Re: [PATCH 0/2] x86/kprobes: Fixes for CONFIG_RETHUNK

Peter Zijlstra
2022-09-07T06:52:46Z

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 :-)