From: Peter Zijlstra <peterz@infradead.org>
To: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Borislav Petkov <bp@suse.de>,
Josh Poimboeuf <jpoimboe@kernel.org>,
linux-kernel@vger.kernel.org,
Steven Rostedt <rostedt@goodmis.org>,
Ingo Molnar <mingo@kernel.org>
Subject: Re: CONFIG_RETHUNK int3 filling prevents kprobes in function body
Date: Mon, 5 Sep 2022 17:09:16 +0200 [thread overview]
Message-ID: <YxYRHHuTHjzgCuE8@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20220904230713.a461f8fe85329663226c755f@kernel.org>
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.
next prev parent reply other threads:[~2022-09-05 15:10 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-04 14:07 CONFIG_RETHUNK int3 filling prevents kprobes in function body Masami Hiramatsu
2022-09-05 14:57 ` Steven Rostedt
2022-09-05 15:10 ` Peter Zijlstra
2022-09-05 15:20 ` Peter Zijlstra
2022-09-06 1:30 ` Masami Hiramatsu
2022-09-05 15:09 ` Peter Zijlstra [this message]
2022-09-05 15:15 ` Peter Zijlstra
2022-09-05 15:52 ` Peter Zijlstra
2022-09-06 1:33 ` Masami Hiramatsu
2022-09-06 8:44 ` Peter Zijlstra
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