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[Qemu-block] iotest 013 failure under clang -fsanitize=undefined

From: John Snow
Subject: [Qemu-block] iotest 013 failure under clang -fsanitize=undefined
Date: Tue, 2 Feb 2016 15:03:18 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0

Recently, qemu iotest 013 has started to fail for me:

Fedora release 22 (Twenty Two)

clang version 3.5.0 (tags/RELEASE_350/final)
Target: x86_64-redhat-linux-gnu
Thread model: posix

+4 KiB/home/jsnow/src/qemu/qemu-io-cmds.c:230:18: runtime error:
division by zero

The problem is that in the print report for read_f, t2 and t1 can
actually be the same exact timestamp, and tdiv will try to divide by 0.0.

Normally this is not a problem as this is defined to be INFINITY in C99
Annex F.

Clang, however, has once again decided to take the pedantic road and
state that Annex F is optional, and therefore division by 0.0 is
actually undefined when using -fsanitize=undefined.


Two workarounds:

(1) Modify the tdiv() function to just return INFINITY manually if the
timestamp provided is 0

(2) Modify tester scripts to also use -fno-sanitize=float-divide-by-zero

I prepared a patch to do the first workaround [1] so I could test
patches with clang in peace as I need to test my pull requests under
clang to make sure I don't break OSX, but it seems so absurd to have to
do this, so I have copied our resident language lawyers (and language
pragmatists) so that they can have a say.

Relevant upstream BZ: https://llvm.org/bugs/show_bug.cgi?id=17000



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