skbuff: Move conditional preprocessor directives out of struct sk_buff

In preparation for using the struct_group() macro in struct sk_buff,
move the conditional preprocessor directives out of the region of struct
sk_buff that will be enclosed by struct_group(). While GCC and Clang are
happy with conditional preprocessor directives here, sparse is not, even
under -Wno-directive-within-macro[1], as would be seen under a C=1 build:

net/core/filter.c: note: in included file (through include/linux/netlink.h, include/linux/sock_diag.h):
./include/linux/skbuff.h:820:1: warning: directive in macro's argument list
./include/linux/skbuff.h:822:1: warning: directive in macro's argument list
./include/linux/skbuff.h:846:1: warning: directive in macro's argument list
./include/linux/skbuff.h:848:1: warning: directive in macro's argument list

Additionally remove empty macro argument definitions and usage.

"objdump -d" shows no object code differences.

[1] https://www.spinics.net/lists/linux-sparse/msg10857.htmlSigned-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -795,7 +795,7 @@ struct sk_buff {
 #else
 #define CLONED_MASK	1
 #endif
-#define CLONED_OFFSET()		offsetof(struct sk_buff, __cloned_offset)
+#define CLONED_OFFSET		offsetof(struct sk_buff, __cloned_offset)
 
 	/* private: */
 	__u8			__cloned_offset[0];
@@ -818,18 +818,10 @@ struct sk_buff {
 	__u32			headers_start[0];
 	/* public: */
 
-/* if you move pkt_type around you also must adapt those constants */
-#ifdef __BIG_ENDIAN_BITFIELD
-#define PKT_TYPE_MAX	(7 << 5)
-#else
-#define PKT_TYPE_MAX	7
-#endif
-#define PKT_TYPE_OFFSET()	offsetof(struct sk_buff, __pkt_type_offset)
-
 	/* private: */
 	__u8			__pkt_type_offset[0];
 	/* public: */
-	__u8			pkt_type:3;
+	__u8			pkt_type:3; /* see PKT_TYPE_MAX */
 	__u8			ignore_df:1;
 	__u8			nf_trace:1;
 	__u8			ip_summed:2;
@@ -845,16 +837,10 @@ struct sk_buff {
 	__u8			encap_hdr_csum:1;
 	__u8			csum_valid:1;
 
-#ifdef __BIG_ENDIAN_BITFIELD
-#define PKT_VLAN_PRESENT_BIT	7
-#else
-#define PKT_VLAN_PRESENT_BIT	0
-#endif
-#define PKT_VLAN_PRESENT_OFFSET()	offsetof(struct sk_buff, __pkt_vlan_present_offset)
 	/* private: */
 	__u8			__pkt_vlan_present_offset[0];
 	/* public: */
-	__u8			vlan_present:1;
+	__u8			vlan_present:1;	/* See PKT_VLAN_PRESENT_BIT */
 	__u8			csum_complete_sw:1;
 	__u8			csum_level:2;
 	__u8			csum_not_inet:1;
@@ -953,6 +939,22 @@ struct sk_buff {
 #endif
 };
 
+/* if you move pkt_type around you also must adapt those constants */
+#ifdef __BIG_ENDIAN_BITFIELD
+#define PKT_TYPE_MAX	(7 << 5)
+#else
+#define PKT_TYPE_MAX	7
+#endif
+#define PKT_TYPE_OFFSET		offsetof(struct sk_buff, __pkt_type_offset)
+
+/* if you move pkt_vlan_present around you also must adapt these constants */
+#ifdef __BIG_ENDIAN_BITFIELD
+#define PKT_VLAN_PRESENT_BIT	7
+#else
+#define PKT_VLAN_PRESENT_BIT	0
+#endif
+#define PKT_VLAN_PRESENT_OFFSET	offsetof(struct sk_buff, __pkt_vlan_present_offset)
+
 #ifdef __KERNEL__
 /*
  *	Handling routines are only of interest to the kernel

net/core/filter.c

@@ -301,7 +301,7 @@ static u32 convert_skb_access(int skb_field, int dst_reg, int src_reg,
 		break;
 
 	case SKF_AD_PKTTYPE:
-		*insn++ = BPF_LDX_MEM(BPF_B, dst_reg, src_reg, PKT_TYPE_OFFSET());
+		*insn++ = BPF_LDX_MEM(BPF_B, dst_reg, src_reg, PKT_TYPE_OFFSET);
 		*insn++ = BPF_ALU32_IMM(BPF_AND, dst_reg, PKT_TYPE_MAX);
 #ifdef __BIG_ENDIAN_BITFIELD
 		*insn++ = BPF_ALU32_IMM(BPF_RSH, dst_reg, 5);
@@ -323,7 +323,7 @@ static u32 convert_skb_access(int skb_field, int dst_reg, int src_reg,
 				      offsetof(struct sk_buff, vlan_tci));
 		break;
 	case SKF_AD_VLAN_TAG_PRESENT:
-		*insn++ = BPF_LDX_MEM(BPF_B, dst_reg, src_reg, PKT_VLAN_PRESENT_OFFSET());
+		*insn++ = BPF_LDX_MEM(BPF_B, dst_reg, src_reg, PKT_VLAN_PRESENT_OFFSET);
 		if (PKT_VLAN_PRESENT_BIT)
 			*insn++ = BPF_ALU32_IMM(BPF_RSH, dst_reg, PKT_VLAN_PRESENT_BIT);
 		if (PKT_VLAN_PRESENT_BIT < 7)
@@ -8029,7 +8029,7 @@ static int bpf_unclone_prologue(struct bpf_insn *insn_buf, bool direct_write,
 	 * (Fast-path, otherwise approximation that we might be
 	 *  a clone, do the rest in helper.)
 	 */
-	*insn++ = BPF_LDX_MEM(BPF_B, BPF_REG_6, BPF_REG_1, CLONED_OFFSET());
+	*insn++ = BPF_LDX_MEM(BPF_B, BPF_REG_6, BPF_REG_1, CLONED_OFFSET);
 	*insn++ = BPF_ALU32_IMM(BPF_AND, BPF_REG_6, CLONED_MASK);
 	*insn++ = BPF_JMP_IMM(BPF_JEQ, BPF_REG_6, 0, 7);
 
@@ -8617,7 +8617,7 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type type,
 	case offsetof(struct __sk_buff, pkt_type):
 		*target_size = 1;
 		*insn++ = BPF_LDX_MEM(BPF_B, si->dst_reg, si->src_reg,
-				      PKT_TYPE_OFFSET());
+				      PKT_TYPE_OFFSET);
 		*insn++ = BPF_ALU32_IMM(BPF_AND, si->dst_reg, PKT_TYPE_MAX);
 #ifdef __BIG_ENDIAN_BITFIELD
 		*insn++ = BPF_ALU32_IMM(BPF_RSH, si->dst_reg, 5);
@@ -8642,7 +8642,7 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type type,
 	case offsetof(struct __sk_buff, vlan_present):
 		*target_size = 1;
 		*insn++ = BPF_LDX_MEM(BPF_B, si->dst_reg, si->src_reg,
-				      PKT_VLAN_PRESENT_OFFSET());
+				      PKT_VLAN_PRESENT_OFFSET);
 		if (PKT_VLAN_PRESENT_BIT)
 			*insn++ = BPF_ALU32_IMM(BPF_RSH, si->dst_reg, PKT_VLAN_PRESENT_BIT);
 		if (PKT_VLAN_PRESENT_BIT < 7)