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Date: Thu, 04 Sep 2014 16:22:53 +0200
From: Hannes Frederic Sowa <hannes@...essinduktion.org>
To: Eric Dumazet <eric.dumazet@...il.com>
Cc: Denis Kirjanov <kirjanov@...il.com>,
Alexei Starovoitov <alexei.starovoitov@...il.com>,
Daniel Borkmann <dborkman@...hat.com>,
Eric Dumazet <edumazet@...gle.com>,
Denis Kirjanov <kda@...ux-powerpc.org>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
Markos Chandras <markos.chandras@...tec.com>,
Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH v2 net-next] net: filter: export pkt_type_offset() helper
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++;
--
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