Re: [patch] Fix disp-step-syscall.exp on some i386 targets

[Yao Qi <yao@codesourcery.com>]

On 02/28/2012 09:42 PM, Jan Kratochvil wrote:
> +/* Linux kernel shows PC value after the 'int $0x80' instruction even if
> +   inferior is still inside the syscall.  On next PTRACE_SINGLESTEP it will
> +   finish the syscall but PC will not change.
> +   
> +   Some vDSOs contain 'int $0x80; ret' and during stepping out of the syscall
> +   i386_displaced_step_fixup would keep PC at the displaced pad location.
> +   As PC is pointing to the 'ret' instruction before the step
> +   i386_displaced_step_fixup would expect inferior has just executed that 'ret'
> +   and PC should not be adjusted.  In reality it finished syscall instead and
> +   PC should get relocated back to its vDSO address.  Hide the 'ret'
> +   instruction by 'nop' so that i386_displaced_step_fixup is not confused.
> +   
> +   It is not fully correct as the bytes in struct displaced_step_closure will
> +   not match the inferior code.  But we would need some new flag in
> +   displaced_step_closure otherwise to keep the state that syscall is finishing
> +   for the later i386_displaced_step_fixup execution as the syscall execution
> +   is already no longer detectable there.  The new flag field would mean
> +   i386-linux-tdep.c needs to wrap all the displacement methods of i386-tdep.c
> +   which does not seem worth it.  The same effect is achieved by patching that
> +   'nop' instruction there instead.  */

These comments are clear.  I like them :)

-- 
Yao (齐尧)