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

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Message-ID: <YxYbPTmN1TCp4En5@hirez.programming.kicks-ass.net>
Date:   Mon, 5 Sep 2022 17:52:29 +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 Mon, Sep 05, 2022 at 05:09:16PM +0200, Peter Zijlstra wrote:

> > 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.

Specifically, kprobe is the only one scribbling random [*] instructions
with int3 in kernel text, so if kprobes doesn't know about the int3, it
must be padding.

[*] there's also static_call, jump_label and ftrace that use
text_poke_bp() to scribble instructions but those are well known
locations.