Re: Unaligned accesses in sha1dc

From: Martin Ågren <martin.agren@gmail.com>
Date: Jun 1, 2017, 10:26 UTC
Message-ID: <CAN0heSrzpwhS3Zf83vTzHAAmi8YVD4CoCh_px5SBXBZhSKPqPQ@mail.gmail.com>
In-Reply-To: <mvmmv9st3yv.fsf@suse.de>

On 1 June 2017 at 12:08, Andreas Schwab <schwab@suse.de> wrote:

Show 10 quoted lines

> On Jun 01 2017, Junio C Hamano <gitster@pobox.com> wrote:
>
>> Depending on the model of "ARM" (or "SPARC") emulated with QEMU, and
>> depending on the OS that runs on such an "ARM" or "SPARC", we may
>> not see this---if the emulated OS has the "software unaligned-access
>> emulation" our userland may not see a SIGBUS.
>
> Even if the architecture implements unaligned accesses in hardware, it
> is still undefined behaviour, and the compiler will (eventually) take
> advantage of it.

I tried to optically follow the macros and ended up on line 87/89 in lib/sha1.c of the sha1dc-library, where there is undefined behavior if the address is unaligned, which it seems it could be. Maybe Git uses some particular combination of macro-definitions and I went down the wrong path... There might also be other spots; I haven't thrown UBSan at the code.

Using memcpy on those lines should not be a performance problem on platforms where unaligned access is ok, of course assuming the compiler sees the opportunity.

Martin