Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | Date | Mon, 24 Aug 2020 16:14:29 +0200 | From | Peter Zijlstra <> | Subject | Re: x86/kprobes: kretprobe fails to triggered if kprobe at function entry is not optimized (trigger by int3 breakpoint) |
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On Mon, Aug 24, 2020 at 12:02:58PM +0000, Eddy_Wu@trendmicro.com wrote: > 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. > > (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. 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?
Masami, would it be possible to have a kretprobe specific recursion count here?
I did the below, but i'm not at all sure that isn't horrible broken. I can't really find many rp->lock sites and this might break things by limiting contention.
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diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h index 9be1bff4f586..0bff314cc800 100644 --- a/include/linux/kprobes.h +++ b/include/linux/kprobes.h @@ -153,6 +153,7 @@ struct kretprobe { size_t data_size; struct hlist_head free_instances; raw_spinlock_t lock; + atomic_t recursion; }; struct kretprobe_instance { diff --git a/kernel/kprobes.c b/kernel/kprobes.c index 287b263c9cb9..27fd096bcb9a 100644 --- a/kernel/kprobes.c +++ b/kernel/kprobes.c @@ -1934,22 +1934,17 @@ unsigned long __weak arch_deref_entry_point(void *entry) static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs) { struct kretprobe *rp = container_of(p, struct kretprobe, kp); - unsigned long hash, flags = 0; struct kretprobe_instance *ri; - - /* - * To avoid deadlocks, prohibit return probing in NMI contexts, - * just skip the probe and increase the (inexact) 'nmissed' - * statistical counter, so that the user is informed that - * something happened: - */ - if (unlikely(in_nmi())) { - rp->nmissed++; - return 0; - } + unsigned long hash, flags; + int rec; /* TODO: consider to only swap the RA after the last pre_handler fired */ hash = hash_ptr(current, KPROBE_HASH_BITS); + rec = atomic_fetch_inc_acquire(&rp->recursion); + if (rec) { + rp->nmissed++; + goto out; + } raw_spin_lock_irqsave(&rp->lock, flags); if (!hlist_empty(&rp->free_instances)) { ri = hlist_entry(rp->free_instances.first, @@ -1964,7 +1959,7 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs) raw_spin_lock_irqsave(&rp->lock, flags); hlist_add_head(&ri->hlist, &rp->free_instances); raw_spin_unlock_irqrestore(&rp->lock, flags); - return 0; + goto out; } arch_prepare_kretprobe(ri, regs); @@ -1978,6 +1973,8 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs) rp->nmissed++; raw_spin_unlock_irqrestore(&rp->lock, flags); } +out: + atomic_dec(&rp->recursion); return 0; } NOKPROBE_SYMBOL(pre_handler_kretprobe);
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