The may_access_direct_pkt_data() helper sets env->seen_direct_write as a
side effect, which creates awkward calling patterns:

- check_special_kfunc() has a comment warning readers about the side effect
- specialize_kfunc() must save and restore the flag around the call

Make the helper a pure function by moving the seen_direct_write flag
setting to call sites that need it.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
---
 kernel/bpf/verifier.c | 33 ++++++++++++---------------------
 1 file changed, 12 insertions(+), 21 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 9394b0de2ef0..52d76a848f65 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6151,13 +6151,9 @@ static bool may_access_direct_pkt_data(struct bpf_verifier_env *env,
 		if (meta)
 			return meta->pkt_access;
 
-		env->seen_direct_write = true;
 		return true;
 
 	case BPF_PROG_TYPE_CGROUP_SOCKOPT:
-		if (t == BPF_WRITE)
-			env->seen_direct_write = true;
-
 		return true;
 
 	default:
@@ -7708,15 +7704,17 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
 			err = check_stack_write(env, regno, off, size,
 						value_regno, insn_idx);
 	} else if (reg_is_pkt_pointer(reg)) {
-		if (t == BPF_WRITE && !may_access_direct_pkt_data(env, NULL, t)) {
-			verbose(env, "cannot write into packet\n");
-			return -EACCES;
-		}
-		if (t == BPF_WRITE && value_regno >= 0 &&
-		    is_pointer_value(env, value_regno)) {
-			verbose(env, "R%d leaks addr into packet\n",
-				value_regno);
-			return -EACCES;
+		if (t == BPF_WRITE) {
+			if (!may_access_direct_pkt_data(env, NULL, BPF_WRITE)) {
+				verbose(env, "cannot write into packet\n");
+				return -EACCES;
+			}
+			if (value_regno >= 0 && is_pointer_value(env, value_regno)) {
+				verbose(env, "R%d leaks addr into packet\n",
+					value_regno);
+				return -EACCES;
+			}
+			env->seen_direct_write = true;
 		}
 		err = check_packet_access(env, regno, off, size, false);
 		if (!err && t == BPF_READ && value_regno >= 0)
@@ -13883,11 +13881,11 @@ static int check_special_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_ca
 		if (meta->func_id == special_kfunc_list[KF_bpf_dynptr_slice]) {
 			regs[BPF_REG_0].type |= MEM_RDONLY;
 		} else {
-			/* this will set env->seen_direct_write to true */
 			if (!may_access_direct_pkt_data(env, NULL, BPF_WRITE)) {
 				verbose(env, "the prog does not allow writes to packet data\n");
 				return -EINVAL;
 			}
+			env->seen_direct_write = true;
 		}
 
 		if (!meta->initialized_dynptr.id) {
@@ -22388,7 +22386,6 @@ static int fixup_call_args(struct bpf_verifier_env *env)
 static int specialize_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_desc *desc, int insn_idx)
 {
 	struct bpf_prog *prog = env->prog;
-	bool seen_direct_write;
 	void *xdp_kfunc;
 	bool is_rdonly;
 	u32 func_id = desc->func_id;
@@ -22404,16 +22401,10 @@ static int specialize_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_desc
 			addr = (unsigned long)xdp_kfunc;
 		/* fallback to default kfunc when not supported by netdev */
 	} else if (func_id == special_kfunc_list[KF_bpf_dynptr_from_skb]) {
-		seen_direct_write = env->seen_direct_write;
 		is_rdonly = !may_access_direct_pkt_data(env, NULL, BPF_WRITE);
 
 		if (is_rdonly)
 			addr = (unsigned long)bpf_dynptr_from_skb_rdonly;
-
-		/* restore env->seen_direct_write to its original value, since
-		 * may_access_direct_pkt_data mutates it
-		 */
-		env->seen_direct_write = seen_direct_write;
 	} else if (func_id == special_kfunc_list[KF_bpf_set_dentry_xattr]) {
 		if (bpf_lsm_has_d_inode_locked(prog))
 			addr = (unsigned long)bpf_set_dentry_xattr_locked;

-- 
2.43.0