[Bug libc/13007] New: have_avx in sysdeps/x68_64/dl-trampoline.S does not check hard enough

[Bug libc/13007] New: have_avx in sysdeps/x68_64/dl-trampoline.S does not check hard enough

[luto at mit dot edu]

http://sourceware.org/bugzilla/show_bug.cgi?id=13007

           Summary: have_avx in sysdeps/x68_64/dl-trampoline.S does not
                    check hard enough
           Product: glibc
           Version: 2.14
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: drepper.fsp@gmail.com
        ReportedBy: luto@mit.edu


have_avx returns 1 if CPUID.01H:ECX.AVX is set.  This is wrong: the AVX bit
indicates that the CPU understands AVX instructions, but it can be set even if
the kernel hasn't enabled YMM state.

The correct check is to also read CPUID.01H:ECX.OSXSAVE to check that xgetbv is
enabled and then to use xgetbv to check XCR0.SSE and XCR0.AVX.

This comes from an Intel blog post here:
http://software.intel.com/en-us/blogs/2011/04/14/is-avx-enabled/

and is borne out by the Exceptions Type 6 section of the SDM, which indicates
that VEX-coded instructions will #UD if XCR0 does not have those two bits set.

This bug presumably caused this crash:
http://comments.gmane.org/gmane.linux.kernel/1166783

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[Bug libc/13007] have_avx in sysdeps/x68_64/dl-trampoline.S does not check hard enough

[drepper.fsp at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=13007

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |WAITING

--- Comment #1 from Ulrich Drepper <drepper.fsp at gmail dot com> 2011-07-20 16:18:26 UTC ---
That's the generic model but why should anyone care on Linux?

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[Bug libc/13007] have_avx in sysdeps/x68_64/dl-trampoline.S does not check hard enough

[luto at mit dot edu]

http://sourceware.org/bugzilla/show_bug.cgi?id=13007

--- Comment #2 from Andy Lutomirski <luto at mit dot edu> 2011-07-20 16:26:07 UTC ---
See the gmane link.  There is apparently at least one person running a legacy
kernel on Sandy Bridge hardware, and glibc gets SIGILL on that box.

I'm not affected myself.  I suspect that only servers will ever see this,
because desktop machines will probably have other problems (like no graphics).

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[Bug libc/13007] have_avx in sysdeps/x68_64/dl-trampoline.S does not check hard enough

[drepper.fsp at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=13007

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

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

--- Comment #3 from Ulrich Drepper <drepper.fsp at gmail dot com> 2011-07-21 01:47:06 UTC ---
I checked in a patch.

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[Bug libc/13007] have_avx in sysdeps/x68_64/dl-trampoline.S does not check hard enough

[schwab@linux-m68k.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=13007

Andreas Schwab <schwab@linux-m68k.org> changed:

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

--- Comment #4 from Andreas Schwab <schwab@linux-m68k.org> 2011-07-21 12:48:31 UTC ---
Which is of course wrong.

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[Bug libc/13007] have_avx in sysdeps/x68_64/dl-trampoline.S does not check hard enough

[drepper.fsp at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=13007

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

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

--- Comment #5 from Ulrich Drepper <drepper.fsp at gmail dot com> 2011-07-23 02:10:57 UTC ---
Don't reopen bugs like this.

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[Bug libc/13007] have_avx in sysdeps/x68_64/dl-trampoline.S does not check hard enough

[luto at mit dot edu]

http://sourceware.org/bugzilla/show_bug.cgi?id=13007

--- Comment #6 from Andy Lutomirski <luto at mit dot edu> 2011-07-23 02:19:18 UTC ---
Should it be a new bug, then?  From my admittedly imperfect recollection of x86
assembly, this:

+       testl   $((1 << 28) | (1 << 27)), %ecx
+       je      2f

is checking whether one of the bits is set, not whether both are set.  I think
you'll get SIGILL when you do xgetbv if OSXSAVE isn't set.  (And you'll get a
different failure if YMM is enabled but AVX isn't present, in the unlikely
event that such a CPU exists.)

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[Bug libc/13007] have_avx in sysdeps/x68_64/dl-trampoline.S does not check hard enough

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=13007

Florian Weimer <fweimer at redhat dot com> changed:

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

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