Re: [PATCH v3 1/2] kretprobe: produce sane stack traces

On 2018-11-11, Masami Hiramatsu [off-list ref] wrote:

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+		addr = kretprobe_ret_addr(current, addr, stack_addr(regs));

But since kretprobe will be an event, which can kick the stackdump.
BTW, from kretprobe, regs->ip should always be the trampoline handler, 
see arch/x86/kernel/kprobes/core.c:772 :-)
So it must be fixed always.

Right, but kretprobe_ret_addr() is returning the *original* return
address (and we need to do an (addr == kretprobe_trampoline)). The
real problem is that stack_addr(regs) isn't the same as it is during
kretprobe setup (but kretprobe_ret_addr() works everywhere else).

I think stack_addr(regs) should be same when this is called from kretprobe
handler context. Otherwise, yes, it is not same, but in that case, regs->ip
is not kretprobe_trampoline too.

I figured it out.

It should be (regs->sp - 1) (just like it is inside the relevant
unwinder function for ORC). I now have a prototype which works under the
frame unwinder[*] -- however under ORC you can only see the top-most
function (the unwinder doesn't see the other function calls). I'm
playing with ORC hints with kretprobe_trampoline to try to improve
things but it's still a bit screwy.


[*]: However, I've noticed that the stack traces between the two traces
	 no longer match. On kprobe you get function_name+1, but on
	 kretprobe you get function_caller+foo. Obviously it's working but
	 the return address results in slightly different stack traces. This
	 means that stack trace aggregation between kprobe and kretprobe
	 won't work anymore -- at least not like it did in my original
	 patch. So I'm really not sure where to go from here.

I can send around another patchset to illustrate the problem if you like
(as well as show how the current unwinding code works).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>