[PATCH] Revert "net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends"

[Mathieu Malaterre]

This reverts commit 88078d98d1bb085d72af8437707279e203524fa5.

It causes regressions for people using chips driven by the sungem
driver. Suspicion is that the skb->csum value isn't being adjusted
properly.

Symptoms as seen on G4+sungem are:

[   34.023281] eth0: hw csum failure
[   34.023438] CPU: 0 PID: 0 Comm: swapper Not tainted 4.17.0+ #2
[   34.023618] Call Trace:
[   34.023707] [dffedbd0] [c069ddac] __skb_checksum_complete+0xf0/0x108 (unreliable)
[   34.023948] [dffedbf0] [c0777a70] tcp_v4_rcv+0x604/0xe00
[   34.024118] [dffedc70] [c0731624] ip_local_deliver_finish+0xa8/0x3c4
[   34.024315] [dffedcb0] [c0732430] ip_local_deliver+0xf0/0x154
[   34.024493] [dffedcf0] [c07328dc] ip_rcv+0x448/0x774
[   34.024653] [dffedd50] [c06aeae0] __netif_receive_skb_core+0x5e8/0x1184
[   34.024857] [dffedde0] [c06bba20] napi_gro_receive+0x160/0x22c
[   34.025044] [dffede10] [e14b2590] gem_poll+0x7fc/0x1ac0 [sungem]
[   34.025228] [dffedee0] [c06bacf0] net_rx_action+0x34c/0x618
[   34.025402] [dffedf60] [c07fd27c] __do_softirq+0x16c/0x5f0
[   34.025575] [dffedfd0] [c0064c7c] irq_exit+0x110/0x1a8
[   34.025738] [dffedff0] [c0016170] call_do_irq+0x24/0x3c
[   34.025903] [c0cf7e80] [c0009a84] do_IRQ+0x98/0x1a0
[   34.026055] [c0cf7eb0] [c001b474] ret_from_except+0x0/0x14
[   34.026225] --- interrupt: 501 at arch_cpu_idle+0x30/0x78
                   LR = arch_cpu_idle+0x30/0x78
[   34.026510] [c0cf7f70] [c0cf6000] 0xc0cf6000 (unreliable)
[   34.026682] [c0cf7f80] [c00a3868] do_idle+0xc4/0x158
[   34.026835] [c0cf7fb0] [c00a3ab0] cpu_startup_entry+0x20/0x28
[   34.027013] [c0cf7fc0] [c0998820] start_kernel+0x47c/0x490
[   34.027181] [c0cf7ff0] [00003444] 0x3444

See commit 7ce5a27f2ef8 ("Revert "net: Handle CHECKSUM_COMPLETE more
adequately in pskb_trim_rcsum()."") for previous reference.

Link: https://lists.ozlabs.org/pipermail/linuxppc-dev/2018-June/174444.html
Reported-by: Meelis Roos <mroos@xxxxxxxx>
Fixes: 88078d98d1bb ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends")
Signed-off-by: Mathieu Malaterre <malat@xxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
---
 include/linux/skbuff.h |  5 +++--
 net/core/skbuff.c      | 14 --------------
 2 files changed, 3 insertions(+), 16 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index c86885954994..cbc753a3e41c 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3134,7 +3134,6 @@ static inline void *skb_push_rcsum(struct sk_buff *skb, unsigned int len)
 	return skb->data;
 }
 
-int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned int len);
 /**
  *	pskb_trim_rcsum - trim received skb and update checksum
  *	@skb: buffer to trim
@@ -3148,7 +3147,9 @@ static inline int pskb_trim_rcsum(struct sk_buff *skb, unsigned int len)
 {
 	if (likely(len >= skb->len))
 		return 0;
-	return pskb_trim_rcsum_slow(skb, len);
+	if (skb->ip_summed == CHECKSUM_COMPLETE)
+		skb->ip_summed = CHECKSUM_NONE;
+	return __pskb_trim(skb, len);
 }
 
 static inline int __skb_trim_rcsum(struct sk_buff *skb, unsigned int len)
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index c642304f178c..360293d1baf3 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1840,20 +1840,6 @@ int ___pskb_trim(struct sk_buff *skb, unsigned int len)
 }
 EXPORT_SYMBOL(___pskb_trim);
 
-/* Note : use pskb_trim_rcsum() instead of calling this directly
- */
-int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned int len)
-{
-	if (skb->ip_summed == CHECKSUM_COMPLETE) {
-		int delta = skb->len - len;
-
-		skb->csum = csum_sub(skb->csum,
-				     skb_checksum(skb, len, delta, 0));
-	}
-	return __pskb_trim(skb, len);
-}
-EXPORT_SYMBOL(pskb_trim_rcsum_slow);
-
 /**
  *	__pskb_pull_tail - advance tail of skb header
  *	@skb: buffer to reallocate
-- 
2.11.0