Messages in this thread | | | Date | Mon, 10 Jul 2023 00:25:07 +0900 | From | Masami Hiramatsu (Google) <> | Subject | Re: [PATCH 2/2] x86/retpoline,kprobes: Avoid treating rethunk as an indirect jump |
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On Sat, 8 Jul 2023 16:18:29 +0200 Petr Pavlu <petr.pavlu@suse.com> wrote:
> On 7/6/23 13:34, Peter Zijlstra wrote: > > On Thu, Jul 06, 2023 at 06:00:14PM +0900, Masami Hiramatsu wrote: > >> On Thu, 6 Jul 2023 09:17:05 +0200 > >> Peter Zijlstra <peterz@infradead.org> wrote: > >> > >>> On Thu, Jul 06, 2023 at 09:47:23AM +0900, Masami Hiramatsu wrote: > >>> > >>>>>> If I understand correctly, all indirect jump will be replaced with JMP_NOSPEC. > >>>>>> If you read the insn_jump_into_range, I onlu jecks the jump code, not call. > >>>>>> So the functions only have indirect call still allow optprobe. > >>>>> > >>>>> With the introduction of kCFI JMP_NOSPEC is no longer an equivalent to a > >>>>> C indirect jump. > >>>> > >>>> If I understand correctly, kCFI is enabled by CFI_CLANG, and clang is not > >>>> using jump-tables by default, so we can focus on gcc. In that case > >>>> current check still work, correct? > >>> > >>> IIRC clang can use jump tables, but like GCC needs RETPOLINE=n and > >>> IBT=n, so effectively nobody has them. > >> > >> So if it requires RETPOLINE=n, current __indirect_thunk_start/end checking > >> is not required, right? (that code is embraced with "#ifdef CONFIG_RETPOLINE") > > > > Correct. > > Thank you both for the explanation. > > If I understand correctly, it means this second patch can be dropped and > I can instead replace it with a removal of the mentioned check. That > will also void the main motivation for the first patch but that one > should be still at least useful to make the LTO_CLANG=y build lay out > the code in the same way as with other configurations.
Yes, something like removing __indirect_thunk_start/end check and disabling insn_is_indirect_jump() when defined(CONFIG_RETPOLINE) || defined(CONFIG_X86_KERNEL_IBT).
kCFI case is also handled later but another series.
Thank you,
> > I will post an updated series with these changes. > > -- Petr >
-- Masami Hiramatsu (Google) <mhiramat@kernel.org>
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