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.
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