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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