path: harden validation of HEAD with non-standard hashes

The `validate_headref()` function takes a path to a supposed "HEAD" file
and checks whether its format is something that we understand. It is
used as part of our repository discovery to check whether a specific
directory is a Git directory or not.

Part of the validation is a check for a detached HEAD that contains a
plain object ID. To do this validation we use `get_oid_hex()`, which
relies on `the_hash_algo`. At this point in time the hash algo cannot
yet be initialized though because we didn't yet read the Git config.
Consequently, it will always be the SHA1 hash algorithm.

In practice this works alright because `get_oid_hex()` only ends up
checking whether the prefix of the buffer is a valid object ID. And
because SHA1 is shorter than SHA256, the function will successfully
parse SHA256 object IDs, as well.

It is somewhat fragile though and not really the intent to only check
for SHA1. With this in mind, harden the code to use `get_oid_hex_any()`
to check whether the "HEAD" file parses as any known hash.

One might be hard pressed to tighten the check even further and fully
validate the file contents, not only the prefix. In practice though that
wouldn't make a lot of sense as it could be that the repository uses a
hash function that produces longer hashes than SHA256, but which the
current version of Git doesn't understand yet. We'd still want to detect
the repository as proper Git repository in that case, and we will fail
eventually with a proper error message that the hash isn't understood
when trying to set up the repository format.

It follows that we could just leave the current code intact, as in
practice the code change doesn't have any user visible impact. But it
also prepares us for `the_hash_algo` being unset when there is no
repository.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/path.c b/path.c
index 67229edb9c..cc02165530 100644
--- a/path.c
+++ b/path.c
@@ -693,7 +693,7 @@ int validate_headref(const char *path)
 	/*
 	 * Is this a detached HEAD?
 	 */
-	if (!get_oid_hex(buffer, &oid))
+	if (get_oid_hex_any(buffer, &oid) != GIT_HASH_UNKNOWN)
 		return 0;
 
 	return -1;