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x86/fpu: Fix boot crash in the early FPU code

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Jan Kara and Thomas Gleixner reported boot crashes in the FPU
code:

  general protection fault: 0000 [#1] SMP
  RIP: 0010:[<ffffffff81048a6c>]  [<ffffffff81048a6c>] mxcsr_feature_mask_init+0x1c/0x40

  2b:*  0f ae 85 00 fe ff ff    fxsave -0x200(%rbp)

and bisected it down to the following FPU commit:

   91a8c2a5b4 ("x86/fpu: Clean up and fix MXCSR handling")

The reason is that the on-stack FPU registers state variable,
used by the FXSAVE instruction, did not have the required
minimum alignment of 16 bytes, causing the general protection
fault.

This is most likely a GCC bug in older GCC versions, but the
offending commit also added a bogus extra 32-byte alignment
(which GCC ignored too).

So fix this bug by making the variable static again, but also
mark it __initdata this time, because fpu__init_system_mxcsr()
is now an __init function.

Reported-and-bisected-by: Jan Kara <jack@suse.cz>
Reported-bisected-and-tested-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150704075819.GA9201@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Ingo Molnar 2015-07-04 09:58:19 +02:00
parent 864d5bb957
commit b96fecbfa8
1 changed files with 4 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
arch/x86/kernel/fpu/init.c
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@ -95,11 +95,12 @@ static void __init fpu__init_system_mxcsr(void)
unsigned int mask = 0;
if (cpu_has_fxsr) {
struct fxregs_state fx_tmp __aligned(32) = { };
/* Static because GCC does not get 16-byte stack alignment right: */
static struct fxregs_state fxregs __initdata;
asm volatile("fxsave %0" : "+m" (fx_tmp));
asm volatile("fxsave %0" : "+m" (fxregs));
mask = fx_tmp.mxcsr_mask;
mask = fxregs.mxcsr_mask;
/*
* If zero then use the default features mask,