[Lguest] [PATCH] Fix out-by-one error in traps.c

[Rusty Russell]

On Fri, 2007-08-31 at 00:51 -0700, Linus Torvalds wrote:
> 
> On Fri, 31 Aug 2007, Rusty Russell wrote:
> > head.S pushes a "$0" on the stack to stop the unwinder, lguest doesn't.
> 
> The unwinder should stop when it sees an invalid frame pointer, and even 
> without the push 0 I'd have expected it to be invalid.
> 
> But I suspect lguest triggers another thing: you actually make the stack 
> start at the *very*top* of the stack area. Afaik, normal x86 does not. A 
> normal x86 kernel will start off with a pt_regs[] setup, I think - ie the 
> kernel stack is always set up so that it has the "return to user mode" 
> information.

This is only for the initial booting stack (init_thread_union); see
arch/i386/kernel/head.S:
	/* Set up the stack pointer */
	lss stack_start,%esp
	...
	pushl $0		# fake return address for unwinder
	...
.data
ENTRY(stack_start)
	.long init_thread_union+THREAD_SIZE
	.long __BOOT_DS

lguest_asm.S missed the pushl $0 (lguest doesn't boot via head.S.  I'd
like to change that for 2.6.24, but it involved perturbing that code so
maybe not).

> But your patch does improve the sanity checking of the frame pointer. That 
> said, I think the following patch improves it more: does this also work 
> for you? (Totally untested, but it looks like the RightThing(tm) to do)

Yes, looks good.  Perhaps one additional magic num removal:

>  #ifdef	CONFIG_FRAME_POINTER
> -	while (valid_stack_ptr(tinfo, (void *)ebp)) {
> +	while (valid_stack_ptr(tinfo, (void *)ebp, 2*sizeof(unsigned long))) {
>  		unsigned long new_ebp;
>  		addr = *(unsigned long *)(ebp + 4);

*((unsigned long *)ebp + 1)?

Thanks,
Rusty.