Re: [PATCH -tip 0/5] kprobes: Fix stacktrace in kretprobes

[Masami Hiramatsu]

On Mon, 8 Mar 2021 11:52:10 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:

> So, here is my idea;
> 
> 1) Change the trampline code to prepare stack frame at first and save
>    registers on it, instead of "push". This will makes ORC easy to setup
>    stackframe information for this code.
> 2) change the return addres fixup timing. Instead of using return value
>    of trampoline handler, before removing the real return address from
>    current->kretprobe_instances.
> 3) Then, if orc_find() finds the ip is in the kretprobe_trampoline, it
>    checks the contents of the end of stackframe (at the place of regs->sp)
>    is same as the address of it. If it is, it can find the correct address
>    from current->kretprobe_instances. If not, there is a correct address.

Another trickly idea is put a call on top of kretprobe_trampoline like this.

        "__kretprobe_trampoline:\n"
        "       call kretprobe_trampoline\n"
        "kretprobe_trampoline:\n"
        "       pushq %rsp\n"
        "       pushfq\n"
        SAVE_REGS_STRING
        "       movq %rsp, %rdi\n"
        "       call trampoline_handler\n"
        /* Replace __kretprobe_trampoline with true return address. */
        "       movq %rax, 20*8(%rsp)\n"
        RESTORE_REGS_STRING
        "       popfq\n"
        "       popq %rsp\n"
        "       ret\n"

This will leave a marker (kretprobe_trampoline or __kretprobe_trampoline+5) on
the top of stack, and the stack frame seems like a normal function. If objtool
can make an ORC info by disassembling kretprobe_trampoline, I guess it is
easy to make a stack frame information.

But anyway, from the inside of target function, it still see "__kretprobe_trampoline"
on the stack instead of caller_func, so orc_kretprobe_find() is still needed.

I'm not familier with the UNWIND_HINT macro, so if it is easy to handle the original
case, I think my first idea will be better.

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>