Re: [PATCH] Don't dereference NULL upon lookup_tree failure.

Junio C Hamano [off-list ref] wrote:

Jim Meyering [off-list ref] writes:

quoted

quoted

While this change may be a prudent safeguard, there is something
else going on.  Can you provide the callchain that led to the
parse_object_buffer() that gave SHA1 of a commit object with
type set to OBJ_TREE?  Which caller does that bogus combination?

Sure.
Here's valgrind output from running this (from my reproducer):

    valgrind --trace-children=yes git clone . k
---------------
  error: Object 0d57588da39d10795486bd5451bc2660832228e6 is a commit, not a tree
  ==9483== Invalid read of size 1
  ==9483==    at 0x405C27: parse_object_buffer (object.c:146)
  ==9483==    by 0x405CE4: parse_object (object.c:187)
  ==9483==    by 0x403185: send_ref (upload-pack.c:561)
  ==9483==    by 0x408EEF: do_for_each_ref (refs.c:546)
  ==9483==    by 0x4036EC: main (upload-pack.c:587)

Sorry, I asked for a wrong thing.  parse_object() reads and
finds out the type, so type there is presumably the right type
of the object (which is OBJ_TREE).  Then parse_object_buffer()
checks if it has already seen the object of the same SHA-1 and
finds that somebody had earlier told that SHA-1 name is of a
commit object.  Either you found a SHA-1 collision (highly
unlikely) or the earlier caller had lied.  And I think what
really needs to be fixed is that lying caller.  That is not in
the above call chain.

I haven't debugged it enough to find/fix the bug in parsecvs.

I presume that parsecvs is the culprit, in constructing
an invalid repository -- especially, since inserting the
single missing tag into the ,v file is enough to make it so
parsecvs creates a repository that no longer triggers the git bug.