Re: [PATCH v2 net-next] net: filter: export pkt_type_offset() helper

[Hannes Frederic Sowa <hannes@stressinduktion.org>]

On Do, 2014-09-04 at 07:11 -0700, Eric Dumazet wrote:
> On Thu, 2014-09-04 at 15:40 +0200, Hannes Frederic Sowa wrote:
> 
> > The type does not matter at all. Actually I wanted to use an empty
> > struct but was afraid it might not work on older compilers and didn't
> > want to check that with each version.
> 
> 
> It matters. Really.
> 
> $ cat try.c
> #include <stdio.h>
> 
> struct S_int {
> 	char a;
> 	int offset[0];
> 	char b:1;
> };
> 
> struct S_char {
> 	char a;
> 	char offset[0];
> 	char b:1;
> };
> 
> int main()
> {
> 	printf("sizeof(S_int) %zu\n", sizeof(struct S_int));
> 	printf("sizeof(S_char) %zu\n", sizeof(struct S_char));
> 	return 0;
> }
> $ gcc -o try try.c && ./try
> sizeof(S_int) 8
> sizeof(S_char) 2

Sure, compiler must make sure to correctly align structure for all
members (so it must round up). But this is not an issue with struct
sk_buff. I agree that switching to __u8 for offset marker seems to
reduce confusion.

Statically compiling skb_pkt_type_mask as I proposed above didn't work
out.

New patch, what do you think?

diff --git a/arch/mips/net/bpf_jit.c b/arch/mips/net/bpf_jit.c
index 05a5661..0fd3f95 100644
--- a/arch/mips/net/bpf_jit.c
+++ b/arch/mips/net/bpf_jit.c
@@ -765,27 +765,6 @@ static u64 jit_get_skb_w(struct sk_buff *skb, unsigned offset)
 	return (u64)err << 32 | ntohl(ret);
 }
 
-#ifdef __BIG_ENDIAN_BITFIELD
-#define PKT_TYPE_MAX	(7 << 5)
-#else
-#define PKT_TYPE_MAX	7
-#endif
-static int pkt_type_offset(void)
-{
-	struct sk_buff skb_probe = {
-		.pkt_type = ~0,
-	};
-	u8 *ct = (u8 *)&skb_probe;
-	unsigned int off;
-
-	for (off = 0; off < sizeof(struct sk_buff); off++) {
-		if (ct[off] == PKT_TYPE_MAX)
-			return off;
-	}
-	pr_err_once("Please fix pkt_type_offset(), as pkt_type couldn't be found\n");
-	return -1;
-}
-
 static int build_body(struct jit_ctx *ctx)
 {
 	void *load_func[] = {jit_get_skb_b, jit_get_skb_h, jit_get_skb_w};
@@ -1332,11 +1311,7 @@ jmp_cmp:
 		case BPF_ANC | SKF_AD_PKTTYPE:
 			ctx->flags |= SEEN_SKB;
 
-			off = pkt_type_offset();
-
-			if (off < 0)
-				return -1;
-			emit_load_byte(r_tmp, r_skb, off, ctx);
+			emit_load_byte(r_tmp, r_skb, skb_pkt_type_offset(), ctx);
 			/* Keep only the last 3 bits */
 			emit_andi(r_A, r_tmp, PKT_TYPE_MAX, ctx);
 #ifdef __BIG_ENDIAN_BITFIELD
diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index 61e45b7..8039bb8 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -223,37 +223,6 @@ static void bpf_jit_epilogue(struct bpf_jit *jit)
 	EMIT2(0x07fe);
 }
 
-/* Helper to find the offset of pkt_type in sk_buff
- * Make sure its still a 3bit field starting at the MSBs within a byte.
- */
-#define PKT_TYPE_MAX 0xe0
-static int pkt_type_offset;
-
-static int __init bpf_pkt_type_offset_init(void)
-{
-	struct sk_buff skb_probe = {
-		.pkt_type = ~0,
-	};
-	char *ct = (char *)&skb_probe;
-	int off;
-
-	pkt_type_offset = -1;
-	for (off = 0; off < sizeof(struct sk_buff); off++) {
-		if (!ct[off])
-			continue;
-		if (ct[off] == PKT_TYPE_MAX)
-			pkt_type_offset = off;
-		else {
-			/* Found non matching bit pattern, fix needed. */
-			WARN_ON_ONCE(1);
-			pkt_type_offset = -1;
-			return -1;
-		}
-	}
-	return 0;
-}
-device_initcall(bpf_pkt_type_offset_init);
-
 /*
  * make sure we dont leak kernel information to user
  */
@@ -753,12 +722,10 @@ call_fn:	/* lg %r1,<d(function)>(%r13) */
 		}
 		break;
 	case BPF_ANC | SKF_AD_PKTTYPE:
-		if (pkt_type_offset < 0)
-			goto out;
 		/* lhi %r5,0 */
 		EMIT4(0xa7580000);
 		/* ic %r5,<d(pkt_type_offset)>(%r2) */
-		EMIT4_DISP(0x43502000, pkt_type_offset);
+		EMIT4_DISP(0x43502000, skb_pkt_type_offset());
 		/* srl %r5,5 */
 		EMIT4_DISP(0x88500000, 5);
 		break;
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 02529fc..944e931 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -548,11 +548,25 @@ struct sk_buff {
 				ip_summed:2,
 				nohdr:1,
 				nfctinfo:3;
+
+	/* pkt_type bitfield is relatively addressed from bpf engines:
+	 * if you reorder skb contents, ensure to update PKT_TYPE_MAX
+	 * and keep __pkt_type_offset marker right in front of bitfield
+	 * which does contain pkt_type member.
+	 */
+
+#ifdef __BIG_ENDIAN_BITFIELD
+#define PKT_TYPE_MAX   (7 << 5)
+#else
+#define PKT_TYPE_MAX   7
+#endif
+	__u8			__pkt_type_offset[0];
 	__u8			pkt_type:3,
 				fclone:2,
 				ipvs_property:1,
 				peeked:1,
 				nf_trace:1;
+
 	kmemcheck_bitfield_end(flags1);
 	__be16			protocol;
 
@@ -647,6 +661,11 @@ struct sk_buff {
 #define SKB_ALLOC_FCLONE	0x01
 #define SKB_ALLOC_RX		0x02
 
+static inline size_t skb_pkt_type_offset(void)
+{
+	return offsetof(struct sk_buff, __pkt_type_offset);
+}
+
 /* Returns true if the skb was allocated from PFMEMALLOC reserves */
 static inline bool skb_pfmemalloc(const struct sk_buff *skb)
 {
diff --git a/net/core/filter.c b/net/core/filter.c
index d814b8a..ca64d7a 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -87,30 +87,6 @@ int sk_filter(struct sock *sk, struct sk_buff *skb)
 }
 EXPORT_SYMBOL(sk_filter);
 
-/* Helper to find the offset of pkt_type in sk_buff structure. We want
- * to make sure its still a 3bit field starting at a byte boundary;
- * taken from arch/x86/net/bpf_jit_comp.c.
- */
-#ifdef __BIG_ENDIAN_BITFIELD
-#define PKT_TYPE_MAX	(7 << 5)
-#else
-#define PKT_TYPE_MAX	7
-#endif
-static unsigned int pkt_type_offset(void)
-{
-	struct sk_buff skb_probe = { .pkt_type = ~0, };
-	u8 *ct = (u8 *) &skb_probe;
-	unsigned int off;
-
-	for (off = 0; off < sizeof(struct sk_buff); off++) {
-		if (ct[off] == PKT_TYPE_MAX)
-			return off;
-	}
-
-	pr_err_once("Please fix %s, as pkt_type couldn't be found!\n", __func__);
-	return -1;
-}
-
 static u64 __skb_get_pay_offset(u64 ctx, u64 a, u64 x, u64 r4, u64 r5)
 {
 	return __skb_get_poff((struct sk_buff *)(unsigned long) ctx);
@@ -191,7 +167,7 @@ static bool convert_bpf_extensions(struct sock_filter *fp,
 
 	case SKF_AD_OFF + SKF_AD_PKTTYPE:
 		*insn = BPF_LDX_MEM(BPF_B, BPF_REG_A, BPF_REG_CTX,
-				    pkt_type_offset());
+				    skb_pkt_type_offset());
 		if (insn->off < 0)
 			return false;
 		insn++;