FAILED: patch "[PATCH] fprobe: Use guard(rcu_sched_notrace) and check" failed to apply to 6.18-stable tree

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x e0a6249190402a28f1e2925a81acf573157d55ef
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026100341-letter-overstate-ddf7@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From e0a6249190402a28f1e2925a81acf573157d55ef Mon Sep 17 00:00:00 2001
From: "Masami Hiramatsu (Google)" <[email protected]>
Date: Tue, 29 Sep 2026 09:19:12 +0900
Subject: [PATCH] fprobe: Use guard(rcu_sched_notrace) and check
rcu_is_watching()

unregister_fprobe() and unregister_fprobe_async() (used by BPF kprobe-multi) rely on standard RCU grace periods (synchronize_rcu() and call_rcu()) to wait until in-flight fprobe handlers complete before freeing the fprobe.

However, if an fprobe handler executes while RCU is not watching (such as in the idle loop or nohz_full extended quiescent states), standard RCU does not track preemption-disabled sections. Consequently, synchronize_rcu() does not wait for those executions, which can lead to a use-after-free if the fprobe is freed immediately after unregistration. Ensure handlers exit early when !rcu_is_watching().

Furthermore, fprobe_fgraph_entry() and fprobe_ftrace_entry() previously used guard(rcu)() and rcu_read_lock(), which invoke lockdep on every hit under CONFIG_PROVE_LOCKING. This adds overhead and can cause lockdep recursion if probed functions interact with lockdep.

Since rhltable_lookup() and rhl_for_each_entry_rcu() use rcu_dereference_all_check() (which checks rcu_read_lock_any_held()), holding preemption disabled via rcu_read_lock_sched_notrace() is fully valid and sufficient so long as rcu_is_watching() is true.

Define and use guard(rcu_sched_notrace)() across fprobe_ftrace_entry(), fprobe_fgraph_entry(), and fprobe_return(). This eliminates fast-path rcu_read_lock() and lockdep overhead while guaranteeing safe grace period synchronization.

index 9f2d98181779..da286619c5d8 100644
@@ -47,6 +47,10 @@ static struct rhltable fprobe_ip_table;
 static DEFINE_MUTEX(fprobe_mutex);
 static struct fgraph_ops fprobe_graph_ops;
 
+DEFINE_LOCK_GUARD_0(rcu_sched_notrace,
+		    rcu_read_lock_sched_notrace(),
+		    rcu_read_unlock_sched_notrace())
+
 static u32 fprobe_node_hashfn(const void *data, u32 len, u32 seed)
 {
 	return hash_ptr(*(unsigned long **)data, 32);

@@ -329,16 +333,14 @@ static void fprobe_ftrace_entry(unsigned long ip, unsigned long parent_ip,
 	struct fprobe *fp;
 	int bit;
 
+	if (!rcu_is_watching())
+		return;
+
 	bit = ftrace_test_recursion_trylock(ip, parent_ip);
 	if (bit < 0)
 		return;
 
-	/*
-	 * ftrace_test_recursion_trylock() disables preemption, but
-	 * rhltable_lookup() checks whether rcu_read_lcok is held.
-	 * So we take rcu_read_lock() here.
-	 */
-	rcu_read_lock();
+	guard(rcu_sched_notrace)();
 	head = rhltable_lookup(&fprobe_ip_table, &ip, fprobe_rht_params);
 
 	rhl_for_each_entry_rcu(node, pos, head, hlist) {

@@ -353,7 +355,6 @@ static void fprobe_ftrace_entry(unsigned long ip, unsigned long parent_ip,
 		else
 			__fprobe_handler(ip, parent_ip, fp, fregs, NULL);
 	}
-	rcu_read_unlock();
 	ftrace_test_recursion_unlock(bit);
 }
 NOKPROBE_SYMBOL(fprobe_ftrace_entry);

@@ -567,10 +568,13 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
 	struct fprobe *fp;
 	int used, ret;
 
+	if (!rcu_is_watching())
+		return 0;
+
 	if (WARN_ON_ONCE(!fregs))
 		return 0;
 
-	guard(rcu)();
+	guard(rcu_sched_notrace)();
 	head = rhltable_lookup(&fprobe_ip_table, &func, fprobe_rht_params);
 	reserved_words = 0;
 	rhl_for_each_entry_rcu(node, pos, head, hlist) {

@@ -665,13 +669,16 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
 	int size, curr;
 	int size_words;
 
+	if (!rcu_is_watching())
+		return;
+
 	fgraph_data = (unsigned long *)fgraph_retrieve_data(gops->idx, &size);
 	if (WARN_ON_ONCE(!fgraph_data))
 		return;
 	size_words = SIZE_IN_LONG(size);
 	ret_ip = ftrace_regs_get_instruction_pointer(fregs);
 
-	preempt_disable_notrace();
+	guard(rcu_sched_notrace)();
 
 	curr = 0;
 	while (size_words > curr) {

@@ -687,7 +694,6 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
 		}
 		curr += size;
 	}
-	preempt_enable_notrace();
 }
 NOKPROBE_SYMBOL(fprobe_return);
 

From: Menglong Dong <[email protected]>

[ Upstream commit ceb5d8d367d6d37a220023df443d8b67c2f4d1cd ]

rcu_read_lock() is not needed in fprobe_entry, but rcu_dereference_check() is used in rhltable_lookup(), which causes suspicious RCU usage warning:

WARNING: suspicious RCU usage
6.17.0-rc1-00001-gdfe0d675df82 #1 Tainted: G S
-----------------------------
include/linux/rhashtable.h:602 suspicious rcu_dereference_check() usage!
......

  stack backtrace:
  CPU: 1 UID: 0 PID: 4652 Comm: ftracetest Tainted: G S
  Tainted: [S]=CPU_OUT_OF_SPEC, [I]=FIRMWARE_WORKAROUND
  Hardware name: Dell Inc. OptiPlex 7040/0Y7WYT, BIOS 1.1.1 10/07/2015
  Call Trace:
   <TASK>
   dump_stack_lvl+0x7c/0x90
   lockdep_rcu_suspicious+0x14f/0x1c0
   __rhashtable_lookup+0x1e0/0x260
   ? __pfx_kernel_clone+0x10/0x10
   fprobe_entry+0x9a/0x450
   ? __lock_acquire+0x6b0/0xca0
   ? find_held_lock+0x2b/0x80
   ? __pfx_fprobe_entry+0x10/0x10
   ? __pfx_kernel_clone+0x10/0x10
   ? lock_acquire+0x14c/0x2d0
   ? __might_fault+0x74/0xc0
   function_graph_enter_regs+0x2a0/0x550
   ? __do_sys_clone+0xb5/0x100
   ? __pfx_function_graph_enter_regs+0x10/0x10
   ? _copy_to_user+0x58/0x70
   ? __pfx_kernel_clone+0x10/0x10
   ? __x64_sys_rt_sigprocmask+0x114/0x180
   ? __pfx___x64_sys_rt_sigprocmask+0x10/0x10
   ? __pfx_kernel_clone+0x10/0x10
   ftrace_graph_func+0x87/0xb0

As we discussed in [1], fix this by using guard(rcu)() in fprobe_entry() to protect the rhltable_lookup() and rhl_for_each_entry_rcu() with rcu_read_lock and suppress this warning.

Stable-dep-of: e0a624919040 ("fprobe: Use guard(rcu_sched_notrace) and check rcu_is_watching()")

---
 kernel/trace/fprobe.c | 1 +
 1 file changed, 1 insertion(+)

index 1c9ae332afad1..2ba7470443551 100644
@@ -428,6 +428,7 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
 	if (WARN_ON_ONCE(!fregs))
 		return 0;
 
+	guard(rcu)();
 	head = rhltable_lookup(&fprobe_ip_table, &func, fprobe_rht_params);
 	reserved_words = 0;
 	rhl_for_each_entry_rcu(node, pos, head, hlist) {

2.53.0

From: "Masami Hiramatsu (Google)" <[email protected]>

[ Upstream commit e0a6249190402a28f1e2925a81acf573157d55ef ]

unregister_fprobe() and unregister_fprobe_async() (used by BPF kprobe-multi) rely on standard RCU grace periods (synchronize_rcu() and call_rcu()) to wait until in-flight fprobe handlers complete before freeing the fprobe.

However, if an fprobe handler executes while RCU is not watching (such as in the idle loop or nohz_full extended quiescent states), standard RCU does not track preemption-disabled sections. Consequently, synchronize_rcu() does not wait for those executions, which can lead to a use-after-free if the fprobe is freed immediately after unregistration. Ensure handlers exit early when !rcu_is_watching().

Furthermore, fprobe_fgraph_entry() and fprobe_ftrace_entry() previously used guard(rcu)() and rcu_read_lock(), which invoke lockdep on every hit under CONFIG_PROVE_LOCKING. This adds overhead and can cause lockdep recursion if probed functions interact with lockdep.

Since rhltable_lookup() and rhl_for_each_entry_rcu() use rcu_dereference_all_check() (which checks rcu_read_lock_any_held()), holding preemption disabled via rcu_read_lock_sched_notrace() is fully valid and sufficient so long as rcu_is_watching() is true.

Define and use guard(rcu_sched_notrace)() across fprobe_ftrace_entry(), fprobe_fgraph_entry(), and fprobe_return(). This eliminates fast-path rcu_read_lock() and lockdep overhead while guaranteeing safe grace period synchronization.

---
 kernel/trace/fprobe.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

index 2ba7470443551..a236baed157a4 100644
@@ -47,6 +47,10 @@ static struct rhltable fprobe_ip_table;
 static DEFINE_MUTEX(fprobe_mutex);
 static struct fgraph_ops fprobe_graph_ops;
 
+DEFINE_LOCK_GUARD_0(rcu_sched_notrace,
+		    rcu_read_lock_sched_notrace(),
+		    rcu_read_unlock_sched_notrace())
+
 static u32 fprobe_node_hashfn(const void *data, u32 len, u32 seed)
 {
 	return hash_ptr(*(unsigned long **)data, 32);

@@ -260,16 +264,14 @@ static void fprobe_ftrace_entry(unsigned long ip, unsigned long parent_ip,
 	struct fprobe *fp;
 	int bit;
 
+	if (!rcu_is_watching())
+		return;
+
 	bit = ftrace_test_recursion_trylock(ip, parent_ip);
 	if (bit < 0)
 		return;
 
-	/*
-	 * ftrace_test_recursion_trylock() disables preemption, but
-	 * rhltable_lookup() checks whether rcu_read_lcok is held.
-	 * So we take rcu_read_lock() here.
-	 */
-	rcu_read_lock();
+	guard(rcu_sched_notrace)();
 	head = rhltable_lookup(&fprobe_ip_table, &ip, fprobe_rht_params);
 
 	rhl_for_each_entry_rcu(node, pos, head, hlist) {

@@ -284,7 +286,6 @@ static void fprobe_ftrace_entry(unsigned long ip, unsigned long parent_ip,
 		else
 			__fprobe_handler(ip, parent_ip, fp, fregs, NULL);
 	}
-	rcu_read_unlock();
 	ftrace_test_recursion_unlock(bit);
 }
 NOKPROBE_SYMBOL(fprobe_ftrace_entry);

@@ -425,10 +426,13 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
 	struct fprobe *fp;
 	int used, ret;
 
+	if (!rcu_is_watching())
+		return 0;
+
 	if (WARN_ON_ONCE(!fregs))
 		return 0;
 
-	guard(rcu)();
+	guard(rcu_sched_notrace)();
 	head = rhltable_lookup(&fprobe_ip_table, &func, fprobe_rht_params);
 	reserved_words = 0;
 	rhl_for_each_entry_rcu(node, pos, head, hlist) {

@@ -521,13 +525,16 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
 	int size, curr;
 	int size_words;
 
+	if (!rcu_is_watching())
+		return;
+
 	fgraph_data = (unsigned long *)fgraph_retrieve_data(gops->idx, &size);
 	if (WARN_ON_ONCE(!fgraph_data))
 		return;
 	size_words = SIZE_IN_LONG(size);
 	ret_ip = ftrace_regs_get_instruction_pointer(fregs);
 
-	preempt_disable_notrace();
+	guard(rcu_sched_notrace)();
 
 	curr = 0;
 	while (size_words > curr) {

@@ -543,7 +550,6 @@ static void fprobe_return(struct ftrace_graph_ret *trace,
 		}
 		curr += size;
 	}
-	preempt_enable_notrace();
 }
 NOKPROBE_SYMBOL(fprobe_return);
 

2.53.0