Re: kprobes: propagate error from arm_kprobe_ftrace()

[Masami Hiramatsu]

On Wed, 3 Jan 2018 22:00:00 +0100
Jessica Yu <jeyu@xxxxxxxxxx> wrote:

> +++ Steven Rostedt [03/01/18 09:33 -0500]:
> >On Wed,  3 Jan 2018 02:40:47 +0100
> >Jessica Yu <jeyu@xxxxxxxxxx> wrote:
> >
> >> Improve error handling when arming ftrace-based kprobes. Specifically, if
> >> we fail to arm a ftrace-based kprobe, register_kprobe()/enable_kprobe()
> >> should report an error instead of success. Previously, this has lead to
> >> confusing situations where register_kprobe() would return 0 indicating
> >> success, but the kprobe would not be functional if ftrace registration
> >> during the kprobe arming process had failed. We should therefore take any
> >> errors returned by ftrace into account and propagate this error so that we
> >> do not register/enable kprobes that cannot be armed. This can happen if,
> >> for example, register_ftrace_function() finds an IPMODIFY conflict (since
> >> kprobe_ftrace_ops has this flag set) and returns an error. Such a conflict
> >> is possible since livepatches also set the IPMODIFY flag for their ftrace_ops.
> >>
> >> arm_all_kprobes() keeps its current behavior and attempts to arm all
> >> kprobes. It returns the last encountered error and gives a warning if
> >> not all probes could be armed.
> >>
> >> This patch is based on Petr Mladek's original patchset (patches 2 and 3)
> >> back in 2015, which improved kprobes error handling, found here:
> >>
> >>    https://lkml.org/lkml/2015/2/26/452
> >>
> >> However, further work on this had been paused since then and the patches
> >> were not upstreamed.
> >>
> >> Based-on-patches-by: Petr Mladek <pmladek@xxxxxxxx>
> >> Signed-off-by: Jessica Yu <jeyu@xxxxxxxxxx>
> >> ---
> >>  kernel/kprobes.c | 94 +++++++++++++++++++++++++++++++++++++++++---------------
> >>  1 file changed, 69 insertions(+), 25 deletions(-)
> >>
> >> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> >> index b4aab48ad258..ae6b6fe79de3 100644
> >> --- a/kernel/kprobes.c
> >> +++ b/kernel/kprobes.c
> >> @@ -988,18 +988,32 @@ static int prepare_kprobe(struct kprobe *p)
> >>  }
> >>
> >>  /* Caller must lock kprobe_mutex */
> >> -static void arm_kprobe_ftrace(struct kprobe *p)
> >> +static int arm_kprobe_ftrace(struct kprobe *p)
> >>  {
> >> -	int ret;
> >> +	int ret = 0;
> >>
> >>  	ret = ftrace_set_filter_ip(&kprobe_ftrace_ops,
> >>  				   (unsigned long)p->addr, 0, 0);
> >> -	WARN(ret < 0, "Failed to arm kprobe-ftrace at %p (%d)\n", p->addr, ret);
> >> -	kprobe_ftrace_enabled++;
> >> -	if (kprobe_ftrace_enabled == 1) {
> >> +	if (WARN(ret < 0, "Failed to arm kprobe-ftrace at %p (%d)\n", p->addr, ret))
> >> +		return ret;
> >
> >I wonder if we should change this from a WARN to a printk(). No reason
> >to do stack dumps here.
> 
> Yeah, I was trying to preserve the current behavior. I'll leave it up
> to Masami.

Thanks Jassica and Steve,

I wonder what are the possible cases of ftrace failure here. If it really rarely
happen, I would like to leave WARN() for debugging or reporting. But if there are
normal cases, we would better make it pr_warn() as Steve said.

> 
> >> +
> >> +	if (kprobe_ftrace_enabled == 0) {
> >>  		ret = register_ftrace_function(&kprobe_ftrace_ops);
> >> -		WARN(ret < 0, "Failed to init kprobe-ftrace (%d)\n", ret);
> >> +		if (WARN(ret < 0, "Failed to init kprobe-ftrace (%d)\n", ret))
> >> +			goto err_ftrace;
> >>  	}
> >> +
> >> +	kprobe_ftrace_enabled++;
> >> +	return ret;
> >> +
> >> +err_ftrace:
> >> +	/*
> >> +	 * Note: Since kprobe_ftrace_ops has IPMODIFY set, and ftrace requires a
> >> +	 * non-empty filter_hash for IPMODIFY ops, we're safe from an accidental
> >> +	 * empty filter_hash which would undesirably trace all functions.
> >> +	 */
> >> +	ftrace_set_filter_ip(&kprobe_ftrace_ops, (unsigned long)p->addr, 1, 0);
> >> +	return ret;
> >>  }
> >>
> >>  /* Caller must lock kprobe_mutex */
> >> @@ -1018,22 +1032,23 @@ static void disarm_kprobe_ftrace(struct kprobe *p)
> >>  }
> >>  #else	/* !CONFIG_KPROBES_ON_FTRACE */
> >>  #define prepare_kprobe(p)	arch_prepare_kprobe(p)
> >> -#define arm_kprobe_ftrace(p)	do {} while (0)
> >> +#define arm_kprobe_ftrace(p)	(0)
> >
> >Hmm. Perhaps we should have arm_kprobe_ftrace() return a failure.

Good catch!

> >
> >>  #define disarm_kprobe_ftrace(p)	do {} while (0)
> >>  #endif
> >>
> >>  /* Arm a kprobe with text_mutex */
> >> -static void arm_kprobe(struct kprobe *kp)
> >> +static int arm_kprobe(struct kprobe *kp)
> >>  {
> >> -	if (unlikely(kprobe_ftrace(kp))) {
> >> -		arm_kprobe_ftrace(kp);
> >> -		return;
> >> -	}
> >> +	if (unlikely(kprobe_ftrace(kp)))
> >> +		return arm_kprobe_ftrace(kp);
> >
> >If CONFIG_KPROBES_ON_FTRACE is not defined, this if should always be
> >false. But if for some reason in the future, it is not false, we just
> >had arm_kprobe_ftrace() return success when it really is a failure.
> >
> > -ENODEV?
> 
> Good point, I will include this change in v4, unless there are
> objections.

I have no objection :)

> 
> >> +
> >>  	cpus_read_lock();
> >>  	mutex_lock(&text_mutex);
> >>  	__arm_kprobe(kp);
> >>  	mutex_unlock(&text_mutex);
> >>  	cpus_read_unlock();
> >> +
> >> +	return 0;
> >>  }
> >>
> >>  /* Disarm a kprobe with text_mutex */
> >> @@ -1372,9 +1387,15 @@ static int register_aggr_kprobe(struct kprobe *orig_p, struct kprobe *p)
> >>
> >>  	if (ret == 0 && kprobe_disabled(ap) && !kprobe_disabled(p)) {
> >>  		ap->flags &= ~KPROBE_FLAG_DISABLED;
> >> -		if (!kprobes_all_disarmed)
> >> +		if (!kprobes_all_disarmed) {
> >>  			/* Arm the breakpoint again. */
> >> -			arm_kprobe(ap);
> >> +			ret = arm_kprobe(ap);
> >> +			if (ret) {
> >> +				ap->flags |= KPROBE_FLAG_DISABLED;
> >> +				list_del_rcu(&p->list);
> >
> >Don't we need to hold the mutex to modify the list?
> 
> It is unfortunately unclear from this snippet, but we do hold the
> kprobe_mutex here. It's held for most of the duration of
> register_kprobe(), where register_aggr_kprobe() is called.

Right, we already hold kprobe_mutex here so it is safe. :)

> 
> >> +				synchronize_sched();
> >> +			}
> >> +		}
> >>  	}
> >>  	return ret;
> >>  }
> >> @@ -1594,8 +1615,14 @@ int register_kprobe(struct kprobe *p)
> >>  	hlist_add_head_rcu(&p->hlist,
> >>  		       &kprobe_table[hash_ptr(p->addr, KPROBE_HASH_BITS)]);
> >>
> >> -	if (!kprobes_all_disarmed && !kprobe_disabled(p))
> >> -		arm_kprobe(p);
> >> +	if (!kprobes_all_disarmed && !kprobe_disabled(p)) {
> >> +		ret = arm_kprobe(p);
> >> +		if (ret) {
> >> +			hlist_del_rcu(&p->hlist);
> >
> >Same here.
> 
> We do hold kprobe_mutex here as well (see above comment).
> 
> >> +			synchronize_sched();
> >> +			goto out;
> >> +		}
> >> +	}
> >>
> >>  	/* Try to optimize kprobe */
> >>  	try_to_optimize_kprobe(p);
> >> @@ -2137,7 +2164,9 @@ int enable_kprobe(struct kprobe *kp)
> >>
> >>  	if (!kprobes_all_disarmed && kprobe_disabled(p)) {
> >>  		p->flags &= ~KPROBE_FLAG_DISABLED;
> >> -		arm_kprobe(p);
> >> +		ret = arm_kprobe(p);
> >> +		if (ret)
> >> +			p->flags |= KPROBE_FLAG_DISABLED;
> >>  	}
> >>  out:
> >>  	mutex_unlock(&kprobe_mutex);
> >> @@ -2565,11 +2594,12 @@ static const struct file_operations debugfs_kprobe_ei_ops = {
> >>  	.release        = seq_release,
> >>  };
> >>
> >> -static void arm_all_kprobes(void)
> >> +static int arm_all_kprobes(void)
> >>  {
> >>  	struct hlist_head *head;
> >>  	struct kprobe *p;
> >> -	unsigned int i;
> >> +	unsigned int i, errors = 0;
> >> +	int err, ret = 0;
> >>
> >>  	mutex_lock(&kprobe_mutex);
> >>
> >> @@ -2586,16 +2616,26 @@ static void arm_all_kprobes(void)
> >>  	/* Arming kprobes doesn't optimize kprobe itself */
> >>  	for (i = 0; i < KPROBE_TABLE_SIZE; i++) {
> >>  		head = &kprobe_table[i];
> >> -		hlist_for_each_entry_rcu(p, head, hlist)
> >> -			if (!kprobe_disabled(p))
> >> -				arm_kprobe(p);
> >> +		/* Arm all kprobes on a best-effort basis */
> >> +		hlist_for_each_entry_rcu(p, head, hlist) {
> >> +			if (!kprobe_disabled(p)) {
> >> +				err = arm_kprobe(p);
> >> +				if (err)  {
> >> +					errors++;
> >> +					ret = err;
> >> +				}
> >> +			}
> >> +		}
> >>  	}
> >>
> >> -	printk(KERN_INFO "Kprobes globally enabled\n");
> >> +	if (errors)
> >> +		pr_warn("Kprobes globally enabled, but failed to arm %d probes\n", errors);
> >
> >Perhaps we should have a count of all kprobes that were tried, and
> >write something like:
> >
> > "Kprobes globally enabled, but failed to arm %d out of %d probes\n",
> >   errors, total

Sounds good to me :)

Thanks!

> 
> Sure, ok.
> 
> Thank you for the review!
> 
> Jessica
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>