Bug 22243 - log2(0) and log10(0) are wrong in downward rounding without the svid compat wrapper
Summary: log2(0) and log10(0) are wrong in downward rounding without the svid compat w...
Status: RESOLVED FIXED
Alias: None
Product: glibc
Classification: Unclassified
Component: math (show other bugs)
Version: 2.27
: P2 normal
Target Milestone: 2.27
Assignee: Not yet assigned to anyone
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2017-10-03 16:03 UTC by Szabolcs Nagy
Modified: 2017-10-04 15:33 UTC (History)
0 users

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Last reconfirmed:
fweimer: security-


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Description Szabolcs Nagy 2017-10-03 16:03:13 UTC
log2(0) and log10(0) return inf instead of -inf in downward rounding when the svid wrapper is suppressed (static linking, new targets).

#include <math.h>
#include <stdio.h>
#include <fenv.h>

int main()
{
	fesetround(FE_DOWNWARD);
	double y1 = log2(0);
	double y2 = log10(0);
	fesetround(FE_TONEAREST);
	printf("%a\n", y1);
	printf("%a\n", y2);
}

prints

inf
inf

with static linking.
Comment 1 Szabolcs Nagy 2017-10-03 16:14:05 UTC
wordsize-64 targets only
Comment 2 cvs-commit@gcc.gnu.org 2017-10-04 09:16:33 UTC
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The branch, master has been updated
       via  86c27ade1e44e29922d33676f950f7334edb37a7 (commit)
       via  8f8f8ef7aba40ef883291e4c4d95a419c3327d70 (commit)
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- Log -----------------------------------------------------------------
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=86c27ade1e44e29922d33676f950f7334edb37a7

commit 86c27ade1e44e29922d33676f950f7334edb37a7
Author: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date:   Tue Oct 3 18:12:42 2017 +0100

    [BZ #22244] Fix yn(n,0) without SVID wrapper
    
    Without SVID compat wrapper yn(n,0) and ynf(n,0) does not raise
    the divide-by-zero excpetion and it may return inf with the wrong
    sign for n < 0.
    
    	[BZ #22244]
    	* sysdeps/ieee754/dbl-64/e_jn.c (__ieee754_yn): Fix x == 0 case.
    	* sysdeps/ieee754/flt-32/e_jnf.c (__ieee754_ynf): Likewise.

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=8f8f8ef7aba40ef883291e4c4d95a419c3327d70

commit 8f8f8ef7aba40ef883291e4c4d95a419c3327d70
Author: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date:   Tue Oct 3 17:13:18 2017 +0100

    [BZ #22243] fix log2(0) and log(10) in downward rounding
    
    On 64bit targets if the SVID compat wrapper is suppressed (e.g. static linking)
    then log2(0) and log10(0) returned inf instead of -inf.
    
    	[BZ #22243]
    	* sysdeps/ieee754/dbl-64/wordsize-64/e_log10.c (__ieee754_log10): Use fabs.
    	* sysdeps/ieee754/dbl-64/wordsize-64/e_log2.c (__ieee754_log2): Likewise.

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog                                    |   12 ++++++++++++
 sysdeps/ieee754/dbl-64/e_jn.c                |   10 +++++-----
 sysdeps/ieee754/dbl-64/wordsize-64/e_log10.c |    2 +-
 sysdeps/ieee754/dbl-64/wordsize-64/e_log2.c  |    2 +-
 sysdeps/ieee754/flt-32/e_jnf.c               |    6 +++---
 5 files changed, 22 insertions(+), 10 deletions(-)
Comment 3 Szabolcs Nagy 2017-10-04 09:33:28 UTC
fixed.