Date:	Thu, 11 Aug 2011 09:22:36 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Jason Baron <jbaron@...hat.com>, yrl.pp-manager.tt@...achi.com
Subject: Re: [PATCH 5/5][RFC] kprobes: Use ftrace hooks when probing ftrace
 nops

On Thu, 2011-08-11 at 16:41 +0900, Masami Hiramatsu wrote:
> Hi Steven,
> 
> As I suggested in another reply, I'm now attracted by the idea
> of using ftrace in kprobe-tracer, because of simplicity.
> Anyway, here is my review.

It may be a little simpler, but it defeats the purpose of what I'm
trying to accomplish. I do want the ability to expand this to do other
things than just trace_kprobe.

> 
> (2011/08/11 1:22), Steven Rostedt wrote:
> > From: Steven Rostedt <srostedt@...hat.com>
> >
> > Currently, if a probe is set at an ftrace nop, the probe will fail.
> >
> > When -mfentry is used by ftrace (in the near future), the nop will be
> > at the beginning of the function. This will prevent kprobes from hooking
> > to the most common place that kprobes are attached.
> >
> > Now that ftrace supports individual users as well as pt_regs passing,
> > kprobes can use the ftrace infrastructure when a probe is placed on
> > a ftrace nop.
> 
> My one general concern is the timing of enabling ftrace-based probe.
> Breakpoint-based kprobe (including optimized kprobe) is enabled
> right after registering. Users might expect that.
> And AFAIK, dynamic ftraces handler will be enabled (activated)
> after a while, because it has to wait for an NMI, doesn't it?

There's no NMI needed. Not sure where you got that idea. When
register_ftrace_function() is called, it will start tracing immediately.
Note, stop_machine() is called, but we could fix that in the future too.

> 
> And theoretically, this ftrace-based probe can't support jprobe,
> because it changes IP address. Nowadays, this may becomes minor
> problem (because someone who wants to trace function args can
> use kprobe-tracer), but still exist.

I want to fix that too. :)

We don't need to worry about that until -mfentry exists. But once that
does, then, yes, I want the ftrace version working for jprobe too.

> 
> 
> > Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> > ---
> >  include/linux/kprobes.h |    6 ++
> >  kernel/kprobes.c        |  163 ++++++++++++++++++++++++++++++++++++++++++++---
> >  2 files changed, 160 insertions(+), 9 deletions(-)
> >
> > diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
> > index dd7c12e..5742071 100644
> > --- a/include/linux/kprobes.h
> > +++ b/include/linux/kprobes.h
> > @@ -37,6 +37,7 @@
> >  #include <linux/spinlock.h>
> >  #include <linux/rcupdate.h>
> >  #include <linux/mutex.h>
> > +#include <linux/ftrace.h>
> >
> >  #ifdef CONFIG_KPROBES
> >  #include <asm/kprobes.h>
> > @@ -112,6 +113,11 @@ struct kprobe {
> >  	/* copy of the original instruction */
> >  	struct arch_specific_insn ainsn;
> >
> > +#ifdef CONFIG_FUNCTION_TRACER
> > +	/* If it is possible to use ftrace to probe */
> > +	struct ftrace_ops fops;
> > +#endif
> > +
> >  	/*
> >  	 * Indicates various status flags.
> >  	 * Protected by kprobe_mutex after this kprobe is registered.
> > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> > index e6c25eb..2160768 100644
> > --- a/kernel/kprobes.c
> > +++ b/kernel/kprobes.c
> > @@ -102,6 +102,31 @@ static struct kprobe_blackpoint kprobe_blacklist[] = {
> >  	{NULL}    /* Terminator */
> >  };
> >
> > +#ifdef CONFIG_FUNCTION_TRACER
> > +/* kprobe stub function for use when probe is not connected to ftrace */
> > +static void
> > +kprobe_ftrace_stub(unsigned long ip, unsigned long pip,
> > +		   struct ftrace_ops *op, struct pt_regs *pt_regs)
> > +{
> > +}
> > +
> > +static bool ftrace_kprobe(struct kprobe *p)
> > +{
> > +	return p->fops.func && p->fops.func != kprobe_ftrace_stub;
> > +}
> > +
> > +static void init_non_ftrace_kprobe(struct kprobe *p)
> > +{
> > +	p->fops.func = kprobe_ftrace_stub;
> > +}
> > +
> > +#else
> > +static bool ftrace_kprobe(struct kprobe *p)
> > +{
> > +	return false;
> > +}
> > +#endif
> > +
> >  #ifdef __ARCH_WANT_KPROBES_INSN_SLOT
> >  /*
> >   * kprobe->ainsn.insn points to the copy of the instruction to be
> > @@ -396,6 +421,9 @@ static __kprobes void free_aggr_kprobe(struct kprobe *p)
> >  {
> >  	struct optimized_kprobe *op;
> >
> > +	if (ftrace_kprobe(p))
> > +		return;
> > +
> >  	op = container_of(p, struct optimized_kprobe, kp);
> >  	arch_remove_optimized_kprobe(op);
> >  	arch_remove_kprobe(p);
> > @@ -759,6 +787,9 @@ static __kprobes void try_to_optimize_kprobe(struct kprobe *p)
> >  	struct kprobe *ap;
> >  	struct optimized_kprobe *op;
> >
> > +	if (ftrace_kprobe(p))
> > +		return;
> > +
> >  	ap = alloc_aggr_kprobe(p);
> >  	if (!ap)
> >  		return;
> > @@ -849,6 +880,10 @@ static void __kprobes __arm_kprobe(struct kprobe *p)
> >  {
> >  	struct kprobe *_p;
> >
> > +	/* Only arm non-ftrace probes */
> > +	if (ftrace_kprobe(p))
> > +		return;
> > +
> >  	/* Check collision with other optimized kprobes */
> >  	_p = get_optimized_kprobe((unsigned long)p->addr);
> >  	if (unlikely(_p))
> > @@ -864,6 +899,10 @@ static void __kprobes __disarm_kprobe(struct kprobe *p,
> bool reopt)
> >  {
> >  	struct kprobe *_p;
> >
> > +	/* Only disarm non-ftrace probes */
> > +	if (ftrace_kprobe(p))
> > +		return;
> > +
> >  	unoptimize_kprobe(p, false);	/* Try to unoptimize */
> >
> >  	if (!kprobe_queued(p)) {
> > @@ -878,13 +917,26 @@ static void __kprobes __disarm_kprobe(struct kprobe *p,
> bool reopt)
> >
> >  #else /* !CONFIG_OPTPROBES */
> >
> > +static void __kprobes __arm_kprobe(struct kprobe *p)
> > +{
> > +	/* Only arm non-ftrace probes */
> > +	if (!ftrace_kprobe(p))
> > +		arch_arm_kprobe(p);
> > +}
> > +
> > +/* Remove the breakpoint of a probe. Must be called with text_mutex locked */
> > +static void __kprobes __disarm_kprobe(struct kprobe *p, bool reopt)
> > +{
> > +	/* Only disarm non-ftrace probes */
> > +	if (!ftrace_kprobe(p))
> > +		arch_disarm_kprobe(p);
> > +}
> 
> If it ignores disabling/enabling, kprobe_ftrace_callback must
> check kprobe_disabled(p) and skip it.

No, the callers of __(dis)arm_kprobe() also call the (dis)arm_kprobe()
later, which calls (un)register_ftrace_function(). The reason we can't
call the ftrace register functions here is because they call
stop_machine() and then we get into a deadlock with the text_mutex. The
places that call __(dis)arm_kprobe() later call (dis)arm_kprobe() after
releasing the text_mutex.


> 
> > +
> >  #define optimize_kprobe(p)			do {} while (0)
> >  #define unoptimize_kprobe(p, f)			do {} while (0)
> >  #define kill_optimized_kprobe(p)		do {} while (0)
> >  #define prepare_optimized_kprobe(p)		do {} while (0)
> >  #define try_to_optimize_kprobe(p)		do {} while (0)
> > -#define __arm_kprobe(p)				arch_arm_kprobe(p)
> > -#define __disarm_kprobe(p, o)			arch_disarm_kprobe(p)
> >  #define kprobe_disarmed(p)			kprobe_disabled(p)
> >  #define wait_for_kprobe_optimizer()		do {} while (0)
> >
> > @@ -897,7 +949,9 @@ static void reuse_unused_kprobe(struct kprobe *ap)
> >
> >  static __kprobes void free_aggr_kprobe(struct kprobe *p)
> >  {
> > -	arch_remove_kprobe(p);
> > +
> > +	if (!ftrace_kprobe(p))
> > +		arch_remove_kprobe(p);
> >  	kfree(p);
> >  }
> >
> > @@ -910,6 +964,12 @@ static __kprobes struct kprobe *alloc_aggr_kprobe(struct
> kprobe *p)
> >  /* Arm a kprobe with text_mutex */
> >  static void __kprobes arm_kprobe(struct kprobe *kp)
> >  {
> > +	/* ftrace probes can skip arch calls */
> > +	if (ftrace_kprobe(kp)) {
> > +		register_ftrace_function(&kp->fops);
> > +		return;
> > +	}
> > +
> >  	/*
> >  	 * Here, since __arm_kprobe() doesn't use stop_machine(),
> >  	 * this doesn't cause deadlock on text_mutex. So, we don't
> > @@ -924,6 +984,12 @@ static void __kprobes arm_kprobe(struct kprobe *kp)
> >  static void __kprobes disarm_kprobe(struct kprobe *kp)
> >  {
> >  	/* Ditto */
> > +
> > +	if (ftrace_kprobe(kp)) {
> > +		unregister_ftrace_function(&kp->fops);
> > +		return;
> > +	}
> > +
> >  	mutex_lock(&text_mutex);
> >  	__disarm_kprobe(kp, true);
> >  	mutex_unlock(&text_mutex);
> > @@ -1313,6 +1379,56 @@ static inline int check_kprobe_rereg(struct kprobe *p)
> >  	return ret;
> >  }
> >
> > +#ifdef CONFIG_FUNCTION_TRACER
> > +static notrace void
> > +kprobe_ftrace_callback(unsigned long ip, unsigned long parent_ip,
> > +		       struct ftrace_ops *op, struct pt_regs *pt_regs)
> > +{
> > +	struct kprobe *p = container_of(op, struct kprobe, fops);
> > +
> 
> Here, we need to set up kprobe_ctlblk and some of pt_regs members,
> ip, cs and orig_ax as optimized_callback()@arch/x86/kernel/kprobes.c
> does.

I'm curious to what this is used for? It doesn't seem to be needed for
the generic kprobes. Because we know the probe was on a nop, there's no
need to simulate the operation. IOW, there's no need for singlestep() or
other gdb like operations.

> 
> > +	/* A nop has been trapped, just run both handlers back to back */
> > +	if (p->pre_handler)
> > +		p->pre_handler(p, pt_regs);
> 
> And increment regs->ip here for NOP.

Does the post_handler() expect the ip to be after the call? Thus a
post_handle() is the same as pre_handle() on rip+next_ins?

> 
> > +	if (p->post_handler)
> > +		p->post_handler(p, pt_regs, 0);
> > +}
> 
> Anyway, above operations strongly depends on arch, so
> kprobe_ftrace_callback should be moved to arch/*/kprobes.c.
> 
> And I think most of the code can be shared with optimized code.

Not sure why. Except if regs->ip needs to be incremented. And that can
be a per arch header file. Remember, the ftrace version has no need for
single stepping like the optimized version does. ftrace replaces a nop,
where as other kprobes replace actual instructions. This is what make
the ftrace version so much less complex.

> 
> > +
> > +static int use_ftrace_hook(struct kprobe *p)
> > +{
> > +	char str[KSYM_SYMBOL_LEN];
> > +
> > +	/* see if this is an ftrace function */
> > +	if (!ftrace_text_reserved(p->addr, p->addr)) {
> > +		/* make sure fops->func is nop */
> > +		init_non_ftrace_kprobe(p);
> > +		return 0;
> > +	}
> > +
> > +	/* If arch does not support pt_regs passing, bail */
> > +	if (!ARCH_SUPPORTS_FTRACE_SAVE_REGS)
> > +		return -EINVAL;
> 
> Hmm, I think this should be checked at build time...

This is actually keeping the same logic that exists now. See below, we
removed the check of ftrace_text_reserved() from the conditions that
make register_kprobe() return -EINVAL. Now if the arch supports
FTRACE_SAVE_REGS, it can handle the ftrace_text_reserved(), if not, then
it goes back to its original behavior.

The only way to remove the later code is with #ifdef ugliness, and I
didn't want to add that.

> 
> > +
> > +	/* Use ftrace hook instead */
> > +
> > +	memset(&p->fops, 0, sizeof(p->fops));
> > +
> > +	/* Find the function this is connected with this addr */
> > +	kallsyms_lookup((unsigned long)p->addr, NULL, NULL, NULL, str);
> > +
> > +	p->fops.flags = FTRACE_OPS_FL_SAVE_REGS;
> > +	p->fops.func = kprobe_ftrace_callback;
> > +
> > +	ftrace_set_filter(&p->fops, str, strlen(str), 1);
> 
> Hmm, IMHO, ftrace_set_filter should not be called here, because
> there can be other kprobes are already registered on the same
> address. In that case, it is natural that we use an aggr_kprobe
> for handling several kprobes on same address. Or, kprobe hash table
> will have several different probes on same address.

The ftrace_set_filter() only updates the p->fops to what it will trace.
It's the register_ftrace_function() that enables the ftrace tracing, and
that is done with the arm_kprobe() call. If that's where the aggr_kprobe
enables its call, then that will be the only probe that is called.

Now I need to re-examine how the aggr_kprobes work. Does it have its own
handler to call multiple probe->handlers?


> 
> > +
> > +	return 0;
> > +}
> > +#else
> > +static int use_ftrace_hook(struct kprobe *p)
> > +{
> > +	return 0;
> > +}
> > +#endif
> > +
> >  int __kprobes register_kprobe(struct kprobe *p)
> >  {
> >  	int ret = 0;
> > @@ -1329,11 +1445,14 @@ int __kprobes register_kprobe(struct kprobe *p)
> >  	if (ret)
> >  		return ret;
> >
> > +	ret = use_ftrace_hook(p);
> > +	if (ret)
> > +		return ret;
> > +
> >  	jump_label_lock();
> >  	preempt_disable();
> >  	if (!kernel_text_address((unsigned long) p->addr) ||
> >  	    in_kprobes_functions((unsigned long) p->addr) ||
> > -	    ftrace_text_reserved(p->addr, p->addr) ||
> >  	    jump_label_text_reserved(p->addr, p->addr))
> >  		goto fail_with_jump_label;
> >
> > @@ -1384,15 +1503,17 @@ int __kprobes register_kprobe(struct kprobe *p)
> >  		goto out;
> >  	}
> >
> > -	ret = arch_prepare_kprobe(p);
> > -	if (ret)
> > -		goto out;
> > +	if (!ftrace_kprobe(p)) {
> > +		ret = arch_prepare_kprobe(p);
> > +		if (ret)
> > +			goto out;
> > +	}
> >
> >  	INIT_HLIST_NODE(&p->hlist);
> >  	hlist_add_head_rcu(&p->hlist,
> >  		       &kprobe_table[hash_ptr(p->addr, KPROBE_HASH_BITS)]);
> >
> > -	if (!kprobes_all_disarmed && !kprobe_disabled(p))
> > +	if (!ftrace_kprobe(p) && !kprobes_all_disarmed && !kprobe_disabled(p))
> >  		__arm_kprobe(p);
> >
> >  	/* Try to optimize kprobe */
> > @@ -1400,6 +1521,12 @@ int __kprobes register_kprobe(struct kprobe *p)
> >
> >  out:
> >  	mutex_unlock(&text_mutex);
> > +
> > +	/* ftrace kprobes must be set outside of text_mutex */
> > +	if (!ret && ftrace_kprobe(p) &&
> > +	    !kprobes_all_disarmed && !kprobe_disabled(p))
> > +		arm_kprobe(p);
> > +
> >  	put_online_cpus();
> >  	jump_label_unlock();
> >  	mutex_unlock(&kprobe_mutex);
> 
> After this, we must handle some fails which can happen when probing
> on a module.

What problems that were added by ftrace that isn't a problem with normal
probes?

> 
> 
> > @@ -2134,6 +2261,14 @@ static void __kprobes arm_all_kprobes(void)
> >  	}
> >  	mutex_unlock(&text_mutex);
> >
> > +	/* ftrace kprobes are enabled outside of text_mutex */
> > +	for (i = 0; i < KPROBE_TABLE_SIZE; i++) {
> > +		head = &kprobe_table[i];
> > +		hlist_for_each_entry_rcu(p, node, head, hlist)
> > +			if (ftrace_kprobe(p) && !kprobe_disabled(p))
> > +				arm_kprobe(p);
> > +	}
> > +
> >  	kprobes_all_disarmed = false;
> >  	printk(KERN_INFO "Kprobes globally enabled\n");
> >
> > @@ -2164,11 +2299,21 @@ static void __kprobes disarm_all_kprobes(void)
> >  	for (i = 0; i < KPROBE_TABLE_SIZE; i++) {
> >  		head = &kprobe_table[i];
> >  		hlist_for_each_entry_rcu(p, node, head, hlist) {
> > -			if (!arch_trampoline_kprobe(p) && !kprobe_disabled(p))
> > +			if (!ftrace_kprobe(p) &&
> > +			    !arch_trampoline_kprobe(p) && !kprobe_disabled(p))
> >  				__disarm_kprobe(p, false);
> >  		}
> >  	}
> >  	mutex_unlock(&text_mutex);
> > +
> > +	/* ftrace kprobes are disabled outside of text_mutex */
> > +	for (i = 0; i < KPROBE_TABLE_SIZE; i++) {
> > +		head = &kprobe_table[i];
> > +		hlist_for_each_entry_rcu(p, node, head, hlist) {
> > +			if (ftrace_kprobe(p) && !kprobe_disabled(p))
> > +				disarm_kprobe(p);
> > +		}
> > +	}
> >  	mutex_unlock(&kprobe_mutex);
> >
> >  	/* Wait for disarming all kprobes by optimizer */
> 
> Thank you,
> 

Thanks for taking the time for your review!

-- Steve


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