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

peterz
2020-09-02T09:36:48Z

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 {