linux-kernel - Re: [PATCH v5 00/21] kprobes: Unify kretprobe trampoline handlers and make kretprobe lockless

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Date:   Wed, 2 Sep 2020 11:36:13 +0200
From:   peterz@...radead.org
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Eddy_Wu@...ndmicro.com, x86@...nel.org, davem@...emloft.net,
        rostedt@...dmis.org, naveen.n.rao@...ux.ibm.com,
        anil.s.keshavamurthy@...el.com, linux-arch@...r.kernel.org,
        cameron@...dycamel.com, oleg@...hat.com, will@...nel.org,
        paulmck@...nel.org
Subject: Re: [PATCH v5 00/21] kprobes: Unify kretprobe trampoline handlers
 and make kretprobe lockless

On Wed, Sep 02, 2020 at 05:17:55PM +0900, Masami Hiramatsu wrote:

> > Ok, but then lockdep will yell at you if you have that enabled and run
> > the unoptimized things.
> 
> Oh, does it warn for all spinlock things in kprobes if it is unoptimized?
> Hmm, it has to be noted in the documentation.

Lockdep will warn about spinlocks used in NMI context that are also used
outside NMI context.

Now, for the kretprobe that kprobe_busy flag prevents the actual
recursion self-deadlock, but lockdep isn't smart enough to see that.

One way around this might be to use SINGLE_DEPTH_NESTING for locks when
we use them from INT3 context. That way they'll have a different class
and lockdep will not see the recursion.

pre_handler_kretprobe() is always called from INT3, right?

Something like the below might then work...

---
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 287b263c9cb9..b78f4fe08e86 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1255,11 +1255,11 @@ __acquires(hlist_lock)
 NOKPROBE_SYMBOL(kretprobe_hash_lock);
 
 static void kretprobe_table_lock(unsigned long hash,
-				 unsigned long *flags)
+				 unsigned long *flags, int nesting)
 __acquires(hlist_lock)
 {
 	raw_spinlock_t *hlist_lock = kretprobe_table_lock_ptr(hash);
-	raw_spin_lock_irqsave(hlist_lock, *flags);
+	raw_spin_lock_irqsave_nested(hlist_lock, *flags, nesting);
 }
 NOKPROBE_SYMBOL(kretprobe_table_lock);
 
@@ -1326,7 +1326,7 @@ void kprobe_flush_task(struct task_struct *tk)
 	INIT_HLIST_HEAD(&empty_rp);
 	hash = hash_ptr(tk, KPROBE_HASH_BITS);
 	head = &kretprobe_inst_table[hash];
-	kretprobe_table_lock(hash, &flags);
+	kretprobe_table_lock(hash, &flags, 0);
 	hlist_for_each_entry_safe(ri, tmp, head, hlist) {
 		if (ri->task == tk)
 			recycle_rp_inst(ri, &empty_rp);
@@ -1361,7 +1361,7 @@ static void cleanup_rp_inst(struct kretprobe *rp)
 
 	/* No race here */
 	for (hash = 0; hash < KPROBE_TABLE_SIZE; hash++) {
-		kretprobe_table_lock(hash, &flags);
+		kretprobe_table_lock(hash, &flags, 0);
 		head = &kretprobe_inst_table[hash];
 		hlist_for_each_entry_safe(ri, next, head, hlist) {
 			if (ri->rp == rp)
@@ -1950,7 +1950,7 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
 
 	/* TODO: consider to only swap the RA after the last pre_handler fired */
 	hash = hash_ptr(current, KPROBE_HASH_BITS);
-	raw_spin_lock_irqsave(&rp->lock, flags);
+	raw_spin_lock_irqsave_nested(&rp->lock, flags, SINGLE_DEPTH_NESTING);
 	if (!hlist_empty(&rp->free_instances)) {
 		ri = hlist_entry(rp->free_instances.first,
 				struct kretprobe_instance, hlist);
@@ -1961,7 +1961,7 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
 		ri->task = current;
 
 		if (rp->entry_handler && rp->entry_handler(ri, regs)) {
-			raw_spin_lock_irqsave(&rp->lock, flags);
+			raw_spin_lock_irqsave_nested(&rp->lock, flags, SINGLE_DEPTH_NESTING);
 			hlist_add_head(&ri->hlist, &rp->free_instances);
 			raw_spin_unlock_irqrestore(&rp->lock, flags);
 			return 0;
@@ -1971,7 +1971,7 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
 
 		/* XXX(hch): why is there no hlist_move_head? */
 		INIT_HLIST_NODE(&ri->hlist);
-		kretprobe_table_lock(hash, &flags);
+		kretprobe_table_lock(hash, &flags, SINGLE_DEPTH_NESTING);
 		hlist_add_head(&ri->hlist, &kretprobe_inst_table[hash]);
 		kretprobe_table_unlock(hash, &flags);
 	} else {