Subject: Re: [PATCH 0/2] compiling with -fsanitize=undefined

From: Jeff King <[email-masked]>
Date: Tue, 29 Dec 2015 01:44:35 -0500

On Tue, Dec 29, 2015 at 01:34:49AM -0500, Jeff King wrote:

>   2. We have some unaligned memory accesses that presumably work OK on
>      x86, but would blow up on ARM or other platforms (I didn't test).
> 
> The latter looks like it's in the untracked cache code (Duy and
> Christian cc'd). Running t7063 gets me this:
> 
> dir.c:2631:45: runtime error: member access within misaligned address 0x7f19806ff185 for type 'const struct ondisk_untracked_cache', which requires 4 byte alignment
> 0x7f19806ff185: note: pointer points here
>  31 33 29 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00
>              ^

I took a brief look at fixing this, and it may actually be a false
positive, as well (in a manner of speaking; if the compiler thinks it is
undefined behavior, it may still be worth fixing so we don't fall prey
to some optimizations-gone-wild).

The line in question is:

  load_sha1_stat(&uc->ss_info_exclude, &ouc->info_exclude_stat,
		 ouc->info_exclude_sha1);

where "ouc" is the on-disk data cast to a struct. So we definitely
generate an unaligned pointer, but then we only access its contents via
get_be32(), which handles alignment. So it might actually be OK. I
dunno.

-Peff