Re: [PATCH v4 0/3] Compile-time stack frame pointer validation

Borislav Petkov
2015-05-20T16:52:30Z

On Wed, May 20, 2015 at 11:25:37AM -0500, Josh Poimboeuf wrote:
> > I've never quite understood what the '?' means.
>
> It basically means "here's a function address we found on the stack,
> which may or may not have been called." It's needed because stack
> walking isn't currently 100% reliable.

Yeah, that was not that trivial to figure out at the time:

unsigned long
print_context_stack(struct thread_info *tinfo,
...

if (__kernel_text_address(addr)) {
if ((unsigned long) stack == bp + sizeof(long)) {
ops->address(data, addr, 1);
frame = frame->next_frame;
bp = (unsigned long) frame;
} else {
ops->address(data, addr, 0);
}

and that ops->address is

print_trace_address()
|-> printk_stack_address()

So if I'm understanding this correctly, if rBP+8 is equal to rSP, i.e.
return address is on the stack, then this frame got called.

Otherwise -> "?".

I might be missing something though...

--
Regards/Gruss,
Boris.

ECO tip #101: Trim your mails when you reply.
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