[PATCH 5.11 534/601] gro: fix napi_gro_frags() Fast GRO breakage due to IP alignment check

Greg Kroah-Hartman
2021-05-12T17:56:47Z

From: Alexander Lobakin <alobakin@pm.me>

[ Upstream commit 7ad18ff6449cbd6beb26b53128ddf56d2685aa93 ]

Commit 38ec4944b593 ("gro: ensure frag0 meets IP header alignment")
did the right thing, but missed the fact that napi_gro_frags() logics
calls for skb_gro_reset_offset() *before* pulling Ethernet header
to the skb linear space.
That said, the introduced check for frag0 address being aligned to 4
always fails for it as Ethernet header is obviously 14 bytes long,
and in case with NET_IP_ALIGN its start is not aligned to 4.

Fix this by adding @nhoff argument to skb_gro_reset_offset() which
tells if an IP header is placed right at the start of frag0 or not.
This restores Fast GRO for napi_gro_frags() that became very slow
after the mentioned commit, and preserves the introduced check to
avoid silent unaligned accesses.

>From v1 [0]:
- inline tiny skb_gro_reset_offset() to let the code be optimized
more efficively (esp. for the !NET_IP_ALIGN case) (Eric);
- pull in Reviewed-by from Eric.

[0] https://lore.kernel.org/netdev/20210418114200.5839-1-alobakin@pm.me

Fixes: 38ec4944b593 ("gro: ensure frag0 meets IP header alignment")
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/core/dev.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 3c0d3b6d674d..633c2d6f1a35 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5867,7 +5867,7 @@ static struct list_head *gro_list_prepare(struct napi_struct *napi,
return head;
}

-static void skb_gro_reset_offset(struct sk_buff *skb)
+static inline void skb_gro_reset_offset(struct sk_buff *skb, u32 nhoff)
{
const struct skb_shared_info *pinfo = skb_shinfo(skb);
const skb_frag_t *frag0 = &pinfo->frags[0];
@@ -5878,7 +5878,7 @@ static void skb_gro_reset_offset(struct sk_buff *skb)

if (!skb_headlen(skb) && pinfo->nr_frags &&
!PageHighMem(skb_frag_page(frag0)) &&
- (!NET_IP_ALIGN || !(skb_frag_off(frag0) & 3))) {
+ (!NET_IP_ALIGN || !((skb_frag_off(frag0) + nhoff) & 3))) {
NAPI_GRO_CB(skb)->frag0 = skb_frag_address(frag0);
NAPI_GRO_CB(skb)->frag0_len = min_t(unsigned int,
skb_frag_size(frag0),
@@ -6111,7 +6111,7 @@ gro_result_t napi_gro_receive(struct napi_struct *napi, struct sk_buff *skb)
skb_mark_napi_id(skb, napi);
trace_napi_gro_receive_entry(skb);

- skb_gro_reset_offset(skb);
+ skb_gro_reset_offset(skb, 0);

ret = napi_skb_finish(napi, skb, dev_gro_receive(napi, skb));
trace_napi_gro_receive_exit(ret);
@@ -6204,7 +6204,7 @@ static struct sk_buff *napi_frags_skb(struct napi_struct *napi)
napi->skb = NULL;

skb_reset_mac_header(skb);
- skb_gro_reset_offset(skb);
+ skb_gro_reset_offset(skb, hlen);

if (unlikely(skb_gro_header_hard(skb, hlen))) {
eth = skb_gro_header_slow(skb, hlen, 0);
--
2.30.2