[PATCH bpf v1] selftests/bpf: Fix test_maps sockmap failure

[Jiayuan Chen <jiayuan.chen@linux.dev>]

test_maps fails in the sockmap test because sockmap_verdict_prog.c
drops the packet when the first 8 bytes are not directly accessible:
	  if (data + 8 > data_end)
			  return SK_DROP;

The blamed commit removed bpf_skb_pull_data() from the stream parser
program so that the parser no longer modifies the skb. That was needed,
but it also removed an implicit side effect: bpf_skb_pull_data()
linearized enough of the skb for later direct packet access.

In this test, the send side goes through the sockmap SK_MSG path. The
skb can have skb->len == 20 while its linear area is empty, so the
verdict program sees data == data_end and drops the packet even though
the payload length is sufficient.

Keep the parser read-only, and pull the first 8 bytes in the verdict
program before reading or writing them. Reload data/data_end after
bpf_skb_pull_data() as required.

Reported-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Closes: https://lore.kernel.org/bpf/e3a91acd-2b4d-4e93-a3bb-a0e9ee5ede0f@linux.dev/
Fixes: 22a0cc10dacb ("selftests/bpf: don't modify the skb in the strparser parser prog")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 .../selftests/bpf/progs/sockmap_verdict_prog.c     | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/progs/sockmap_verdict_prog.c b/tools/testing/selftests/bpf/progs/sockmap_verdict_prog.c
index 0660f29dca955..3177bc5b733ac 100644
--- a/tools/testing/selftests/bpf/progs/sockmap_verdict_prog.c
+++ b/tools/testing/selftests/bpf/progs/sockmap_verdict_prog.c
@@ -44,8 +44,18 @@ int bpf_prog2(struct __sk_buff *skb)
 	__sink(lport);
 	__sink(rport);
 
-	if (data + 8 > data_end)
-		return SK_DROP;
+	if (data + 8 > data_end) {
+		if (bpf_skb_pull_data(skb, 8))
+			return SK_DROP;
+
+		data = (void *)(long)skb->data;
+		data_end = (void *)(long)skb->data_end;
+
+		if (data + 8 > data_end)
+			return SK_DROP;
+
+		d = data;
+	}
 
 	map = d[0];
 	sk = d[1];
-- 
2.43.0