bpf: support nested rcu critical sections

Currently, nested rcu critical sections are rejected by the verifier and
rcu_lock state is managed by a boolean variable. Add support for nested
rcu critical sections by make active_rcu_locks a counter similar to
active_preempt_locks. bpf_rcu_read_lock() increments this counter and
bpf_rcu_read_unlock() decrements it, MEM_RCU -> PTR_UNTRUSTED transition
happens when active_rcu_locks drops to 0.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251117200411.25563-2-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: puranjaymohan

include/linux/bpf_verifier.h

@@ -416,7 +416,7 @@ struct bpf_verifier_state {
 	u32 active_irq_id;
 	u32 active_lock_id;
 	void *active_lock_ptr;
-	bool active_rcu_lock;
+	u32 active_rcu_locks;
 
 	bool speculative;
 	bool in_sleepable;

kernel/bpf/verifier.c

@@ -1437,7 +1437,7 @@ static int copy_reference_state(struct bpf_verifier_state *dst, const struct bpf
 	dst->acquired_refs = src->acquired_refs;
 	dst->active_locks = src->active_locks;
 	dst->active_preempt_locks = src->active_preempt_locks;
-	dst->active_rcu_lock = src->active_rcu_lock;
+	dst->active_rcu_locks = src->active_rcu_locks;
 	dst->active_irq_id = src->active_irq_id;
 	dst->active_lock_id = src->active_lock_id;
 	dst->active_lock_ptr = src->active_lock_ptr;
@@ -5889,7 +5889,7 @@ static bool in_sleepable(struct bpf_verifier_env *env)
  */
 static bool in_rcu_cs(struct bpf_verifier_env *env)
 {
-	return env->cur_state->active_rcu_lock ||
+	return env->cur_state->active_rcu_locks ||
 	       env->cur_state->active_locks ||
 	       !in_sleepable(env);
 }
@@ -10744,7 +10744,7 @@ static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		}
 
 		if (env->subprog_info[subprog].might_sleep &&
-		    (env->cur_state->active_rcu_lock || env->cur_state->active_preempt_locks ||
+		    (env->cur_state->active_rcu_locks || env->cur_state->active_preempt_locks ||
 		     env->cur_state->active_irq_id || !in_sleepable(env))) {
 			verbose(env, "global functions that may sleep are not allowed in non-sleepable context,\n"
 				     "i.e., in a RCU/IRQ/preempt-disabled section, or in\n"
@@ -11327,7 +11327,7 @@ static int check_resource_leak(struct bpf_verifier_env *env, bool exception_exit
 		return -EINVAL;
 	}
 
-	if (check_lock && env->cur_state->active_rcu_lock) {
+	if (check_lock && env->cur_state->active_rcu_locks) {
 		verbose(env, "%s cannot be used inside bpf_rcu_read_lock-ed region\n", prefix);
 		return -EINVAL;
 	}
@@ -11465,7 +11465,7 @@ static int get_helper_proto(struct bpf_verifier_env *env, int func_id,
 /* Check if we're in a sleepable context. */
 static inline bool in_sleepable_context(struct bpf_verifier_env *env)
 {
-	return !env->cur_state->active_rcu_lock &&
+	return !env->cur_state->active_rcu_locks &&
 	       !env->cur_state->active_preempt_locks &&
 	       !env->cur_state->active_irq_id &&
 	       in_sleepable(env);
@@ -11531,7 +11531,7 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 		return err;
 	}
 
-	if (env->cur_state->active_rcu_lock) {
+	if (env->cur_state->active_rcu_locks) {
 		if (fn->might_sleep) {
 			verbose(env, "sleepable helper %s#%d in rcu_read_lock region\n",
 				func_id_name(func_id), func_id);
@@ -14038,36 +14038,33 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 	preempt_disable = is_kfunc_bpf_preempt_disable(&meta);
 	preempt_enable = is_kfunc_bpf_preempt_enable(&meta);
 
-	if (env->cur_state->active_rcu_lock) {
+	if (rcu_lock) {
+		env->cur_state->active_rcu_locks++;
+	} else if (rcu_unlock) {
 		struct bpf_func_state *state;
 		struct bpf_reg_state *reg;
 		u32 clear_mask = (1 << STACK_SPILL) | (1 << STACK_ITER);
 
-		if (in_rbtree_lock_required_cb(env) && (rcu_lock || rcu_unlock)) {
-			verbose(env, "Calling bpf_rcu_read_{lock,unlock} in unnecessary rbtree callback\n");
-			return -EACCES;
-		}
-
-		if (rcu_lock) {
-			verbose(env, "nested rcu read lock (kernel function %s)\n", func_name);
+		if (env->cur_state->active_rcu_locks == 0) {
+			verbose(env, "unmatched rcu read unlock (kernel function %s)\n", func_name);
 			return -EINVAL;
-		} else if (rcu_unlock) {
+		}
+		if (--env->cur_state->active_rcu_locks == 0) {
 			bpf_for_each_reg_in_vstate_mask(env->cur_state, state, reg, clear_mask, ({
 				if (reg->type & MEM_RCU) {
 					reg->type &= ~(MEM_RCU | PTR_MAYBE_NULL);
 					reg->type |= PTR_UNTRUSTED;
 				}
 			}));
-			env->cur_state->active_rcu_lock = false;
-		} else if (sleepable) {
-			verbose(env, "kernel func %s is sleepable within rcu_read_lock region\n", func_name);
-			return -EACCES;
 		}
-	} else if (rcu_lock) {
-		env->cur_state->active_rcu_lock = true;
-	} else if (rcu_unlock) {
-		verbose(env, "unmatched rcu read unlock (kernel function %s)\n", func_name);
-		return -EINVAL;
+	} else if (sleepable && env->cur_state->active_rcu_locks) {
+		verbose(env, "kernel func %s is sleepable within rcu_read_lock region\n", func_name);
+		return -EACCES;
+	}
+
+	if (in_rbtree_lock_required_cb(env) && (rcu_lock || rcu_unlock)) {
+		verbose(env, "Calling bpf_rcu_read_{lock,unlock} in unnecessary rbtree callback\n");
+		return -EACCES;
 	}
 
 	if (env->cur_state->active_preempt_locks) {
@@ -19387,7 +19384,7 @@ static bool refsafe(struct bpf_verifier_state *old, struct bpf_verifier_state *c
 	if (old->active_preempt_locks != cur->active_preempt_locks)
 		return false;
 
-	if (old->active_rcu_lock != cur->active_rcu_lock)
+	if (old->active_rcu_locks != cur->active_rcu_locks)
 		return false;
 
 	if (!check_ids(old->active_irq_id, cur->active_irq_id, idmap))

tools/testing/selftests/bpf/prog_tests/rcu_read_lock.c

@@ -28,6 +28,7 @@ static void test_success(void)
 	bpf_program__set_autoload(skel->progs.two_regions, true);
 	bpf_program__set_autoload(skel->progs.non_sleepable_1, true);
 	bpf_program__set_autoload(skel->progs.non_sleepable_2, true);
+	bpf_program__set_autoload(skel->progs.nested_rcu_region, true);
 	bpf_program__set_autoload(skel->progs.task_trusted_non_rcuptr, true);
 	bpf_program__set_autoload(skel->progs.rcu_read_lock_subprog, true);
 	bpf_program__set_autoload(skel->progs.rcu_read_lock_global_subprog, true);
@@ -78,7 +79,6 @@ static const char * const inproper_region_tests[] = {
 	"non_sleepable_rcu_mismatch",
 	"inproper_sleepable_helper",
 	"inproper_sleepable_kfunc",
-	"nested_rcu_region",
 	"rcu_read_lock_global_subprog_lock",
 	"rcu_read_lock_global_subprog_unlock",
 	"rcu_read_lock_sleepable_helper_global_subprog",