From: Song Liu <song@kernel.org>
To: bpf@vger.kernel.org,
	linux-trace-kernel@vger.kernel.org,
	live-patching@vger.kernel.org
Cc: ast@kernel.org,
	daniel@iogearbox.net,
	andrii@kernel.org,
	rostedt@goodmis.org,
	andrey.grodzovsky@crowdstrike.com,
	mhiramat@kernel.org,
	kernel-team@meta.com,
	olsajiri@gmail.com,
	Song Liu <song@kernel.org>,
	stable@vger.kernel.org,
	Jiri Olsa <jolsa@kernel.org>
Subject: [PATCH v4 bpf 1/3] ftrace: Fix BPF fexit with livepatch
Date: Mon, 27 Oct 2025 10:50:21 -0700
Message-ID: <20251027175023.1521602-2-song@kernel.org>
In-Reply-To: <20251027175023.1521602-1-song@kernel.org>
References: <20251027175023.1521602-1-song@kernel.org>
Precedence: bulk
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit

When livepatch is attached to the same function as bpf trampoline with
a fexit program, bpf trampoline code calls register_ftrace_direct()
twice. The first time will fail with -EAGAIN, and the second time it
will succeed. This requires register_ftrace_direct() to unregister
the address on the first attempt. Otherwise, the bpf trampoline cannot
attach. Here is an easy way to reproduce this issue:

  insmod samples/livepatch/livepatch-sample.ko
  bpftrace -e 'fexit:cmdline_proc_show {}'
  ERROR: Unable to attach probe: fexit:vmlinux:cmdline_proc_show...

Fix this by cleaning up the hash when register_ftrace_function_nolock hits
errors.

Also, move the code that resets ops->func and ops->trampoline to the error
path of register_ftrace_direct(); and add a helper function reset_direct()
in register_ftrace_direct() and unregister_ftrace_direct().

Fixes: d05cb470663a ("ftrace: Fix modification of direct_function hash while in use")
Cc: stable@vger.kernel.org # v6.6+
Reported-by: Andrey Grodzovsky <andrey.grodzovsky@crowdstrike.com>
Closes: https://lore.kernel.org/live-patching/c5058315a39d4615b333e485893345be@crowdstrike.com/
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Cc: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-and-tested-by: Andrey Grodzovsky <andrey.grodzovsky@crowdstrike.com>
Signed-off-by: Song Liu <song@kernel.org>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
---
 kernel/bpf/trampoline.c |  5 -----
 kernel/trace/ftrace.c   | 20 ++++++++++++++------
 2 files changed, 14 insertions(+), 11 deletions(-)

diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 5949095e51c3..f2cb0b097093 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -479,11 +479,6 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mut
 		 * BPF_TRAMP_F_SHARE_IPMODIFY is set, we can generate the
 		 * trampoline again, and retry register.
 		 */
-		/* reset fops->func and fops->trampoline for re-register */
-		tr->fops->func = NULL;
-		tr->fops->trampoline = 0;
-
-		/* free im memory and reallocate later */
 		bpf_tramp_image_free(im);
 		goto again;
 	}
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 42bd2ba68a82..cbeb7e833131 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5953,6 +5953,17 @@ static void register_ftrace_direct_cb(struct rcu_head *rhp)
 	free_ftrace_hash(fhp);
 }
 
+static void reset_direct(struct ftrace_ops *ops, unsigned long addr)
+{
+	struct ftrace_hash *hash = ops->func_hash->filter_hash;
+
+	remove_direct_functions_hash(hash, addr);
+
+	/* cleanup for possible another register call */
+	ops->func = NULL;
+	ops->trampoline = 0;
+}
+
 /**
  * register_ftrace_direct - Call a custom trampoline directly
  * for multiple functions registered in @ops
@@ -6048,6 +6059,8 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
 	ops->direct_call = addr;
 
 	err = register_ftrace_function_nolock(ops);
+	if (err)
+		reset_direct(ops, addr);
 
  out_unlock:
 	mutex_unlock(&direct_mutex);
@@ -6080,7 +6093,6 @@ EXPORT_SYMBOL_GPL(register_ftrace_direct);
 int unregister_ftrace_direct(struct ftrace_ops *ops, unsigned long addr,
 			     bool free_filters)
 {
-	struct ftrace_hash *hash = ops->func_hash->filter_hash;
 	int err;
 
 	if (check_direct_multi(ops))
@@ -6090,13 +6102,9 @@ int unregister_ftrace_direct(struct ftrace_ops *ops, unsigned long addr,
 
 	mutex_lock(&direct_mutex);
 	err = unregister_ftrace_function(ops);
-	remove_direct_functions_hash(hash, addr);
+	reset_direct(ops, addr);
 	mutex_unlock(&direct_mutex);
 
-	/* cleanup for possible another register call */
-	ops->func = NULL;
-	ops->trampoline = 0;
-
 	if (free_filters)
 		ftrace_free_filter(ops);
 	return err;
-- 
2.47.3