[PATCH] dir: fix malloc of root untracked_cache_dir

From: Jeff Hostetler via GitGitGadget <gitgitgadget@gmail.com>
Date: Feb 24, 2021, 14:31 UTC
Message-ID: <pull.884.git.1614177117508.gitgitgadget@gmail.com>

From: Jeff Hostetler <jeffhost@microsoft.com>

Use FLEX_ALLOC_STR() to allocate the `struct untracked_cache_dir` for the root directory. Get rid of unsafe code that might fail to initialize the `name` field (if FLEX_ARRAY is not 1). This will make it clear that we intend to have a structure with an empty string following it.

A problem was observed on Windows where the length of the memset() was too short, so the first byte of the name field was not zeroed. This resulted in the name field having garbage from a previous use of that area of memory.

The record for the root directory was then written to the untracked-cache extension in the index. This garbage would then be visible to future commands when they reloaded the untracked-cache extension.

Since the directory record for the root directory had garbage in the `name` field, the `t/helper/test-tool dump-untracked-cache` tool printed this garbage as the path prefix (rather than '/') for each directory in the untracked cache as it recursed.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
    dir: fix malloc of root untracked_cache_dir

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-884%2Fjeffhostetler%2Funtracked-cache-corruption-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-884/jeffhostetler/untracked-cache-corruption-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/884

 dir.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/dir.c b/dir.c
index d153a63bbd14..fd8aa7c40faa 100644
--- a/dir.c
+++ b/dir.c
@@ -2730,11 +2730,8 @@ static struct untracked_cache_dir *validate_untracked_cache(struct dir_struct *d
 		return NULL;
 	}
 
-	if (!dir->untracked->root) {
-		const int len = sizeof(*dir->untracked->root);
-		dir->untracked->root = xmalloc(len);
-		memset(dir->untracked->root, 0, len);
-	}
+	if (!dir->untracked->root)
+		FLEX_ALLOC_STR(dir->untracked->root, name, "");
 
 	/* Validate $GIT_DIR/info/exclude and core.excludesfile */
 	root = dir->untracked->root;

base-commit: 966e671106b2fd38301e7c344c754fd118d0bb07

-- 
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