck.c
+++ b/fsck.c
@@ -237,6 +237,26 @@ static int fsck_tree(struct tree *item, int strict, fsck_error error_func)
 	return retval;
 }
 
+static int require_end_of_header(const void *data, unsigned long size,
+	struct object *obj, fsck_error error_func)
+{
+	const char *buffer = (const char *)data;
+	int i;
+
+	for (i = 0; i < size; i++) {
+		switch (buffer[i]) {
+		case '\0':
+			return error_func(obj, FSCK_ERROR,
+				"invalid message: NUL at offset %d", i);
+		case '\n':
+			if (i + 1 < size && buffer[i + 1] == '\n')
+				return 0;
+		}
+	}
+
+	return error_func(obj, FSCK_ERROR, "invalid buffer: missing empty line");
+}
+
 static int fsck_ident(const char **ident, struct object *obj, fsck_error error_func)
 {
 	char *end;
@@ -284,6 +304,9 @@ static int fsck_commit_buffer(struct commit *commit, const char *buffer,
 	unsigned parent_count, parent_line_count = 0;
 	int err;
 
+	if (require_end_of_header(buffer, size, &commit->object, error_func))
+		return -1;
+
 	if (!skip_prefix(buffer, "tree ", &buffer))
 		return error_func(&commit->object, FSCK_ERROR, "invalid format - expected 'tree' line");
 	if (get_sha1_hex(buffer, tree_sha1) || buffer[40] != '\n')

diff --git a/fsck.c b/fsck.c
index 9dd7d12..8341a30 100644
--- a/fsck.c
+++ b/fsck.c
@@ -6,6 +6,7 @@
 #include "commit.h"
 #include "tag.h"
 #include "fsck.h"
+#include "refs.h"
 
 static int fsck_walk_tree(struct tree *tree, fsck_walk_func walk, void *data)
 {
@@ -355,6 +356,87 @@ static int fsck_commit(struct commit *commit, const char *data,
 	return ret;
 }
 
+static int fsck_tag_buffer(struct tag *tag, const char *data,
+	unsigned long size, fsck_error error_func)
+{
+	unsigned char sha1[20];
+	int ret = 0;
+	const char *buffer;
+	char *to_free = NULL, *eol;
+	struct strbuf sb = STRBUF_INIT;
+
+	if (data)
+		buffer = data;
+	else {
+		enum object_type type;
+
+		buffer = to_free =
+			read_sha1_file(tag->object.sha1, &type, &size);
+		if (!buffer)
+			return error_func(&tag->object, FSCK_ERROR,
+				"cannot read tag object");
+
+		if (type != OBJ_TAG) {
+			ret = error_func(&tag->object, FSCK_ERROR,
+				"expected tag got %s",
+			    typename(type));
+			goto done;
+		}
+	}
+
+	if (require_end_of_header(buffer, size, &tag->object, error_func))
+		goto done;
+
+	if (!skip_prefix(buffer, "object ", &buffer)) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - expected 'object' line");
+		goto done;
+	}
+	if (get_sha1_hex(buffer, sha1) || buffer[40] != '\n') {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid 'object' line format - bad sha1");
+		goto done;
+	}
+	buffer += 41;
+
+	if (!skip_prefix(buffer, "type ", &buffer)) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - expected 'type' line");
+		goto done;
+	}
+	eol = strchr(buffer, '\n');
+	if (!eol) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - unexpected end after 'type' line");
+		goto done;
+	}
+	if (type_from_string_gently(buffer, eol - buffer, 1) < 0)
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid 'type' value");
+	if (ret)
+		goto done;
+	buffer = eol + 1;
+
+	if (!skip_prefix(buffer, "tag ", &buffer)) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - expected 'tag' line");
+		goto done;
+	}
+	eol = strchr(buffer, '\n');
+	if (!eol) {
+		ret = error_func(&tag->object, FSCK_ERROR, "invalid format - unexpected end after 'type' line");
+		goto done;
+	}
+	strbuf_addf(&sb, "refs/tags/%.*s", (int)(eol - buffer), buffer);
+	if (check_refname_format(sb.buf, 0))
+		error_func(&tag->object, FSCK_WARN, "invalid 'tag' name: %s", buffer);
+	buffer = eol + 1;
+
+	if (!skip_prefix(buffer, "tagger ", &buffer)) {
+		/* early tags do not contain 'tagger' lines; warn only */
+		error_func(&tag->object, FSCK_WARN, "invalid format - expected 'tagger' line");
+	}
+	ret = fsck_ident(&buffer, &tag->object, error_func);
+
+done:
+	free(to_free);
+	return ret;
+}
+
 static int fsck_tag(struct tag *tag, const char *data,
 	unsigned long size, fsck_error error_func)
 {
@@ -362,7 +444,8 @@ static int fsck_tag(struct tag *tag, const char *data,
 
 	if (!tagged)
 		return error_func(&tag->object, FSCK_ERROR, "could not load tagged object");
-	return 0;
+
+	return fsck_tag_buffer(tag, data, size, error_func);
 }
 
 int fsck_object(struct object *obj, void *data, unsigned long size,

diff --git a/t/t1450-fsck.sh b/t/t1450-fsck.sh
index 8c739c9..16b3e4a 100755
--- a/t/t1450-fsck.sh
+++ b/t/t1450-fsck.sh
@@ -194,6 +194,45 @@ test_expect_success 'tag pointing to something else than its type' '
 	test_must_fail git fsck --tags
 '
 
+test_expect_success 'tag with incorrect tag name' '
+	sha=$(git rev-parse HEAD) &&
+	cat >wrong-tag <<-EOF &&
+	object $sha
+	type commit
+	tag wrong name format
+	tagger T A Gger <tagger@example.com> 1234567890 -0000
+
+	This is an invalid tag.
+	EOF
+
+	tag=$(git hash-object -t tag -w --stdin <wrong-tag) &&
+	test_when_finished "remove_object $tag" &&
+	echo $tag >.git/refs/tags/wrong &&
+	test_when_finished "git update-ref -d refs/tags/wrong" &&
+	git fsck --tags 2>out &&
+	cat out &&
+	grep "invalid .tag. name" out
+'
+
+test_expect_success 'tag with missing tagger' '
+	sha=$(git rev-parse HEAD) &&
+	cat >wrong-tag <<-EOF &&
+	object $sha
+	type commit
+	tag gutentag
+
+	This is an invalid tag.
+	EOF
+
+	tag=$(git hash-object -t tag -w --stdin <wrong-tag) &&
+	test_when_finished "remove_object $tag" &&
+	echo $tag >.git/refs/tags/wrong &&
+	test_when_finished "git update-ref -d refs/tags/wrong" &&
+	git fsck --tags 2>out &&
+	cat out &&
+	grep "expected .tagger. line" out
+'
+
 test_expect_success 'cleaned up' '
 	git fsck >actual 2>&1 &&
 	test_cmp empty actual

diff --git a/t/t5302-pack-index.sh b/t/t5302-pack-index.sh
index 4bbb718..80bd3ee 100755
--- a/t/t5302-pack-index.sh
+++ b/t/t5302-pack-index.sh
@@ -243,4 +243,23 @@ test_expect_success 'running index-pack in the object store' '
     test -f .git/objects/pack/pack-${pack1}.idx
 '
 
+test_expect_success 'index-pack --strict fails upon invalid tag' '
+    sha=$(git rev-parse HEAD) &&
+    cat >wrong-tag <<EOF &&
+object $sha
+type commit
+tag guten tag
+
+This is an invalid tag.
+EOF
+
+    tag=$(git hash-object -t tag -w --stdin <wrong-tag) &&
+    pack1=$(echo $tag | git pack-objects tag-test) &&
+    echo remove tag object &&
+    thirtyeight=${tag#??} &&
+    rm -f .git/objects/${tag%$thirtyeight}/$thirtyeight &&
+    test_must_fail git index-pack --strict tag-test-${pack1}.pack 2> err &&
+    grep "invalid .tag. name" err
+'
+
 test_done