Date: Mon, 31 Dec 2018 22:32:06 -0500 From: Pedro Giffuni <pfg@FreeBSD.org> To: freebsd-numerics@freebsd.org Cc: sgk@troutmask.apl.washington.edu Subject: Undefined Behavior in lib/msun/src/e_pow.c (was Re: New math library from ARM) Message-ID: <06c8b6a2-ed26-f255-3947-c79b593a9dea@FreeBSD.org> In-Reply-To: <20181231152230.GC823@troutmask.apl.washington.edu> References: <797a7755-db93-1b9c-f3b9-8850d948e098@FreeBSD.org> <20181231151904.GB823@troutmask.apl.washington.edu> <20181231152230.GC823@troutmask.apl.washington.edu>
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This is a multi-part message in MIME format. --------------77F699FB60D2895086164B43 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Hmm ... Looking at the changes in musl's libc I found this issue which seems real (although somewhat theoretical): https://git.musl-libc.org/cgit/musl/commit/src/math?id=688d3da0f1730daddbc954bbc2d27cc96ceee04c Is the attached patch acceptable? Also, their code is bit different here: https://git.musl-libc.org/cgit/musl/commit/src/math?id=282b1cd26649d69de038111f5876853df6ddc345 but we may also have to check fmaf(-0x1.26524ep-54, -0x1.cb7868p+11, 0x1.d10f5ep-29). Cheers, Pedro. --------------77F699FB60D2895086164B43 Content-Type: text/x-patch; name="msun-pow-ub.diff" Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename="msun-pow-ub.diff" Index: lib/msun/src/e_pow.c =================================================================== --- lib/msun/src/e_pow.c (revision 342665) +++ lib/msun/src/e_pow.c (working copy) @@ -130,6 +130,7 @@ if(hx<0) { if(iy>=0x43400000) yisint = 2; /* even integer y */ else if(iy>=0x3ff00000) { + uint32_t j; /* Avoid UB in bit operations below. */ k = (iy>>20)-0x3ff; /* exponent */ if(k>20) { j = ly>>(52-k); --------------77F699FB60D2895086164B43--
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