Commit b546b575 authored by Eduard Zingerman's avatar Eduard Zingerman Committed by Alexei Starovoitov
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selftests/bpf: update tcp_custom_syncookie to use scalar packet offset 



This commit updates tcp_custom_syncookie.c:tcp_parse_option() to use
explicit packet offset (ctx->off) for packet access instead of ever
moving pointer (ctx->ptr), this reduces verification complexity:
- the tcp_parse_option() is passed as a callback to bpf_loop();
- suppose a checkpoint is created each time at function entry;
- the ctx->ptr is tracked by verifier as PTR_TO_PACKET;
- the ctx->ptr is incremented in tcp_parse_option(),
  thus umax_value field tracked for it is incremented as well;
- on each next iteration of tcp_parse_option()
  checkpoint from a previous iteration can't be reused
  for state pruning, because PTR_TO_PACKET registers are
  considered equivalent only if old->umax_value >= cur->umax_value;
- on the other hand, the ctx->off is a SCALAR,
  subject to widen_imprecise_scalars();
- it's exact bounds are eventually forgotten and it is tracked as
  unknown scalar at entry to tcp_parse_option();
- hence checkpoints created at the start of the function eventually
  converge.

The change is similar to one applied in [0] to xdp_synproxy_kern.c.

Comparing before and after with veristat yields following results:

File                             Insns (A)  Insns (B)  Insns      (DIFF)
-------------------------------  ---------  ---------  -----------------
test_tcp_custom_syncookie.bpf.o     466657      12423  -454234 (-97.34%)

[0] commit 977bc146 ("selftests/bpf: track tcp payload offset as scalar in xdp_synproxy")

Acked-by: default avatarYonghong Song <yonghong.song@linux.dev>
Signed-off-by: default avatarEduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240222150300.14909-2-eddyz87@gmail.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent a3c70a3c

tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c

+53 −30
Original line number Diff line number Diff line
@@ -10,6 +10,8 @@ 
#include "test_siphash.h"

#include "test_tcp_custom_syncookie.h"



#define MAX_PACKET_OFF 0xffff



/* Hash is calculated for each client and split into ISN and TS.

 *

 *       MSB                                   LSB
@@ -52,16 +54,15 @@ static siphash_key_t test_key_siphash = { 


struct tcp_syncookie {

	struct __sk_buff *skb;

	void *data;

	void *data_end;

	struct ethhdr *eth;

	struct iphdr *ipv4;

	struct ipv6hdr *ipv6;

	struct tcphdr *tcp;

	union {

		char *ptr;

	__be32 *ptr32;

	};

	struct bpf_tcp_req_attrs attrs;

	u32 off;

	u32 cookie;

	u64 first;

};
@@ -70,6 +71,7 @@ bool handled_syn, handled_ack; 


static int tcp_load_headers(struct tcp_syncookie *ctx)

{

	ctx->data = (void *)(long)ctx->skb->data;

	ctx->data_end = (void *)(long)ctx->skb->data_end;

	ctx->eth = (struct ethhdr *)(long)ctx->skb->data;
@@ -134,6 +136,7 @@ static int tcp_reload_headers(struct tcp_syncookie *ctx) 
	if (bpf_skb_change_tail(ctx->skb, data_len + 60 - ctx->tcp->doff * 4, 0))

		goto err;



	ctx->data = (void *)(long)ctx->skb->data;

	ctx->data_end = (void *)(long)ctx->skb->data_end;

	ctx->eth = (struct ethhdr *)(long)ctx->skb->data;

	if (ctx->ipv4) {
@@ -195,47 +198,68 @@ static int tcp_validate_header(struct tcp_syncookie *ctx) 
	return -1;

}



static int tcp_parse_option(__u32 index, struct tcp_syncookie *ctx)

static __always_inline void *next(struct tcp_syncookie *ctx, __u32 sz)

{

	char opcode, opsize;

	__u64 off = ctx->off;

	__u8 *data;



	if (ctx->ptr + 1 > ctx->data_end)

		goto stop;

	/* Verifier forbids access to packet when offset exceeds MAX_PACKET_OFF */

	if (off > MAX_PACKET_OFF - sz)

		return NULL;



	data = ctx->data + off;

	barrier_var(data);

	if (data + sz >= ctx->data_end)

		return NULL;



	opcode = *ctx->ptr++;

	ctx->off += sz;

	return data;

}



	if (opcode == TCPOPT_EOL)

static int tcp_parse_option(__u32 index, struct tcp_syncookie *ctx)

{

	__u8 *opcode, *opsize, *wscale;

	__u32 *tsval, *tsecr;

	__u16 *mss;

	__u32 off;



	off = ctx->off;

	opcode = next(ctx, 1);

	if (!opcode)

		goto stop;



	if (opcode == TCPOPT_NOP)

	if (*opcode == TCPOPT_EOL)

		goto stop;



	if (*opcode == TCPOPT_NOP)

		goto next;



	if (ctx->ptr + 1 > ctx->data_end)

	opsize = next(ctx, 1);

	if (!opsize)

		goto stop;



	opsize = *ctx->ptr++;



	if (opsize < 2)

	if (*opsize < 2)

		goto stop;



	switch (opcode) {

	switch (*opcode) {

	case TCPOPT_MSS:

		if (opsize == TCPOLEN_MSS && ctx->tcp->syn &&

		    ctx->ptr + (TCPOLEN_MSS - 2) < ctx->data_end)

			ctx->attrs.mss = get_unaligned_be16(ctx->ptr);

		mss = next(ctx, 2);

		if (*opsize == TCPOLEN_MSS && ctx->tcp->syn && mss)

			ctx->attrs.mss = get_unaligned_be16(mss);

		break;

	case TCPOPT_WINDOW:

		if (opsize == TCPOLEN_WINDOW && ctx->tcp->syn &&

		    ctx->ptr + (TCPOLEN_WINDOW - 2) < ctx->data_end) {

		wscale = next(ctx, 1);

		if (*opsize == TCPOLEN_WINDOW && ctx->tcp->syn && wscale) {

			ctx->attrs.wscale_ok = 1;

			ctx->attrs.snd_wscale = *ctx->ptr;

			ctx->attrs.snd_wscale = *wscale;

		}

		break;

	case TCPOPT_TIMESTAMP:

		if (opsize == TCPOLEN_TIMESTAMP &&

		    ctx->ptr + (TCPOLEN_TIMESTAMP - 2) < ctx->data_end) {

			ctx->attrs.rcv_tsval = get_unaligned_be32(ctx->ptr);

			ctx->attrs.rcv_tsecr = get_unaligned_be32(ctx->ptr + 4);

		tsval = next(ctx, 4);

		tsecr = next(ctx, 4);

		if (*opsize == TCPOLEN_TIMESTAMP && tsval && tsecr) {

			ctx->attrs.rcv_tsval = get_unaligned_be32(tsval);

			ctx->attrs.rcv_tsecr = get_unaligned_be32(tsecr);



			if (ctx->tcp->syn && ctx->attrs.rcv_tsecr)

				ctx->attrs.tstamp_ok = 0;
@@ -244,13 +268,12 @@ static int tcp_parse_option(__u32 index, struct tcp_syncookie *ctx) 
		}

		break;

	case TCPOPT_SACK_PERM:

		if (opsize == TCPOLEN_SACK_PERM && ctx->tcp->syn &&

		    ctx->ptr + (TCPOLEN_SACK_PERM - 2) < ctx->data_end)

		if (*opsize == TCPOLEN_SACK_PERM && ctx->tcp->syn)

			ctx->attrs.sack_ok = 1;

		break;

	}



	ctx->ptr += opsize - 2;

	ctx->off = off + *opsize;

next:

	return 0;

stop:
@@ -259,7 +282,7 @@ static int tcp_parse_option(__u32 index, struct tcp_syncookie *ctx) 


static void tcp_parse_options(struct tcp_syncookie *ctx)

{

	ctx->ptr = (char *)(ctx->tcp + 1);

	ctx->off = (__u8 *)(ctx->tcp + 1) - (__u8 *)ctx->data,



	bpf_loop(40, tcp_parse_option, ctx, 0);

}