Re: [PATCH][gdb/testsuite] Fix gdb.arch/amd64-stap-three-arg-disp.S

[Sergio Durigan Junior via Gdb-patches <gdb-patches@sourceware.org>]

On Monday, January 11 2021, Tom de Vries wrote:

> Hi,
>
> On SLE-11 I ran into:
> ...
> (gdb) print $_probe_arg0^M
> Cannot access memory at address 0x8000003fe05c^M
> (gdb) FAIL: gdb.arch/amd64-stap-special-operands.exp: probe: three_arg: \
>   print $_probe_arg0
> ...
>
> The memory cannot be accessed because the address used to evaluate
> $_probe_arg0 at the probe point is incorrect.
>
> The address is calculated using this expression:
> ...
> .asciz "-4@-4(%rbp,%ebx,0)"
> ...
> which uses $ebx, but $ebx is uninitialized at the probe point.
>
> The test-case does contain a "movl $0, %ebx" insn to set $ebx to 0, but that
> insn is placed after the probe point.  We could fix this by moving the insn
> to before the probe point.  But, $ebx is also a callee-save register, so
> normally, if we modify it, we also need to save and restore it, which is
> currently not done.  This is currently not harmful, because we don't run the
> test-case further than the probe point, but it's bound to cause confusion.
>
> So, fix this instead by using $eax instead in the expression, and moving the
> insn setting $eax to 0 to before the probe point.

Thanks for the patch.

> Any comments?

The fix makes sense to me as-is.

Eventually, I'd like to convert this testcase to use the same format as
gdb.arch/amd64-stap-optional-prefix.S, which is much simpler to
read/hack.  It shouldn't be a hard thing to do, but I don't want to
impose on you and ask you to do that, so if you want to go ahead and
push this patch, feel free to do so.

Thanks,

> Thanks,
> - Tom
>
> [gdb/testsuite] Fix gdb.arch/amd64-stap-three-arg-disp.S
>
> gdb/testsuite/ChangeLog:
>
> 2021-01-10  Tom de Vries  <tdevries@suse.de>
>
> 	PR testsuite/26968
> 	* gdb.arch/amd64-stap-three-arg-disp.S: Remove insn modifying $ebx.
> 	Move insn setting $eax to before probe point.
>
> ---
>  gdb/testsuite/gdb.arch/amd64-stap-three-arg-disp.S | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/gdb/testsuite/gdb.arch/amd64-stap-three-arg-disp.S b/gdb/testsuite/gdb.arch/amd64-stap-three-arg-disp.S
> index cf3856f41e9..17b64048082 100644
> --- a/gdb/testsuite/gdb.arch/amd64-stap-three-arg-disp.S
> +++ b/gdb/testsuite/gdb.arch/amd64-stap-three-arg-disp.S
> @@ -15,6 +15,7 @@ main:
>  	movl	%edi, -20(%rbp)
>  	movq	%rsi, -32(%rbp)
>  	movl	$10, -4(%rbp)
> +	movl	$0, %eax
>  #APP
>  # 8 "amd64-stap-three-arg-disp.c" 1
>  	990: nop
> @@ -28,7 +29,7 @@ main:
>  .8byte 0
>  .asciz "test"
>  .asciz "three_arg"
> -.asciz "-4@-4(%rbp,%ebx,0)"
> +.asciz "-4@-4(%rbp,%eax,0)"
>  994: .balign 4
>  .popsection
>  
> @@ -45,8 +46,6 @@ _.stapsdt.base: .space 1
>  
>  # 0 "" 2
>  #NO_APP
> -	movl	$0, %eax
> -	movl	$0, %ebx
>  	popq	%rbp
>  	.cfi_def_cfa 7, 8
>  # SUCC: EXIT [100.0%] 

-- 
Sergio
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