On Sat, 24 Sep 2022 09:52:28 +0800
"liaochang (A)" <liaochang1@xxxxxxxxxx> wrote:
>
>
> 在 2022/9/23 20:39, Mark Rutland 写道:
> > On Fri, Sep 23, 2022 at 04:46:58PM +0800, Liao Chang wrote:
> >> Single-step slot would not be used until kprobe is enabled, that means
> >> no race condition occurs on it under SMP, hence it is safe to pacth ss
> >> slot without stopping machine.
> >
> > I think this is correct, but this depends on a couple of subtleties,
> > importantly:
> >
> > * That the I-cache maintenance for these instructions is complete *before* the
> > kprobe BRK is written (and aarch64_insn_patch_text_nosync() ensures this, but
> > just omits causing a Context-Synchronization-Event on all CPUS).
>
> So in order to guarantee the I-cache maintenance is observed on all CPUS,
> it needs to be followed by a explicit Context-Synchronization-Event, perhaps
> it is better to place ISB before kprobe BRK is written.
>
> >
> > * That the kprobe BRK results in an exception (and consequently a
> > Context-Synchronoization-Event), which ensures that the CPU will fetch the
> > single-step slot instructions *after* this, ensuring that the new
> > instructions are used.
>
> Yes, because of single-step slot is installed int the BRK execption handler,
> so it is not necessary to generate Context-Synchronization-Event via ISB mentioned above...
Can you update the patch including above as comments in the code?
Maybe you also have to ensure it on other patches too.
Thank you,
>
> Thanks.
>
> >
> > It would be good if we could call that out explicitly.
> >
> > Thanks,
> > Mark.
> >
> >> Since I and D caches are coherent within single-step slot from
> >> aarch64_insn_patch_text_nosync(), hence no need to do it again via
> >> flush_icache_range().
> >>
> >> Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> >> Signed-off-by: Liao Chang <liaochang1@xxxxxxxxxx>
> >> ---
> >> arch/arm64/kernel/probes/kprobes.c | 7 ++-----
> >> 1 file changed, 2 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> >> index d1d182320245..29b98bc12833 100644
> >> --- a/arch/arm64/kernel/probes/kprobes.c
> >> +++ b/arch/arm64/kernel/probes/kprobes.c
> >> @@ -44,13 +44,10 @@ post_kprobe_handler(struct kprobe *, struct kprobe_ctlblk *, struct pt_regs *);
> >> static void __kprobes arch_prepare_ss_slot(struct kprobe *p)
> >> {
> >> kprobe_opcode_t *addr = p->ainsn.api.insn;
> >> - void *addrs[] = {addr, addr + 1};
> >> - u32 insns[] = {p->opcode, BRK64_OPCODE_KPROBES_SS};
> >>
> >> /* prepare insn slot */
> >> - aarch64_insn_patch_text(addrs, insns, 2);
> >> -
> >> - flush_icache_range((uintptr_t)addr, (uintptr_t)(addr + MAX_INSN_SIZE));
> >> + aarch64_insn_patch_text_nosync(addr, p->opcode);
> >> + aarch64_insn_patch_text_nosync(addr + 1, BRK64_OPCODE_KPROBES_SS);
> >>
> >> /*
> >> * Needs restoring of return address after stepping xol.
> >> --
> >> 2.17.1
> >>
> >
> > .
>
> --
> BR,
> Liao, Chang
--
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
