Backtraces broken on i386 by unorthodox encoding of push %ebp

[Richard Smith]

I have an i386 program on Linux where some of the functions 
start with a slightly unorthodox encoding of the usual 
function prologue:

   08048104 <foo>:
    8048104:	ff f5                	push   %ebp
    8048106:	89 e5                	mov    %esp,%ebp

However, when I have this form of prologue, gdb doesn't 
recongnise it, and doesn't give a meaningful backtrace.

In this case, the push instruction is encoded using the 
two-byte PUSH r/m32 (FF /6) form, instead of the more common 
one-byte PUSH r32 (50+rd) form.  The two instructions 
execute identically, and both have been around since the 
8086.  So far as I'm aware, the ABI doesn't require the use 
of the shorter encoding of the instruction in the function 
prologue.  And for this particular application, I would 
prefer to continue using the two-byte form.

I think it should be pretty straightforward to make gdb 
accept both versions of the instruction.  I'm not familiar 
with gdb's source code, but at a guess, modifying the start 
of i386_analyze_frame_setup in gdb/i386-tdep.c so that it 
does something like this:

   /* Check for `pushl %ebp', either encoded using `push r32' (55)
      or less commonly using `push r/m32' (FF F5). */
   if (op == 0x55 || op == 0xff
       && read_memory_unsigned_integer (pc + 1, 1, byte_order) == 0xf5)
     {
       /* Take into account that we've executed the `pushl %ebp' that
          starts this instruction sequence.  */
       cache->saved_regs[I386_EBP_REGNUM] = 0;
       cache->sp_offset += 4;
       pc += op == 0x55 ? 1 : 2;

I expect the amd64 version of the code could do with a 
similar change.

Richard