Re: Unaligned accesses in sha1dc
From: Junio C Hamano <hidden>
Date: 2017-06-02 00:15:24
Possibly related (same subject, not in this thread)
- 2017-06-03 · Re: Unaligned accesses in sha1dc · Junio C Hamano <hidden>
- 2017-06-03 · Re: Unaligned accesses in sha1dc · Junio C Hamano <hidden>
- 2017-06-01 · Re: Unaligned accesses in sha1dc · Martin Ågren <hidden>
- 2017-06-01 · Re: Unaligned accesses in sha1dc · Martin Ågren <hidden>
- 2017-06-01 · Re: Unaligned accesses in sha1dc · Ævar Arnfjörð Bjarmason <hidden>
Martin Ågren [off-list ref] writes:
quoted hunk
I looked into this some more. It turns out it is possible to trigger undefined behavior on "next". Here's what I did: ... This "fixes" the problem: ...diff --git a/sha1dc/sha1.c b/sha1dc/sha1.c index 3dff80a..d6f4c44 100644 --- a/sha1dc/sha1.c +++ b/sha1dc/sha1.c@@ -66,9 +66,9 @@... With this diff, various tests which seem relevant for SHA-1 pass, including t0013, and the UBSan-error is gone. The second diff is just a monkey-patch. I have no reason to believe I will be able to come up with a proper and complete patch for sha1dc. And I guess such a thing would not really be Git's patch to carry, either. But at least Git could consider whether to keep relying on undefined behavior or not. There's a fair chance I've mangled the whitespace. I'm using gmail's web interface... Sorry about that.
Thanks. I see Marc Stevens is CC'ed in the thread, so I'd expect that the final "fix" would come from his sha1collisiondetection repository via Ævar. In the meantime, I am wondering if it makes sense to merge the earlier update with #ifdef ALLOW_UNALIGNED_ACCESS and #ifdef SHA1DC_FORCE_LITTLEENDIAN for the v2.13.x maintenance track, which would at least unblock those on platforms v2.13.0 did not work correctly at all. Ævar, thoughts?