Re: [PATCH 18/18] tracing/kprobes: Dump the culprit kprobe in caseof kprobe recursion

From: Masami Hiramatsu
Date: Thu Aug 27 2009 - 11:48:56 EST

Frederic Weisbecker wrote:
On Thu, Aug 27, 2009 at 11:30:24AM -0400, Masami Hiramatsu wrote:
Hi Frederic,

Frederic Weisbecker wrote:
Kprobes can enter into a probing recursion, ie: a kprobe that does an
endless loop because one of its core mechanism function used during
probing is also probed itself.

This patch helps pinpointing the kprobe that raised such recursion
by dumping it and raising a BUG instead of a warning (we also disarm
the kprobe to try avoiding recursion in BUG itself). Having a BUG
instead of a warning stops the stacktrace in the right place and
doesn't pollute the logs with hundreds of traces that eventually end
up in a stack overflow.

Thanks, but I also found similar bug cases.

Signed-off-by: Frederic Weisbecker<fweisbec@xxxxxxxxx>
Cc: Masami Hiramatsu<mhiramat@xxxxxxxxxx>
Cc: Ananth N Mavinakayanahalli<ananth@xxxxxxxxxx>
arch/x86/kernel/kprobes.c | 8 ++++++--
include/linux/kprobes.h | 2 ++
kernel/kprobes.c | 7 +++++++
3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
index 16ae961..ecee3d2 100644
--- a/arch/x86/kernel/kprobes.c
+++ b/arch/x86/kernel/kprobes.c
@@ -490,9 +490,13 @@ static int __kprobes reenter_kprobe(struct kprobe *p, struct pt_regs *regs,

Before this, kprobes checks p != kprobe_running(), but it's a
meaningless branch. Hitting a kprobe while KPROBES_HIT_SS always
treated as unrecoverable.

Yeah, but that's the place where a probe ends up when bad reentrancy happens

No, a place which is shared by kprobes and other subsystems, will cause a

for example, I found an irq_return case which will be p == kprobe_running()
on x86-64.

-> <some irq occurs>
-> irq_return
-> <hit int3>
-> do_int3
-> <handling kprobe (set kprobe_running)>
-> irq_return (from do_int3)
-> <hit int3>
-> do_int3
<handling kprobe (kprobe_running == p)> <- here!

Perhaps, the original code assumes that it will be caused by an int3
which another subsystem inserted on out-of-line singlestep buffer
if the hitting probe is same as current probe.

However, in that case, int3 hitting address is on the out-of-line
buffer and should be different from first (current) int3 address.

So, I think this part should also be removed.

if (p == kprobe_running()) {
regs->flags &= ~X86_EFLAGS_TF;
regs->flags |= kcb->kprobe_saved_flags;
return 0;
} else {

Thank you,

/* A probe has been hit in the codepath leading up
* to, or just after, single-stepping of a probed
* instruction. This entire codepath should strictly
- * reside in .kprobes.text section. Raise a warning
- * to highlight this peculiar case.
+ * reside in .kprobes.text section.
+ * Raise a BUG or we'll continue in an endless
+ * reentering loop and eventually a stack overflow.
+ arch_disarm_kprobe(p);
+ dump_kprobe(p);
+ BUG();
/* impossible cases */
diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
index bcd9c07..87eb79c 100644
--- a/include/linux/kprobes.h
+++ b/include/linux/kprobes.h
@@ -296,6 +296,8 @@ void recycle_rp_inst(struct kretprobe_instance *ri, struct hlist_head *head);
int disable_kprobe(struct kprobe *kp);
int enable_kprobe(struct kprobe *kp);

+void dump_kprobe(struct kprobe *kp);
#else /* !CONFIG_KPROBES: */

static inline int kprobes_built_in(void)
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index ef177d6..f72e96c 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1141,6 +1141,13 @@ static void __kprobes kill_kprobe(struct kprobe *p)

+void __kprobes dump_kprobe(struct kprobe *kp)
+ printk(KERN_WARNING "Dumping kprobe:\n");
+ printk(KERN_WARNING "Name: %s\nAddress: %p\nOffset: %x\n",
+ kp->symbol_name, kp->addr, kp->offset);

Since kp->symbol_name + kp->offset = kp->addr, I recommend to show it
as "Kprobe at %s+%x:<%p>\n", kp->symbol_name, kp->offset, kp->addr.

Ok I'll fix this, thanks.

/* Module notifier call back, checking kprobes on the module */
static int __kprobes kprobes_module_callback(struct notifier_block *nb,
unsigned long val, void *data)

Thank you,

Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@xxxxxxxxxx

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