net: filter: constify detection of pkt_type_offset

Currently we have 2 pkt_type_offset functions doing the same thing and
spread across the architecture files. Remove those and replace them
with a PKT_TYPE_OFFSET macro helper which gets the constant value from a
zero sized sk_buff member right in front of the bitfield with offsetof.
This new offset marker does not change size of struct sk_buff.

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Markos Chandras <markos.chandras@imgtec.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Daniel Borkmann <dborkman@redhat.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Denis Kirjanov <kda@linux-powerpc.org>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

4 files changed, 14 insertions, 89 deletions

diff --git a/arch/mips/net/bpf_jit.c b/arch/mips/net/bpf_jit.c
index 0e97ccd29fe3..7edc08398c4a 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, 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 555f5c7e83ab..c52ac77408ca 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -227,37 +227,6 @@ 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
  */
@@ -757,12 +726,10 @@ 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, PKT_TYPE_OFFSET());
                 /* srl %r5,5 */
                 EMIT4_DISP(0x88500000, 5);
                 break;
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 07c9fdd0c126..756e3d057e84 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -548,6 +548,16 @@ struct sk_buff {
                                 ip_summed:2,
                                 nohdr:1,
                                 nfctinfo:3;
+
+/* 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)
+
+        __u8                    __pkt_type_offset[0];
         __u8                    pkt_type:3,
                                 fclone:2,
                                 ipvs_property:1,
diff --git a/net/core/filter.c b/net/core/filter.c
index dfc716ffa44b..601f28de7311 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);
@@ -190,11 +166,8 @@ static bool convert_bpf_extensions(struct sock_filter *fp,
                 break;
 
         case SKF_AD_OFF + SKF_AD_PKTTYPE:
-                *insn = BPF_LDX_MEM(BPF_B, BPF_REG_A, BPF_REG_CTX,
-                                    pkt_type_offset());
-                if (insn->off < 0)
-                        return false;
-                insn++;
+                *insn++ = BPF_LDX_MEM(BPF_B, BPF_REG_A, BPF_REG_CTX,
+                                      PKT_TYPE_OFFSET());
                 *insn = BPF_ALU32_IMM(BPF_AND, BPF_REG_A, PKT_TYPE_MAX);
 #ifdef __BIG_ENDIAN_BITFIELD
                 insn++;