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* [Bug libc/27697] New: Technically-undefined shifts in sysdeps/x86/dl-cacheinfo.h
@ 2021-04-05 23:11 brooks at gcc dot gnu.org
2021-04-06 18:29 ` [Bug libc/27697] " joseph at codesourcery dot com
2021-04-08 6:35 ` fweimer at redhat dot com
0 siblings, 2 replies; 3+ messages in thread
From: brooks at gcc dot gnu.org @ 2021-04-05 23:11 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=27697
Bug ID: 27697
Summary: Technically-undefined shifts in
sysdeps/x86/dl-cacheinfo.h
Product: glibc
Version: unspecified
Status: NEW
Severity: minor
Priority: P2
Component: libc
Assignee: unassigned at sourceware dot org
Reporter: brooks at gcc dot gnu.org
CC: drepper.fsp at gmail dot com
Target Milestone: ---
We recently got some positives on a "shift sanitizer" check, in
sysdeps/x86/dl-cacheinfo.h.
Specifically, at lines 611 and 628, we have:
count_mask = ~(-1 << (count_mask + 1));
Technically, left shifts of negative numbers are undefined; we can avoid the
undefined behavior by respelling this as
count_mask = ~(~0U << (count_mask + 1));
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* [Bug libc/27697] Technically-undefined shifts in sysdeps/x86/dl-cacheinfo.h
2021-04-05 23:11 [Bug libc/27697] New: Technically-undefined shifts in sysdeps/x86/dl-cacheinfo.h brooks at gcc dot gnu.org
@ 2021-04-06 18:29 ` joseph at codesourcery dot com
2021-04-08 6:35 ` fweimer at redhat dot com
1 sibling, 0 replies; 3+ messages in thread
From: joseph at codesourcery dot com @ 2021-04-06 18:29 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=27697
--- Comment #1 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
See <https://sourceware.org/legacy-ml/libc-alpha/2019-02/msg00177.html> -
I failed to get consensus for that patch (my goal there was fixing
warnings that appeared with -Wextra).
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* [Bug libc/27697] Technically-undefined shifts in sysdeps/x86/dl-cacheinfo.h
2021-04-05 23:11 [Bug libc/27697] New: Technically-undefined shifts in sysdeps/x86/dl-cacheinfo.h brooks at gcc dot gnu.org
2021-04-06 18:29 ` [Bug libc/27697] " joseph at codesourcery dot com
@ 2021-04-08 6:35 ` fweimer at redhat dot com
1 sibling, 0 replies; 3+ messages in thread
From: fweimer at redhat dot com @ 2021-04-08 6:35 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=27697
Florian Weimer <fweimer at redhat dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |fweimer at redhat dot com
--- Comment #2 from Florian Weimer <fweimer at redhat dot com> ---
GCC makes these shifts defined as an extension, but still flags them in the
sanitizers. So we are in a bit of an awkward situation here.
I think we should make the change if it helps with testing glibc with the
sanitizers. This piece of information wasn't available when the patch was
proposed the last time, I think.
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2021-04-05 23:11 [Bug libc/27697] New: Technically-undefined shifts in sysdeps/x86/dl-cacheinfo.h brooks at gcc dot gnu.org
2021-04-06 18:29 ` [Bug libc/27697] " joseph at codesourcery dot com
2021-04-08 6:35 ` fweimer at redhat dot com
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