[PATCH v3 0/3] Add commit-graph fuzzer and fix buffer overflow

Ad a new fuzz test for the commit graph and fix a buffer read-overflow that it discovered. Additionally, fix the Makefile instructions for building fuzzers.

Changes since V2:
  * Avoid pointer arithmetic overflow when checking the graph's chunk
    count.
  * Merge the corrupt_graph_and_verify and
    corrupt_and_zero_graph_then_verify test functions.

Josh Steadmon (3):
  commit-graph, fuzz: Add fuzzer for commit-graph
  commit-graph: fix buffer read-overflow
  Makefile: correct example fuzz build

 .gitignore              |  1 +
 Makefile                |  3 +-
 commit-graph.c          | 67 +++++++++++++++++++++++++++++------------
 commit-graph.h          |  3 ++
 fuzz-commit-graph.c     | 16 ++++++++++
 t/t5318-commit-graph.sh | 15 +++++++--
 6 files changed, 83 insertions(+), 22 deletions(-)
 create mode 100644 fuzz-commit-graph.c

Range-diff against v2:
1:  af45c2337f ! 1:  675d58ecea commit-graph: fix buffer read-overflow
    @@ -22,8 +22,8 @@
     +		uint64_t chunk_offset;
      		int chunk_repeated = 0;
      
    -+		if (chunk_lookup + GRAPH_CHUNKLOOKUP_WIDTH >
    -+		    data + graph_size) {
    ++		if (data + graph_size - chunk_lookup <
    ++		    GRAPH_CHUNKLOOKUP_WIDTH) {
     +			error(_("chunk lookup table entry missing; graph file may be incomplete"));
     +			free(graph);
     +			return NULL;
    @@ -40,31 +40,34 @@
      --- a/t/t5318-commit-graph.sh
      +++ b/t/t5318-commit-graph.sh
     @@
    - 	test_i18ngrep "$grepstr" err
    - }
    + GRAPH_BYTE_OCTOPUS=$(($GRAPH_OCTOPUS_DATA_OFFSET + 4))
    + GRAPH_BYTE_FOOTER=$(($GRAPH_OCTOPUS_DATA_OFFSET + 4 * $NUM_OCTOPUS_EDGES))
      
    -+
    -+# usage: corrupt_and_zero_graph_then_verify <corrupt_position> <data> <zero_position> <string>
    -+# Manipulates the commit-graph file at <corrupt_position> by inserting the data,
    -+# then zeros the file starting at <zero_position>. Finally, runs
    -+# 'git commit-graph verify' and places the output in the file 'err'. Tests 'err'
    -+# for the given string.
    -+corrupt_and_zero_graph_then_verify() {
    -+	corrupt_pos=$1
    -+	data="${2:-\0}"
    -+	zero_pos=$3
    -+	grepstr=$4
    +-# usage: corrupt_graph_and_verify <position> <data> <string>
    ++# usage: corrupt_graph_and_verify <position> <data> <string> [<zero_pos>]
    + # Manipulates the commit-graph file at the position
    +-# by inserting the data, then runs 'git commit-graph verify'
    ++# by inserting the data, optionally zeroing the file
    ++# starting at <zero_pos>, then runs 'git commit-graph verify'
    + # and places the output in the file 'err'. Test 'err' for
    + # the given string.
    + corrupt_graph_and_verify() {
    + 	pos=$1
    + 	data="${2:-\0}"
    + 	grepstr=$3
     +	orig_size=$(stat --format=%s $objdir/info/commit-graph)
    -+	cd "$TRASH_DIRECTORY/full" &&
    -+	test_when_finished mv commit-graph-backup $objdir/info/commit-graph &&
    -+	cp $objdir/info/commit-graph commit-graph-backup &&
    -+	printf "$data" | dd of="$objdir/info/commit-graph" bs=1 seek="$corrupt_pos" conv=notrunc &&
    ++	zero_pos=${4:-${orig_size}}
    + 	cd "$TRASH_DIRECTORY/full" &&
    + 	test_when_finished mv commit-graph-backup $objdir/info/commit-graph &&
    + 	cp $objdir/info/commit-graph commit-graph-backup &&
    + 	printf "$data" | dd of="$objdir/info/commit-graph" bs=1 seek="$pos" conv=notrunc &&
     +	truncate --size=$zero_pos $objdir/info/commit-graph &&
     +	truncate --size=$orig_size $objdir/info/commit-graph &&
    -+	test_must_fail git commit-graph verify 2>test_err &&
    -+	grep -v "^+" test_err >err &&
    -+	test_i18ngrep "$grepstr" err
    -+}
    + 	test_must_fail git commit-graph verify 2>test_err &&
    + 	grep -v "^+" test_err >err
    + 	test_i18ngrep "$grepstr" err
    + }
    + 
     +
      test_expect_success 'detect bad signature' '
      	corrupt_graph_and_verify 0 "\0" \
    @@ -73,9 +76,9 @@
      		"incorrect checksum"
      '
      
    -+test_expect_success 'detect truncated graph' '
    -+	corrupt_and_zero_graph_then_verify $GRAPH_BYTE_CHUNK_COUNT "\xff" \
    -+		$GRAPH_CHUNK_LOOKUP_OFFSET "chunk lookup table entry missing"
    ++test_expect_success 'detect incorrect chunk count' '
    ++	corrupt_graph_and_verify $GRAPH_BYTE_CHUNK_COUNT "\xff" \
    ++		"chunk lookup table entry missing" $GRAPH_CHUNK_LOOKUP_OFFSET
     +'
     +
      test_expect_success 'git fsck (checks commit-graph)' '
2:  7519fc76df = 2:  06a32bfe8b Makefile: correct example fuzz build