Re: [PATCH,v2][AArch64] Fix step-over-syscall.exp failure

[Alan Hayward <Alan.Hayward@arm.com>]

This is ok too.

Alan.

> On 13 Jan 2020, at 17:42, Luis Machado <luis.machado@linaro.org> wrote:
> 
> New in v2:
> 
> - Reverted to using pc_adjust as bool/offset and added more comments to explain
> how it is being used.
> 
> --
> 
> In particular, this one:
> 
> FAIL: gdb.base/step-over-syscall.exp: fork: displaced=on: check_pc_after_cross_syscall: single step over fork final pc
> 
> When ptrace fork event reporting is enabled, GDB gets a PTRACE_EVENT_FORK
> event whenever the inferior executes the fork syscall.
> 
> Then the logic is that GDB needs to step the inferior yet again in order to
> receive a predetermined SIGTRAP, but no execution takes place because the
> signal was already queued for delivery. That means the PC should stay the same.
> 
> I noticed the aarch64 code is currently adjusting the PC in this situation,
> making the inferior skip an instruction without executing it.
> 
> The following change checks if we did not execute the instruction
> (pc - to == 0), making proper adjustments for such case.
> 
> Regression tested on aarch64-linux-gnu.
> 
> gdb/ChangeLog:
> 
> 2020-01-13  Luis Machado  <luis.machado@linaro.org>
> 
> 	* aarch64-tdep.c (struct aarch64_displaced_step_closure )
> 	<pc_adjust>: Adjust the documentation.
> 	(aarch64_displaced_step_fixup): Check if PC really moved before
> 	adjusting it.
> ---
> gdb/aarch64-tdep.c | 17 +++++++++++++----
> 1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
> index da41e22130..6a9d34dc67 100644
> --- a/gdb/aarch64-tdep.c
> +++ b/gdb/aarch64-tdep.c
> @@ -2737,7 +2737,8 @@ struct aarch64_displaced_step_closure : public displaced_step_closure
>      is being displaced stepping.  */
>   int cond = 0;
> 
> -  /* PC adjustment offset after displaced stepping.  */
> +  /* PC adjustment offset after displaced stepping.  If 0, then we don't
> +     write the PC back, assuming the PC is already the right address.  */
>   int32_t pc_adjust = 0;
> };
> 
> @@ -3032,11 +3033,12 @@ aarch64_displaced_step_fixup (struct gdbarch *gdbarch,
> {
>   aarch64_displaced_step_closure *dsc = (aarch64_displaced_step_closure *) dsc_;
> 
> +  ULONGEST pc;
> +
> +  regcache_cooked_read_unsigned (regs, AARCH64_PC_REGNUM, &pc);
> +
>   if (dsc->cond)
>     {
> -      ULONGEST pc;
> -
> -      regcache_cooked_read_unsigned (regs, AARCH64_PC_REGNUM, &pc);
>       if (pc - to == 8)
> 	{
> 	  /* Condition is true.  */
> @@ -3052,6 +3054,13 @@ aarch64_displaced_step_fixup (struct gdbarch *gdbarch,
> 
>   if (dsc->pc_adjust != 0)
>     {
> +      /* Make sure the previous instruction was executed (that is, the PC
> +	 has changed).  If the PC didn't change, then discard the adjustment
> +	 offset.  Otherwise we may skip an instruction before its execution
> +	 took place.  */
> +      if ((pc - to) == 0)
> +	dsc->pc_adjust = 0;
> +
>       if (debug_displaced)
> 	{
> 	  debug_printf ("displaced: fixup: set PC to %s:%d\n",
> -- 
> 2.17.1
>