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

[Daniel Xu]

Hi Masami,

Just want to clarify a few points:

On Mon, Mar 08, 2021 at 11:52:10AM +0900, Masami Hiramatsu wrote:
> On Sun, 7 Mar 2021 13:23:33 -0800
> Daniel Xu <dxu@xxxxxxxxx> wrote:
> To help your understanding, let me explain.
> 
> If we have a code here
> 
> caller_func:
> 0x00 add sp, 0x20	/* 0x20 bytes stack frame allocated */
> ...
> 0x10 call target_func
> 0x15 ... /* return address */
> 
> On the stack in the entry of target_func, we have
> 
> [stack]
> 0x0e0 caller_func+0x15
> ... /* 0x20 bytes = 4 entries  are stack frame of caller_func */
> 0x100 /* caller_func return address */
> 
> And when we put a kretprobe on the target_func, the stack will be
> 
> [stack]
> 0x0e0 kretprobe_trampoline
> ... /* 0x20 bytes = 4 entries  are stack frame of caller_func */
> 0x100 /* caller_func return address */
> 
> * "caller_func+0x15" is saved in current->kretprobe_instances.first.
> 
> When returning from the target_func, call consumed the 0x0e0 and
> jump to kretprobe_trampoline. Let's see the assembler code.
> 
>         ".text\n"
>         ".global kretprobe_trampoline\n"
>         ".type kretprobe_trampoline, @function\n"
>         "kretprobe_trampoline:\n"
>         /* We don't bother saving the ss register */
>         "       pushq %rsp\n"
>         "       pushfq\n"
>         SAVE_REGS_STRING
>         "       movq %rsp, %rdi\n"
>         "       call trampoline_handler\n"
>         /* Replace saved sp with true return address. */
>         "       movq %rax, 19*8(%rsp)\n"
>         RESTORE_REGS_STRING
>         "       popfq\n"
>         "       ret\n"
> 
> When the entry of trampoline_handler, stack is like this;
> 
> [stack]
> 0x040 kretprobe_trampoline+0x25
> 0x048 r15
> ...     /* pt_regs */
> 0x0d8 flags
> 0x0e0 rsp (=0x0e0)
> ... /* 0x20 bytes = 4 entries  are stack frame of caller_func */
> 0x100 /* caller_func return address */
> 
> And after returned from trampoline_handler, "movq" changes the
> stack like this.
> 
> [stack]
> 0x040 kretprobe_trampoline+0x25
> 0x048 r15
> ...     /* pt_regs */
> 0x0d8 flags
> 0x0e0 caller_func+0x15
> ... /* 0x20 bytes = 4 entries  are stack frame of caller_func */
> 0x100 /* caller_func return address */

Thanks for the detailed explanation. I think I understand kretprobe
mechanics from a somewhat high level (kprobe saves real return address
on entry, overwrites return address to trampoline, then trampoline
runs handler and finally resets return address to real return address).

I don't usually write much assembly so the details escape me somewhat.

> So at the kretprobe handler, we have 2 issues.
> 1) the return address (caller_func+0x15) is not on the stack.
>    this can be solved by searching from current->kretprobe_instances.

Yes, agreed.

> 2) the stack frame size of kretprobe_trampoline is unknown
>    Since the stackframe is fixed, the fixed number (0x98) can be used.

I'm confused why this is relevant. Is it so ORC knows where to find
saved return address in the frame?

> However, those solutions are only for the kretprobe handler. The stacktrace
> from interrupt handler hit in the kretprobe_trampoline still doesn't work.
> 
> 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.

I'm confused on the details here. But this is what Josh solves in his
patch, right?

> 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.

Is the idea to have `kretprobe_trampoline` place the real return address
on the stack (while telling ORC where to find it) _before_ running `call
trampoline_handler` ? So that an unwind from inside the user defined
kretprobe handler simply unwinds correctly?

And to be extra clear, this would only work for stack_trace_save() and
not stack_trace_save_regs()?

> 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.

What do you mean by "it" w.r.t. "is the same address of it"? I'm
confused on this point.

Thanks,
Daniel