[tip: perf/urgent] uprobes: Remove too strict lockdep_assert() condition in hprobe_expire()

["tip-bot2 for Andrii Nakryiko" <tip-bot2@linutronix.de>]

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     f8c857238a392f21d5726d07966f6061007c8d4f
Gitweb:        https://git.kernel.org/tip/f8c857238a392f21d5726d07966f6061007c8d4f
Author:        Andrii Nakryiko <andrii@kernel.org>
AuthorDate:    Tue, 25 Feb 2025 14:32:14 -08:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Tue, 25 Feb 2025 23:36:19 +01:00

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
---
 kernel/events/uprobes.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index af53fbd..b4ca889 100644
--- 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) {