[Bug libc/12420] New: On AMD64 linux, getcontext resets FPU exception mask.

[Bug libc/12420] New: On AMD64 linux, getcontext resets FPU exception mask.

[jean.claude.beaudoin at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=12420

           Summary: On AMD64 linux, getcontext resets FPU exception mask.
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: drepper.fsp@gmail.com
        ReportedBy: jean.claude.beaudoin@gmail.com


Created attachment 5202
  --> http://sourceware.org/bugzilla/attachment.cgi?id=5202
small demo to be linked with -lm

The attached program demonstrates that the floating-point exception
mask is being reset to 0 (effectively cleared) by getcontext().

The problem does not exist on i386 since the implementation of
getcontext() on it properly executes a fldenv right after the
fnstenv (as justified by the very informative comment to be seen
in the source code getcontext.S).

Seen on Ubuntu 10.10, Fedora 12 and Fedora 14 at least.
>From reading the current source code, current git code should have
the same problem.

Cheers,

Jean-Claude Beaudoin

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[Bug libc/12420] On AMD64 linux, getcontext resets FPU exception mask.

[drepper.fsp at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=12420

Ulrich Drepper <drepper.fsp at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

--- Comment #1 from Ulrich Drepper <drepper.fsp at gmail dot com> 2011-04-18 01:54:05 UTC ---
Seems to be a problem with the processor.  I worked around it.

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[Bug libc/12420] On AMD64 linux, getcontext resets FPU exception mask.

[hjl.tools at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=12420

--- Comment #2 from H.J. Lu <hjl.tools at gmail dot com> 2011-04-18 16:16:05 UTC ---
>From Intel64 SDM:

These instructions are often used by exception handlers because they provide
access to the FPU instruction and data pointers. The environment is typically
saved in the stack. Masking all exceptions after saving the environment
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
prevents floating-point exceptions from interrupting the exception handler.

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[Bug libc/12420] On AMD64 linux, getcontext resets FPU exception mask.

[hjl.tools at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=12420

--- Comment #3 from H.J. Lu <hjl.tools at gmail dot com> 2011-04-19 16:48:43 UTC ---
Inte64 SDM has

Store FPU environment to m14byte or m28byte without checking for pending
unmasked floating-point exceptions. Then mask all floatingpoint exceptions.
                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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[Bug libc/12420] On AMD64 linux, getcontext resets FPU exception mask.

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=12420

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

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