Re: Pb when calling a nested function in the inferior

[Kevin Buettner <kevinb@cygnus.com>]

On Jul 30, 11:24pm, Joel Brobecker wrote:

> > > I've personally never used (or even seen) this
> > > nested function syntax, so I don't know anything
> > > useful about it.		-- Michael
> 
> Yes, I hesitated a bit before posting this example, knowing that it was
> a gcc extension. However, I thought it would make an easier to undertand
> example rather than posting Ada95 code.

Have you verified that the mechanisms used by gcc and by Ada95 for
passing the static chain are the same?

> > It's a gcc extension.  Apparently, on i386, a pointer to the static
> > chain is passed in ecx.  If Joel wants to fix this problem, it'd be a
> > good idea to see if the relevant ABI addresses this issue and then
> > make the appropriate changes.  (The trick, I think, is to figure out
> > the correct value to load into ecx.)
> 
> OK, I'll have a look and see what can be done. But before doing
> anything, is there a way for gdb to detect that the function it is about
> to call is nested? I think that, as a first step, having gdb diagnose
> such cases and report a warning or an error would be an improvement.
> Right now, it gives either an incorrect value or even crashes the
> inferior.

A couple of ideas come to mind:

    1) In the version of gcc that I'm using the symbol that's associated
       with get_value() is get_value.0.  You could look for such symbols
       and refuse to allow them to be called as inferior functions.

    2) You could scan the prologue and look for a sequence of instructions
       which looks like a save of the static chain.  E.g, in your example,
       I see:

	0x804842c <get_value.0>:        push   %ebp
	0x804842d <get_value.0+1>:      mov    %esp,%ebp
	0x804842f <get_value.0+3>:      sub    $0x4,%esp
	0x8048432 <get_value.0+6>:      mov    %ecx,0xfffffffc(%ebp)
	0x8048435 <get_value.0+9>:      mov    0xfffffffc(%ebp),%ecx
	0x8048438 <get_value.0+12>:     mov    %ecx,%ecx
	0x804843a <get_value.0+14>:     mov    0xfffffffc(%ecx),%eax
	0x804843d <get_value.0+17>:     mov    %eax,%eax
	0x804843f <get_value.0+19>:     leave  
	0x8048440 <get_value.0+20>:     ret    

       It appears to me that ``mov %ecx,0xfffffffc(%ebp)'' is
       responsible for saving the static chain pointer.  If you could
       detect this, you could print your error or warning.  (You'd
       want to make sure that no other instruction with a destination
       of %ecx appears before this instruction in the prologue though;
       if it does, it means it's doing something else.)

BTW, GDB isn't particularly graceful in its handling of the ``get_value.0''
symbol.  E.g, observe what happens when I do ``x/i get_value.0'':

    (gdb) x/i get_value.0
    No symbol "get_value" in current context.

Kevin