Re: [RFA] x86 - jump instruction after the prologue

[Jerome Guitton <guitton at act-europe dot fr>]

Ping? Still waiting for approval...

(I have a testcase for that, coming soon...)

Jerome Guitton (guitton@act-europe.fr):

> For what I understand from the skip_prologue algorithm in i386-tdep.c,
> there are cases when the first instruction of a function is a jump to
> the prologue code, which is located somewhere else in the function
> (e.g.  the end of the function).  The last instruction of the prologue
> in this case is a branch to the "real" code.
> 
> To take this case into account, GDB applies two corrections:
> 
> C1: GDB tests if the first instruction of the function is a jump; if
> so, GDB jumps to the target of the branch (i.e.  it follows the branch).
> 
> C2: GDB tests if the next instruction after the prologue is a jump; if
> so, it considers that it is a branch back to the "real" beginning of
> the program and follows the branch.
> 
> A problem appears if we are in the "usual" case and if the first instruction
> of the "real" code is a branch instruction:
> 
> 0x8049454 <_ada_b>:     push   %ebp
> 0x8049455 <_ada_b+1>:   mov    %esp,%ebp
> 0x8049457 <_ada_b+3>:   jmp    0x8049460 <_ada_b+12>
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> In this case, GDB will not apply C1 but will apply C2. That seems wrong to
> me, I cannot see how the pair (not C1, C2) can be correct.
> 
> See a possible fix in attachment. Tested on x86-linux, no regression.
> 
> OK to apply?
> 
> -- 
> Jerome

> 2004-04-19  Jerome Guitton  <guitton@gnat.com>
> 
> 	* i386-tdep.c (i386_skip_prologue): follow the last jump only if the
> 	function begins with a branch instruction.
> 
> Index: i386-tdep.c
> ===================================================================
> RCS file: /cvs/src/src/gdb/i386-tdep.c,v
> retrieving revision 1.187
> diff -u -p -r1.187 i386-tdep.c
> --- i386-tdep.c	18 Apr 2004 18:38:04 -0000	1.187
> +++ i386-tdep.c	19 Apr 2004 17:08:36 -0000
> @@ -750,7 +750,15 @@ i386_skip_prologue (CORE_ADDR start_pc)
>  	}
>      }
>  
> -  return i386_follow_jump (pc);
> +  /* If the first instruction of the function is a branch, then the
> +     setup sequence is at the end of the function and the instruction
> +     at pc is branch back to the start. In this case, follow the
> +     jump.  */
> +
> +  if (i386_follow_jump (start_pc) != start_pc)
> +    return i386_follow_jump (pc);
> +  else
> +    return pc;
>  }
>  
>  /* This function is 64-bit safe.  */