bpf: Support narrow loads from bpf_sock_addr.user_port

bpf_sock_addr.user_port supports only 4-byte load and it leads to ugly
code in BPF programs, like:

	volatile __u32 user_port = ctx->user_port;
	__u16 port = bpf_ntohs(user_port);

Since otherwise clang may optimize the load to be 2-byte and it's
rejected by verifier.

Add support for 1- and 2-byte loads same way as it's supported for other
fields in bpf_sock_addr like user_ip4, msg_src_ip4, etc.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/c1e983f4c17573032601d0b2b1f9d1274f24bc16.1589420814.git.rdna@fb.com

include/uapi/linux/bpf.h

@@ -3728,7 +3728,7 @@ struct bpf_sock_addr {
 	__u32 user_ip6[4];	/* Allows 1,2,4,8-byte read and 4,8-byte write.
 				 * Stored in network byte order.
 				 */
-	__u32 user_port;	/* Allows 4-byte read and write.
+	__u32 user_port;	/* Allows 1,2,4-byte read and 4-byte write.
 				 * Stored in network byte order
 				 */
 	__u32 family;		/* Allows 4-byte read, but no write */

net/core/filter.c

@@ -7029,6 +7029,7 @@ static bool sock_addr_is_valid_access(int off, int size,
 	case bpf_ctx_range(struct bpf_sock_addr, msg_src_ip4):
 	case bpf_ctx_range_till(struct bpf_sock_addr, msg_src_ip6[0],
 				msg_src_ip6[3]):
+	case bpf_ctx_range(struct bpf_sock_addr, user_port):
 		if (type == BPF_READ) {
 			bpf_ctx_record_field_size(info, size_default);
 
@@ -7059,10 +7060,6 @@ static bool sock_addr_is_valid_access(int off, int size,
 				return false;
 		}
 		break;
-	case bpf_ctx_range(struct bpf_sock_addr, user_port):
-		if (size != size_default)
-			return false;
-		break;
 	case offsetof(struct bpf_sock_addr, sk):
 		if (type != BPF_READ)
 			return false;
@@ -7958,8 +7955,8 @@ static u32 sock_addr_convert_ctx_access(enum bpf_access_type type,
 					struct bpf_insn *insn_buf,
 					struct bpf_prog *prog, u32 *target_size)
 {
+	int off, port_size = sizeof_field(struct sockaddr_in6, sin6_port);
 	struct bpf_insn *insn = insn_buf;
-	int off;
 
 	switch (si->off) {
 	case offsetof(struct bpf_sock_addr, user_family):
@@ -7994,9 +7991,11 @@ static u32 sock_addr_convert_ctx_access(enum bpf_access_type type,
 			     offsetof(struct sockaddr_in6, sin6_port));
 		BUILD_BUG_ON(sizeof_field(struct sockaddr_in, sin_port) !=
 			     sizeof_field(struct sockaddr_in6, sin6_port));
-		SOCK_ADDR_LOAD_OR_STORE_NESTED_FIELD(struct bpf_sock_addr_kern,
-						     struct sockaddr_in6, uaddr,
-						     sin6_port, tmp_reg);
+		/* Account for sin6_port being smaller than user_port. */
+		port_size = min(port_size, BPF_LDST_BYTES(si));
+		SOCK_ADDR_LOAD_OR_STORE_NESTED_FIELD_SIZE_OFF(
+			struct bpf_sock_addr_kern, struct sockaddr_in6, uaddr,
+			sin6_port, bytes_to_bpf_size(port_size), 0, tmp_reg);
 		break;
 
 	case offsetof(struct bpf_sock_addr, family):

tools/include/uapi/linux/bpf.h

@@ -3728,7 +3728,7 @@ struct bpf_sock_addr {
 	__u32 user_ip6[4];	/* Allows 1,2,4,8-byte read and 4,8-byte write.
 				 * Stored in network byte order.
 				 */
-	__u32 user_port;	/* Allows 4-byte read and write.
+	__u32 user_port;	/* Allows 1,2,4-byte read and 4-byte write.
 				 * Stored in network byte order
 				 */
 	__u32 family;		/* Allows 4-byte read, but no write */