[PATCH 4/5] describe: handle blob traversal with no commits
- From
Jeff King <peff@peff.net>
- Date
- Aug 18, 2025, 21:03 UTC
- Message-ID
- <20250818210312.GD1024556@coredump.intra.peff.net>
- In-Reply-To
- <20250818205812.GA1018043@coredump.intra.peff.net>
When describing a blob, we traverse from HEAD, remembering each commit we saw, and then checking each blob to report the containing commit. But if we haven't seen any commits at all, we'll segfault (we store the "current" commit as an oid initialized to the null oid, causing lookup_commit_reference() to return NULL).
This shouldn't be able to happen normally. We always start our traversal at HEAD, which must be a commit (a property which is enforced by the refs code). But you can trigger the segfault like this:
blob=$(echo foo | git hash-object -w --stdin) echo $blob >.git/HEAD git describe $blob
We can instead catch this case and return an empty result, which hits the usual "we didn't find $blob while traversing HEAD" error.
This is a minor lie in that we did "find" the blob. And this even hints at a bigger problem in this code: what if the traversal pointed to the blob as _not_ part of a commit at all, but we had previously filled in the recorded "current commit"? One could imagine this happening due to a tag pointing directly to the blob in question.
But that can't happen, because we only traverse from HEAD, never from any other refs. And the intent of the blob-describing code is to find blobs within commits.
So I think this matches the original intent as closely as we can (and again, this segfault cannot be triggered without corrupting your repository!).
I didn't include a test here because it requires corrupting the repository in a way that is only easy to do using the files ref backend. It doesn't seem worth carrying a REFFILES test just for this oddity.
Signed-off-by: Jeff King <peff@peff.net> --- builtin/describe.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/builtin/describe.c b/builtin/describe.c index f7bea3c8c5..72b2e1162c 100644 --- a/builtin/describe.c +++ b/builtin/describe.c @@ -507,8 +507,10 @@ static void process_object(struct object *obj, const char *path, void *data) if (oideq(pcd->looking_for, &obj->oid) && !pcd->dst->len) { reset_revision_walk(); - describe_commit(&pcd->current_commit, pcd->dst); - strbuf_addf(pcd->dst, ":%s", path); + if (!is_null_oid(&pcd->current_commit)) { + describe_commit(&pcd->current_commit, pcd->dst); + strbuf_addf(pcd->dst, ":%s", path); + } free_commit_list(pcd->revs->commits); pcd->revs->commits = NULL; }
-- 2.51.0.326.gecbb38d78e