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SubjectRe: [PATCH 18/18] tracing/kprobes: Dump the culprit kprobe in case of kprobe recursion
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@gmail.com>
> Cc: Masami Hiramatsu<mhiramat@redhat.com>
> Cc: Ananth N Mavinakayanahalli<ananth@in.ibm.com>
> ---
> 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.

> /* 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();
> }
> default:
> /* 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)
> arch_remove_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.

> +
> /* 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@redhat.com



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