[SRU][Xenial][PATCH 1/1] bpf, test: fix ld_abs + vlan push/pop stress test

[Kleber Sacilotto de Souza]

From: Daniel Borkmann <daniel at iogearbox.net>

BugLink: https://bugs.launchpad.net/bugs/1879752

After commit 636c2628086e ("net: skbuff: Remove errornous length
validation in skb_vlan_pop()") mentioned test case stopped working,
throwing a -12 (ENOMEM) return code. The issue however is not due to
636c2628086e, but rather due to a buggy test case that got uncovered
from the change in behaviour in 636c2628086e.

The data_size of that test case for the skb was set to 1. In the
bpf_fill_ld_abs_vlan_push_pop() handler bpf insns are generated that
loop with: reading skb data, pushing 68 tags, reading skb data,
popping 68 tags, reading skb data, etc, in order to force a skb
expansion and thus trigger that JITs recache skb->data. Problem is
that initial data_size is too small.

While before 636c2628086e, the test silently bailed out due to the
skb->len < VLAN_ETH_HLEN check with returning 0, and now throwing an
error from failing skb_ensure_writable(). Set at least minimum of
ETH_HLEN as an initial length so that on first push of data, equivalent
pop will succeed.

Fixes: 4d9c5c53ac99 ("test_bpf: add bpf_skb_vlan_push/pop() tests")
Signed-off-by: Daniel Borkmann <daniel at iogearbox.net>
Acked-by: Alexei Starovoitov <ast at kernel.org>
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit 0d906b1e8d4002cdd59590fec630f4e75023e288)
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>
---
 lib/test_bpf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/test_bpf.c b/lib/test_bpf.c
index c78e9842f6bc..16faf45452cf 100644
--- a/lib/test_bpf.c
+++ b/lib/test_bpf.c
@@ -4571,7 +4571,7 @@ static struct bpf_test tests[] = {
 		{ },
 		INTERNAL,
 		{ 0x34 },
-		{ { 1, 0xbef } },
+		{ { ETH_HLEN, 0xbef } },
 		.fill_helper = bpf_fill_ld_abs_vlan_push_pop,
 	},
 	/*
-- 
2.20.1