Re: [PATCH] ref-cache: fix invalid free operation in `free_ref_entry`
From: Patrick Steinhardt <hidden>
Date: 2024-11-27 05:50:27
On Tue, Nov 26, 2024 at 10:40:57PM +0800, shejialuo wrote:
In cfd971520e (refs: keep track of unresolved reference value in iterators, 2024-08-09), we added a new field "referent" into the "struct ref" structure. In order to free the "referent", we unconditionally freed the "referent" by simply adding a "free" statement. However, this is a bad usage. Because when ref entry is either directory or loose ref, we will always execute the following statement: free(entry->u.value.referent); This does not make sense. We should never access the "entry->u.value" field when "entry" is a directory. However, the change obviously doesn't break the tests. Let's analysis why. The anonymous union in the "ref_entry" has two members: one is "struct ref_value", another is "struct ref_dir". On a 64-bit machine, the size of "struct ref_dir" is 32 bytes, which is smaller than the 48-byte size of "struct ref_value". And the offset of "referent" field in "struct ref_value" is 40 bytes. So, whenever we create a new "ref_entry" for a directory, we will leave the offset from 40 bytes to 48 bytes untouched, which means the value for this memory is zero (NULL). It's OK to free a NULL pointer, but this is merely a coincidence of memory layout.
Makes sense.
quoted hunk ↗ jump to hunk
To fix this issue, we now ensure that "free(entry->u.value.referent)" is only called when "entry->flag" indicates that it represents a loose reference and not a directory to avoid the invalid memory operation. Signed-off-by: shejialuo <redacted> --- refs/ref-cache.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)diff --git a/refs/ref-cache.c b/refs/ref-cache.c index 35bae7e05d..02f09e4df8 100644 --- a/refs/ref-cache.c +++ b/refs/ref-cache.c@@ -68,8 +68,9 @@ static void free_ref_entry(struct ref_entry *entry) * trigger the reading of loose refs. */ clear_ref_dir(&entry->u.subdir); + } else { + free(entry->u.value.referent); } - free(entry->u.value.referent); free(entry); }
And the fix looks obviously good to me. Thanks for catching this! Patrick