linux-kernel - Re: CONFIG_RETHUNK int3 filling prevents kprobes in function body

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Message-ID: <YxYRHHuTHjzgCuE8@hirez.programming.kicks-ass.net>
Date:   Mon, 5 Sep 2022 17:09:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Borislav Petkov <bp@...e.de>, Josh Poimboeuf <jpoimboe@...nel.org>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: CONFIG_RETHUNK int3 filling prevents kprobes in function body

On Sun, Sep 04, 2022 at 11:07:13PM +0900, Masami Hiramatsu wrote:
> Hi Peter,
> 
> I found that the CONFIG_RETHUNK code (path_return) fills the unused bytes
> with int3 for padding. Unfortunately, this prevents kprobes on the function
> body after the return code (e.g. branch blocks placed behind the return.)

Prior to that CONFIG_SLS would already use "ret; int3"

> This is because kprobes decodes function body to ensure the probed address
> is an instruction boundary, and if it finds the 0xcc (int3), it stops
> decoding and reject probing because the int3 is usually used for a
> software breakpoint and is replacing some other instruction. Without
> recovering the instruction, it can not continue decoding safely.

I can't follow this logic. Decoding the single byte int3 instruction is
trivial. If you want a sanity check, follow the branches you found while
decoding the instruction starting at +0.

> Can we use another instruction for padding instead of INT3? (e.g. NOP or UD2)

No. NOP is not a trap instruction and UD2 is longer than it needs to be.