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

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Date:   Tue, 6 Sep 2022 10:33:29 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Peter Zijlstra <peterz@...radead.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, 5 Sep 2022 17:52:29 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> 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.

No, kgdb is also handles int3 for its breakpoint. Of course we can
ignore it or ask kgdb to expose the API to decode it.

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

Yeah, but it is a temporal use, so user can try it again.

Thank you,

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