bpf: Allow calling static subprogs while holding a bpf_spin_lock

Currently, calling any helpers, kfuncs, or subprogs except the graph
data structure (lists, rbtrees) API kfuncs while holding a bpf_spin_lock
is not allowed. One of the original motivations of this decision was to
force the BPF programmer's hand into keeping the bpf_spin_lock critical
section small, and to ensure the execution time of the program does not
increase due to lock waiting times. In addition to this, some of the
helpers and kfuncs may be unsafe to call while holding a bpf_spin_lock.

However, when it comes to subprog calls, atleast for static subprogs,
the verifier is able to explore their instructions during verification.
Therefore, it is similar in effect to having the same code inlined into
the critical section. Hence, not allowing static subprog calls in the
bpf_spin_lock critical section is mostly an annoyance that needs to be
worked around, without providing any tangible benefit.

Unlike static subprog calls, global subprog calls are not safe to permit
within the critical section, as the verifier does not explore them
during verification, therefore whether the same lock will be taken
again, or unlocked, cannot be ascertained.

Therefore, allow calling static subprogs within a bpf_spin_lock critical
section, and only reject it in case the subprog linkage is global.
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: David Vernet <void@manifault.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20240204222349.938118-2-memxor@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -9493,6 +9493,13 @@ static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 	if (subprog_is_global(env, subprog)) {
 		const char *sub_name = subprog_name(env, subprog);
 
+		/* Only global subprogs cannot be called with a lock held. */
+		if (env->cur_state->active_lock.ptr) {
+			verbose(env, "global function calls are not allowed while holding a lock,\n"
+				     "use static function instead\n");
+			return -EINVAL;
+		}
+
 		if (err) {
 			verbose(env, "Caller passes invalid args into func#%d ('%s')\n",
 				subprog, sub_name);
@@ -17644,7 +17651,6 @@ static int do_check(struct bpf_verifier_env *env)
 
 				if (env->cur_state->active_lock.ptr) {
 					if ((insn->src_reg == BPF_REG_0 && insn->imm != BPF_FUNC_spin_unlock) ||
-					    (insn->src_reg == BPF_PSEUDO_CALL) ||
 					    (insn->src_reg == BPF_PSEUDO_KFUNC_CALL &&
 					     (insn->off != 0 || !is_bpf_graph_api_kfunc(insn->imm)))) {
 						verbose(env, "function calls are not allowed while holding a lock\n");
@@ -17692,8 +17698,7 @@ static int do_check(struct bpf_verifier_env *env)
 					return -EINVAL;
 				}
 process_bpf_exit_full:
-				if (env->cur_state->active_lock.ptr &&
-				    !in_rbtree_lock_required_cb(env)) {
+				if (env->cur_state->active_lock.ptr && !env->cur_state->curframe) {
 					verbose(env, "bpf_spin_unlock is missing\n");
 					return -EINVAL;
 				}

tools/testing/selftests/bpf/progs/verifier_spin_lock.c

@@ -330,7 +330,7 @@ l1_%=:	r7 = r0;					\
 
 SEC("cgroup/skb")
 __description("spin_lock: test10 lock in subprog without unlock")
-__failure __msg("unlock is missing")
+__success
 __failure_unpriv __msg_unpriv("")
 __naked void lock_in_subprog_without_unlock(void)
 {