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    SubjectRe: [PATCH v3 1/2] kretprobe: produce sane stack traces
    On 2018-11-11, Masami Hiramatsu <mhiramat@kernel.org> wrote:
    > > > > + 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/>
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