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net: macb: copy shared skbs before appending the FCS

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macb_pad_and_fcs() appends the FCS in place when the skb has tailroom.
A shared skb, as pktgen sends in clone_skb mode, grows by one FCS per
transmit. BQL then completes more bytes than were queued and
dql_completed() hits its BUG_ON.

On a Raspberry Pi CM5 (RP1 GEM) pktgen with clone_skb 1000 burst 32 at
60 bytes kills the box within seconds.

Copy shared skbs before appending the FCS. Clearing IFF_TX_SKB_SHARING
would also fix it but makes pktgen refuse clone_skb on macb.

Fixes: 653e92a917 ("net: macb: add support for padding and fcs computation")
Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
Link: https://patch.msgid.link/20261006-nb-macb-shared-skb-net-v1-2-a80641479041@tipi-net.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Nicolai Buchwitz 2026-10-06 07:41:33 +02:00 committed by Jakub Kicinski
parent 6b48ed85ae
commit 9151d6c427
1 changed files with 4 additions and 2 deletions
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6
drivers/net/ethernet/cadence/macb_main.c
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@ -2435,6 +2435,7 @@ static int macb_pad_and_fcs(struct sk_buff **skb, bool add_fcs)
skb_is_nonlinear(*skb);
int padlen = ETH_ZLEN - (*skb)->len;
int tailroom = skb_tailroom(*skb);
bool shared = skb_shared(*skb);
struct sk_buff *nskb;
u32 fcs;
@ -2443,7 +2444,8 @@ static int macb_pad_and_fcs(struct sk_buff **skb, bool add_fcs)
if (padlen <= 0) {
/* FCS could be appended to tailroom. */
if (!skb_is_nonlinear(*skb) && tailroom >= ETH_FCS_LEN)
if (!shared && !skb_is_nonlinear(*skb) &&
tailroom >= ETH_FCS_LEN)
goto add_fcs;
/* Reallocate with room for the FCS. */
padlen = ETH_FCS_LEN;
@ -2452,7 +2454,7 @@ static int macb_pad_and_fcs(struct sk_buff **skb, bool add_fcs)
padlen += ETH_FCS_LEN;
}
if (cloned || tailroom < padlen) {
if (shared || cloned || tailroom < padlen) {
nskb = skb_copy_expand(*skb, 0, padlen, GFP_ATOMIC);
if (!nskb)
return -ENOMEM;