uprobes: Remove too strict lockdep_assert() condition in hprobe_expire()

hprobe_expire() is used to atomically switch pending uretprobe instance
(struct return_instance) from being SRCU protected to be refcounted.
This can be done from background timer thread, or synchronously within
current thread when task is forked.

In the former case, return_instance has to be protected through RCU read
lock, and that's what hprobe_expire() used to check with
lockdep_assert(rcu_read_lock_held()).

But in the latter case (hprobe_expire() called from dup_utask()) there
is no RCU lock being held, and it's both unnecessary and incovenient.
Inconvenient due to the intervening memory allocations inside
dup_return_instance()'s loop. Unnecessary because dup_utask() is called
synchronously in current thread, and no uretprobe can run at that point,
so return_instance can't be freed either.

So drop rcu_read_lock_held() condition, and expand corresponding comment
to explain necessary lifetime guarantees. lockdep_assert()-detected
issue is a false positive.

Fixes: dd1a7567784e ("uprobes: SRCU-protect uretprobe lifetime (with timeout)")
Reported-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250225223214.2970740-1-andrii@kernel.org

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index af53fbd2d12c452f93cab8f555a77db4c2123947..b4ca8898fe178ecc91e4b15aa24987eafb2854cc 100644 (file)
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -767,10 +767,14 @@ static struct uprobe *hprobe_expire(struct hprobe *hprobe, bool get)
        enum hprobe_state hstate;
 
        /*
-        * return_instance's hprobe is protected by RCU.
-        * Underlying uprobe is itself protected from reuse by SRCU.
+        * Caller should guarantee that return_instance is not going to be
+        * freed from under us. This can be achieved either through holding
+        * rcu_read_lock() or by owning return_instance in the first place.
+        *
+        * Underlying uprobe is itself protected from reuse by SRCU, so ensure
+        * SRCU lock is held properly.
         */
-       lockdep_assert(rcu_read_lock_held() && srcu_read_lock_held(&uretprobes_srcu));
+       lockdep_assert(srcu_read_lock_held(&uretprobes_srcu));
 
        hstate = READ_ONCE(hprobe->state);
        switch (hstate) {