On Mon, 24 Aug 2020 12:02:58 +0000
"eddy...@trendmicro.com" <eddy...@trendmicro.com> wrote:
> Greetings!
>
> Starting from kernel 5.8 (x86_64), kretprobe handler will always missed if
> corresponding kprobe on function entry is not optimized (using break point
> instead).
Oops, good catch. I always enabled ftrace hook for kretprobe, I didn't noticed
that.
> Step to reproduce this:
> 1) Build the kretprobe example module (CONFIG_SAMPLE_KRETPROBES=m)
> 2) Disable jump optimization (`sysctl debug.kprobes-optimization=0` or
> register any kprobe.post_handler at same location)
> 3) Insert the kretprobe_example module
> 4) Launch some process to trigger _do_fork
> 5) Remove kretprobe_example module
> 6) dmesg shows that all probing instances are missed
>
> Example output:
> # sysctl debug.kprobes-optimization=0
> debug.kprobes-optimization = 0
> # insmod samples/kprobes/kretprobe_example.ko
> # ls > /dev/null
> # rmmod kretprobe_example
> # dmesg
> [48555.067295] Planted return probe at _do_fork: 0000000038ae0211
> [48560.229459] kretprobe at 0000000038ae0211 unregistered
> [48560.229460] Missed probing 3 instances of _do_fork
>
> After bisecting, I found this behavior seems to introduce by this commit:
> (5.8-rc1)
> 0d00449c7a28a1514595630735df383dec606812 x86: Replace ist_enter() with
> nmi_enter()
> This make kprobe_int3_handler() effectively running as NMI context, which
> pre_handler_kretprobe() explicitly checked to prevent recursion.
Thanks for the bisecting!
>
> (in_nmi() check appears from v3.17)
> f96f56780ca584930bb3a2769d73fd9a101bcbbe kprobes: Skip kretprobe hit in NMI
> context to avoid deadlock
>
> To make kretprobe work again with int3 breakpoint, I think we can replace the
> in_nmi() check with in_nmi() == (1 << NMI_SHIFT) at kprobe_int3_handler() and
> skip kretprobe if nested NMI.
Ah, I see. Now int3 is a kind of NMI, so in the handler in_nmi() always returns
!0.
> Did a quick test on 5.9-rc2 and it seems to be working.
> I'm not sure if it is the best way to do since it may also require change to
> other architecture as well, any thought?
Hmm, this behavior is arch-dependent. So I think we need an weak function like
this.
@kernel/kprobes.c
bool __weak arch_kprobe_in_nmi(void)
{
return in_nmi()
}
@arch/x86/kernel/kprobes/core.c
bool arch_kprobe_in_nmi(void)
{
/*
* Since the int3 is one of NMI, we have to check in_nmi() is
* bigger than 1 << NMI_SHIFT instead of !0.
*/
return in_nmi() > (1 << NMI_SHIFT);
}
And use arch_kprobe_in_nmi() instead of in_nmi() in kprobes.c.
Thanks,
--
Masami Hiramatsu <mhira...@kernel.org>
