Messages in this thread | | | Date | Mon, 5 Sep 2022 17:52:29 +0200 | From | Peter Zijlstra <> | Subject | Re: CONFIG_RETHUNK int3 filling prevents kprobes in function body |
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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.
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