Bug#519391: libgcrypt11: SIGILL on _gcry_detect_hw_features

[Ben Hutchings]

On Thu, 2009-03-12 at 11:20 +0100, Werner Koch wrote:
> On Thu, 12 Mar 2009 09:32, dexter at debian.org said:
> 
> > Program received signal SIGILL, Illegal instruction.
> > [Switching to Thread 0xb7b9c6e0 (LWP 2196)]
> > 0xb7bae10e in _gcry_detect_hw_features () at hwfeatures.c:78
> 
> That seems to be the cpuid opcode which is not available, Linux knows
> about it:
> 
> > cpuid level     : -1
> 
> but my code gets the detection wrong:
> 
>   /* Detect the CPUID feature by testing some undefined behaviour (16
>      vs 32 bit pushf/popf). */
>   asm volatile
>     ("pushf\n\t"                 /* Copy flags to EAX.  */
>      "popl %%eax\n\t"
>      "movl %%eax, %%ecx\n\t"     /* Save flags into ECX.  */
>      "xorl $0x200000, %%eax\n\t" /* Toggle ID bit and copy it to the flags.  */
>      "pushl %%eax\n\t"            
>      "popf\n\t"                
>      "pushf\n\t"                 /* Copy changed flags again to EAX.  */    
>      "popl %%eax\n\t"
>      "pushl %%ecx\n\t"           /* Restore flags from ECX.  */
>      "popf\n\t"
>      "xorl %%eax, %%ecx\n\t"     /* Compare flags against saved flags.  */
>      "jz .Lno_cpuid%=\n\t"       /* Toggling did not work, thus no CPUID.  */
>      "movl $1, %0\n"             /* Worked. true -> HAS_CPUID.  */
>      ".Lno_cpuid%=:\n\t"
>      : "=r" (has_cpuid)
[...]

"=r" means that the code *will* write to register %0, which is then
written to has_cpuid.  But if this code does not detect cpuid, the
register is uninitialised and probably has a non-zero value.

You must either change the detection code to set %0 to 0 if the
condition is not met, or change "=r" to "+r" so that %0 will be
initialised to the previous value of has_cpuid (i.e. 0).

Ben.

-- 
Ben Hutchings
Quantity is no substitute for quality, but it's the only one we've got.
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