bpf: Derive global subprog stack liveness from BTF signatures

Stack liveness currently analyzes global callees in the context of their
callers. The resulting read/write marks describe only the current body,
but freplace can replace it after the caller has been verified.

A no-op callee taking a stack pointer can be replaced with one writing
through that pointer, leaving the call's may_write marks incomplete.
SCEV could then preserve a stack counter recurrence across a call that
resets it, invalidating a loop termination proof.

Instead, derive global call effects from BTF argument types instead,
same is it done on the main verification pass.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261009-scev-minimal-rebase-v3-10-bcba9bc53ac9@gmail.com

include/linux/bpf_verifier.h

@@ -1744,6 +1744,9 @@ bpf_helper_stack_access_bytes(struct bpf_verifier_env *env,
 struct arg_access_info
 bpf_kfunc_stack_access_bytes(struct bpf_verifier_env *env,
 			     struct bpf_insn *insn, int arg, int insn_idx);
+struct arg_access_info
+bpf_global_subprog_stack_access_bytes(struct bpf_verifier_env *env,
+				      struct bpf_insn *insn, int arg, int insn_idx);
 int bpf_compute_subprog_arg_access(struct bpf_verifier_env *env);
 
 int bpf_stack_liveness_init(struct bpf_verifier_env *env);

kernel/bpf/liveness.c

@@ -1715,6 +1715,8 @@ static int record_arg_access(struct bpf_verifier_env *env,
 		info = bpf_helper_stack_access_bytes(env, insn, arg_idx, insn_idx);
 	} else if (bpf_pseudo_kfunc_call(insn)) {
 		info = bpf_kfunc_stack_access_bytes(env, insn, arg_idx, insn_idx);
+	} else if (bpf_pseudo_call(insn)) {
+		info = bpf_global_subprog_stack_access_bytes(env, insn, arg_idx, insn_idx);
 	} else {
 		for (int f = 0; f <= depth; f++) {
 			err = mark_stack_read_all(env, instance, f, insn_idx);
@@ -1736,7 +1738,7 @@ static int record_arg_access(struct bpf_verifier_env *env,
 	return err;
 }
 
-/* Record stack access for a given 'at' state of helper/kfunc 'insn' */
+/* Record stack access for a given 'at' state of helper/kfunc/global subprog 'insn' */
 static int record_call_access(struct bpf_verifier_env *env,
 			      struct func_instance *instance,
 			      struct arg_track *at,
@@ -1744,12 +1746,19 @@ static int record_call_access(struct bpf_verifier_env *env,
 {
 	struct bpf_insn *insn = &env->prog->insnsi[insn_idx];
 	struct bpf_call_summary cs;
-	int r, err, arg_slot_cnt = 5;
+	int r, err, callee, arg_slot_cnt = 5;
 
-	if (bpf_pseudo_call(insn))
-		return 0;
-
-	if (bpf_is_callx(insn))
+	if (bpf_pseudo_call(insn)) {
+		/*
+		 * analyze_subprog() handles static callees. For global callees,
+		 * derive effects from argument types so may_write marks remain
+		 * conservative under freplace.
+		 */
+		callee = bpf_find_subprog(env, insn_idx + insn->imm + 1);
+		if (callee < 0 || !bpf_subprog_is_global(env, callee))
+			return 0;
+		arg_slot_cnt = MAX_BPF_FUNC_ARGS;
+	} else if (bpf_is_callx(insn))
 		/*
 		 * The callee is not known statically. Assume that all arg
 		 * slots are passed and let record_arg_access() conservatively
@@ -2282,7 +2291,12 @@ static int analyze_subprog(struct bpf_verifier_env *env,
 		if (bpf_pseudo_call(insn)) {
 			target = idx + insn->imm + 1;
 			callee = bpf_find_subprog(env, target);
-			if (callee < 0)
+			/*
+			 * Global subprograms can be freplaced, so conservatively derive
+			 * their stack effects from their BTF signatures instead of
+			 * analyzing them in the context of their callers.
+			 */
+			if (callee < 0 || bpf_subprog_is_global(env, callee))
 				continue;
 
 			/* Build entry args: R1-R5 and stack args from at_in at call site */

kernel/bpf/verifier.c

@@ -14571,6 +14571,72 @@ bpf_kfunc_stack_access_bytes(struct bpf_verifier_env *env, struct bpf_insn *insn
 	return info;
 }
 
+/*
+ * Called before dead code elimination, some subprograms may be genuinely unreachable.
+ * Suppress diagnostics so callers can use a conservative result on error.
+ * The main pass would report errors for reachable subprograms.
+ */
+static bool try_btf_prepare_func_args(struct bpf_verifier_env *env, int subprog)
+{
+	struct bpf_prog *prog = env->prog;
+	u32 log_level = env->log.level;
+	int err;
+
+	if (!prog->aux->func_info || !prog->aux->func_info[subprog].type_id)
+		return false;
+
+	env->log.level = 0;
+	err = btf_prepare_func_args(env, subprog);
+	env->log.level = log_level;
+	return !err;
+}
+
+/*
+ * Derive stack effects for a global subprog's argument slot @arg (0-based)
+ * from its declared type, matching the main pass's argument checks.
+ */
+struct arg_access_info
+bpf_global_subprog_stack_access_bytes(struct bpf_verifier_env *env, struct bpf_insn *insn,
+				      int arg, int insn_idx)
+{
+	struct arg_access_info info = {
+		.size = U32_MAX,
+		.may_read = true,
+		.may_write = true,
+	};
+	struct bpf_subprog_info *sub;
+	int callee;
+
+	callee = bpf_find_subprog(env, insn_idx + insn->imm + 1);
+	if (callee < 0)
+		return info;
+	sub = subprog_info(env, callee);
+	if (!try_btf_prepare_func_args(env, callee))
+		return info;
+
+	switch (base_type(sub->args[arg].arg_type)) {
+	case ARG_UNUSED:	/* Not an argument slot. */
+	case ARG_ANYTHING:	/* A scalar, the callee can't dereference it. */
+		return (struct arg_access_info) {};
+	case ARG_PTR_TO_BTF_ID:
+		/*
+		 * Trusted arguments cannot be stack pointers.
+		 * Untrusted arguments use ARG_IGNORE:
+		 * probe reads need no caller stack state, writes are prohibited.
+		 */
+		return (struct arg_access_info) {};
+	case ARG_PTR_TO_MEM:
+		/* mem_size is zero for untrusted pointers of unknown size. */
+		if (sub->args[arg].mem_size) {
+			info.size = sub->args[arg].mem_size;
+			info.must_write = true;
+		}
+		return info;
+	default:
+		return info;
+	}
+}
+
 /* check special kfuncs and return:
  *  1  - not fall-through to 'else' branch, continue verification
  *  0  - fall-through to 'else' branch