[Bug libc/9726] New: Incorrect fp reg specified in mtfsf instruction used by powepc32 [set|swap]context().

["rsa at us dot ibm dot com" <sourceware-bugzilla at sourceware dot org>]

The [set|swap]context-common.S assembly code:

glibc/sysdeps/unix/sysv/linux/powerpc/powerpc32/setcontext-common.S
glibc/sysdeps/unix/sysv/linux/powerpc/powerpc32/swapcontext-common.S

is doing:

# ifdef _ARCH_PWR6
        /* Use the extended four-operand version of the mtfsf insn.  */
        mtfsf  0xff,fp0,1,0
# else

when it should be doing:

# ifdef _ARCH_PWR6
        /* Use the extended four-operand version of the mtfsf insn.  */
        mtfsf  0xff,fp31,1,0
# else

Because the FPSCR is restored into fp31, not fp0.  This is a copy-paste error.

The testcase called 'tst-setcontext-fpscr.c' is supposed to catch these bugs
but it stored a temporary (but valid) FPSCR value into fp0.  When
[set|swap]context() was called it set the FPSCR, but from the stale FPSCR value
in fp0, not from the saved value in fp31.  So unfortunately the correct (but
stale) FPSCR value was restored and the test passed erroneously.

I will include a patch which corrects the assembly code in
[set|swap]context-common.S.  I will fix the test-case to clobber fp0 after it
is no longer used so that temporary data won't make future tests pass
erroneously.

I will also attach a corrective patch.

The impact of not having this patch is that applications running on Power6, with
an older kernel, that use the setcontext() and swapcontext() routines in
powerpc32 will not restore the correct value of the FPSCR.

-- 
           Summary: Incorrect fp reg specified in mtfsf instruction used by
                    powepc32 [set|swap]context().
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P1
         Component: libc
        AssignedTo: drepper at redhat dot com
        ReportedBy: rsa at us dot ibm dot com
                CC: glibc-bugs at sources dot redhat dot com
 GCC build triplet: powerpc64-linux
  GCC host triplet: powerpc64-linux
GCC target triplet: powerpc64-linux


http://sourceware.org/bugzilla/show_bug.cgi?id=9726

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