[net] bpf, test: fix ld_abs + vlan push/pop stress test

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@iogearbox.net>
---
 lib/test_bpf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On Thu, Oct 20, 2016 at 05:13:53PM +0200, Daniel Borkmann wrote:
> 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@iogearbox.net>

Impressive sleuthing.
Acked-by: Alexei Starovoitov <ast@kernel.org>

From: Daniel Borkmann <daniel@iogearbox.net>
Date: Thu, 20 Oct 2016 17:13:53 +0200

> 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@iogearbox.net>

Applied, thanks Daniel.