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

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Date: Mon, 8 Apr 2024 12:16:57 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Jiri Olsa <olsajiri@...il.com>
Cc: Oleg Nesterov <oleg@...hat.com>, Andrii Nakryiko
 <andrii.nakryiko@...il.com>, Steven Rostedt <rostedt@...dmis.org>, Alexei
 Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org, Song Liu
 <songliubraving@...com>, Yonghong Song <yhs@...com>, John Fastabend
 <john.fastabend@...il.com>, Peter Zijlstra <peterz@...radead.org>, Thomas
 Gleixner <tglx@...utronix.de>, "Borislav Petkov (AMD)" <bp@...en8.de>,
 x86@...nel.org, linux-api@...r.kernel.org
Subject: Re: [PATCHv2 1/3] uprobe: Add uretprobe syscall to speed up return
 probe

On Fri, 5 Apr 2024 10:56:15 +0200
Jiri Olsa <olsajiri@...il.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@...nel.org>