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Date: Sat, 9 Jan 2016 00:59:55 +0300
From: Alexander Cherepanov <ch3root@...nwall.com>
To: musl@...ts.openwall.com
Subject: Re: string word-at-a-time and atomic.h FAQ on twitter
On 2016-01-05 20:50, Rich Felker wrote:
> So we could just consider trying to drop the OOB
> accesses. Do we have a list of affected functions? That might be nice
> to include.
I think it would be nice to have a full list of intentional UB. For
example, this:
http://git.musl-libc.org/cgit/musl/tree/src/stdio/vsnprintf.c#n33
if (n > (char *)0+SIZE_MAX-s-1) n = (char *)0+SIZE_MAX-s-1;
If I understand the code correctly, fixing it will require changes to
the FILE structure. Are there such plans?
>> this takes care of oob access, but the bytes outside the passed
>> object might change concurrently i.e. strlen might introduce a
>> data race: again this is a problem on the abstract c language
>> level that may be solved e.g. by making all accesses to those
>> bytes relaxed atomic, but user code is not under libc control.
>> in practice the code works if HASZERO reads the word once so it
>> does arithmetics with a consistent value (because the memory
>> model of the underlying machine does not treat such race
>> undefined and it does not propagate unspecified value bits nor
>> has trap representations).
>
> Indeed, this seems like less of a practical concern.
HASZERO reads the word twice so this should be a problem for unoptimized
code on big-endian platforms.
--
Alexander Cherepanov
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