Merge branch 'fix-bpf_link-grace-period-wait-for-tracepoints'

Kumar Kartikeya Dwivedi says:

====================
Fix bpf_link grace period wait for tracepoints

A recent change to non-faultable tracepoints switched from
preempt-disabled critical sections to SRCU-fast, which breaks
assumptions in the bpf_link_free() path. Use call_srcu() to fix the
breakage.

Changelog:
----------
v3 -> v4
v3: https://lore.kernel.org/bpf/20260331005215.2813492-1-memxor@gmail.com

 * Introduce call_tracepoint_unregister_{atomic,syscall} instead. (Alexei, Steven)

v2 -> v3
v2: https://lore.kernel.org/bpf/20260330143102.1265391-1-memxor@gmail.com

 * Introduce and switch to call_tracepoint_unregister_non_faultable(). (Steven)
 * Address Andrii's comment and add Acked-by. (Andrii)
 * Drop rcu_trace_implies_rcu_gp() conversion. (Alexei)

v1 -> v2
v1: https://lore.kernel.org/bpf/20260330032124.3141001-1-memxor@gmail.com

 * Add Reviewed-by tags. (Paul, Puranjay)
 * Adjust commit descriptions and comments to clarify intent. (Puranjay)
====================

Link: https://patch.msgid.link/20260331211021.1632902-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Alexei Starovoitov

include/linux/bpf.h

@@ -1854,6 +1854,10 @@ struct bpf_link_ops {
 	 * target hook is sleepable, we'll go through tasks trace RCU GP and
 	 * then "classic" RCU GP; this need for chaining tasks trace and
 	 * classic RCU GPs is designated by setting bpf_link->sleepable flag
+	 *
+	 * For non-sleepable tracepoint links we go through SRCU gp instead,
+	 * since RCU is not used in that case. Sleepable tracepoints still
+	 * follow the scheme above.
 	 */
 	void (*dealloc_deferred)(struct bpf_link *link);
 	int (*detach)(struct bpf_link *link);

include/linux/tracepoint.h

@@ -122,13 +122,33 @@ static inline bool tracepoint_is_faultable(struct tracepoint *tp)
 {
 	return tp->ext && tp->ext->faultable;
 }
+/*
+ * Run RCU callback with the appropriate grace period wait for non-faultable
+ * tracepoints, e.g., those used in atomic context.
+ */
+static inline void call_tracepoint_unregister_atomic(struct rcu_head *rcu, rcu_callback_t func)
+{
+	call_srcu(&tracepoint_srcu, rcu, func);
+}
+/*
+ * Run RCU callback with the appropriate grace period wait for faultable
+ * tracepoints, e.g., those used in syscall context.
+ */
+static inline void call_tracepoint_unregister_syscall(struct rcu_head *rcu, rcu_callback_t func)
+{
+	call_rcu_tasks_trace(rcu, func);
+}
 #else
 static inline void tracepoint_synchronize_unregister(void)
 { }
 static inline bool tracepoint_is_faultable(struct tracepoint *tp)
 {
 	return false;
 }
+static inline void call_tracepoint_unregister_atomic(struct rcu_head *rcu, rcu_callback_t func)
+{  }
+static inline void call_tracepoint_unregister_syscall(struct rcu_head *rcu, rcu_callback_t func)
+{  }
 #endif
 
 #ifdef CONFIG_HAVE_SYSCALL_TRACEPOINTS

kernel/bpf/syscall.c

@@ -3261,6 +3261,18 @@ static void bpf_link_defer_dealloc_rcu_gp(struct rcu_head *rcu)
 	bpf_link_dealloc(link);
 }
 
+static bool bpf_link_is_tracepoint(struct bpf_link *link)
+{
+	/*
+	 * Only these combinations support a tracepoint bpf_link.
+	 * BPF_LINK_TYPE_TRACING raw_tp progs are hardcoded to use
+	 * bpf_raw_tp_link_lops and thus dealloc_deferred(), see
+	 * bpf_raw_tp_link_attach().
+	 */
+	return link->type == BPF_LINK_TYPE_RAW_TRACEPOINT ||
+	       (link->type == BPF_LINK_TYPE_TRACING && link->attach_type == BPF_TRACE_RAW_TP);
+}
+
 static void bpf_link_defer_dealloc_mult_rcu_gp(struct rcu_head *rcu)
 {
 	if (rcu_trace_implies_rcu_gp())
@@ -3279,16 +3291,25 @@ static void bpf_link_free(struct bpf_link *link)
 	if (link->prog)
 		ops->release(link);
 	if (ops->dealloc_deferred) {
-		/* Schedule BPF link deallocation, which will only then
+		/*
+		 * Schedule BPF link deallocation, which will only then
 		 * trigger putting BPF program refcount.
 		 * If underlying BPF program is sleepable or BPF link's target
 		 * attach hookpoint is sleepable or otherwise requires RCU GPs
 		 * to ensure link and its underlying BPF program is not
 		 * reachable anymore, we need to first wait for RCU tasks
-		 * trace sync, and then go through "classic" RCU grace period
+		 * trace sync, and then go through "classic" RCU grace period.
+		 *
+		 * For tracepoint BPF links, we need to go through SRCU grace
+		 * period wait instead when non-faultable tracepoint is used. We
+		 * don't need to chain SRCU grace period waits, however, for the
+		 * faultable case, since it exclusively uses RCU Tasks Trace.
 		 */
 		if (link->sleepable || (link->prog && link->prog->sleepable))
 			call_rcu_tasks_trace(&link->rcu, bpf_link_defer_dealloc_mult_rcu_gp);
+		/* We need to do a SRCU grace period wait for non-faultable tracepoint BPF links. */
+		else if (bpf_link_is_tracepoint(link))
+			call_tracepoint_unregister_atomic(&link->rcu, bpf_link_defer_dealloc_rcu_gp);
 		else
 			call_rcu(&link->rcu, bpf_link_defer_dealloc_rcu_gp);
 	} else if (ops->dealloc) {