Re: [PATCHv2 1/3] uprobe: Add uretprobe syscall to speed up return probe

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Fri, 5 Apr 2024 10:56:15 +0200
Jiri Olsa <olsajiri@gmail.com> wrote:

> > 
> > Can we avoid this with below strict check?
> > 
> > if (ri->stack != regs->sp + expected_offset)
> > 	goto sigill;
> 
> hm the current uprobe 'alive' check makes sure the return_instance is above
> or at the same stack address, not sure we can match it exactly, need to think
> about that more
> 
> > 
> > expected_offset should be 16 (push * 3 - ret) on x64 if we ri->stack is the
> > regs->sp right after call.
> 
> the syscall trampoline already updates the regs->sp before calling
> handle_trampoline
> 
>         regs->sp += sizeof(r11_cx_ax);

Yes, that is "push * 3" part. And "- ret"  is that the stack entry is consumed
by the "ret", which is stored by call.

1: |--------| <- sp at function entry == ri->stack
0: |ret-addr| <- call pushed it

0: |ret-addr| <- sp at return trampoline

3: |r11     | <- regs->sp at syscall
2: |rcx     |
1: |rax     | <- ri->stack
0: |ret-addr|

(Note: The lower the line, the larger the address.)

Thus, we can check the stack address by (regs->sp + 16 == ri->stack).

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>