linux-kernel - Re: [PATCH 0/2] x86/kprobes: Fixes for CONFIG

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Message-ID: <Yxg/rZDPc1fKaS7H@hirez.programming.kicks-ass.net>
Date:   Wed, 7 Sep 2022 08:52:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        bpf <bpf@...r.kernel.org>, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...e.de>,
        Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org
Subject: Re: [PATCH 0/2] x86/kprobes: Fixes for CONFIG_RETHUNK

On Wed, Sep 07, 2022 at 09:55:11AM +0900, Masami Hiramatsu (Google) wrote:
> Hi,
> 
> Here is a couple of patches to fix kprobes and optprobe to work
> on the kernel with CONFIG_RETHUNK and CONFIG_SLS.
> 
> With these configs, the kernel functions may includes padding INT3 in
> the function code block (body) in addition to the gaps between functions.
> 
> Since kprobes on x86 has to ensure the probe address is a function

s/function/instruction/

> bondary, it decodes the instructions in the function until the address.
> If it finds an INT3 which is not embedded by kprobe, it stops decoding
> because usually the INT3 is used for debugging as a software breakpoint
> and such INT3 will replace the first byte of an original instruction.
> Without recovering it, kprobes can not continue to decode it. Thus the
> kprobes returns -EILSEQ as below.

In the absence of kgdb nobody else except kprobes itself will do this.

>  # echo "p:probe/vfs_truncate_L19 vfs_truncate+98" >> kprobe_events 
>  sh: write error: Invalid or incomplete multibyte or wide character
> 
> 
> Actually, those INT3s are just for padding and can be ignored.

They are speculations stops, not mere padding.


Anyway, let me get on with reading the actual patches :-)