On 2021-02-01 06:57, Xie He wrote:
When sending a packet, we will prepend it with an LAPB header.
This modifies the shared parts of a cloned skb, so we should copy the
skb rather than just clone it, before we prepend the header.
In "Documentation/networking/driver.rst" (the 2nd point), it states
that drivers shouldn't modify the shared parts of a cloned skb when
transmitting.
The "dev_queue_xmit_nit" function in "net/core/dev.c", which is called
when an skb is being sent, clones the skb and sents the clone to
AF_PACKET sockets. Because the LAPB drivers first remove a 1-byte
pseudo-header before handing over the skb to us, if we don't copy the
skb before prepending the LAPB header, the first byte of the packets
received on AF_PACKET sockets can be corrupted.
What kind of packages do you mean are corrupted?
ETH_P_X25 or ETH_P_HDLC?
I have also sent a patch here in the past that addressed corrupted
ETH_P_X25 frames on an AF_PACKET socket:
https://lkml.org/lkml/2020/1/13/388
Unfortunately I could not track and describe exactly where the problem
was.
I just wonder when/where is the logically correct place to copy the skb.
Shouldn't it be copied before removing the pseudo header (as I did in my
patch)?
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: Martin Schiller <ms@xxxxxxxxxx>
Signed-off-by: Xie He <xie.he.0141@xxxxxxxxx>
---
net/lapb/lapb_out.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/net/lapb/lapb_out.c b/net/lapb/lapb_out.c
index 7a4d0715d1c3..a966d29c772d 100644
--- a/net/lapb/lapb_out.c
+++ b/net/lapb/lapb_out.c
@@ -82,7 +82,8 @@ void lapb_kick(struct lapb_cb *lapb)
skb = skb_dequeue(&lapb->write_queue);
do {
- if ((skbn = skb_clone(skb, GFP_ATOMIC)) == NULL) {
+ skbn = skb_copy(skb, GFP_ATOMIC);
+ if (!skbn) {
skb_queue_head(&lapb->write_queue, skb);
break;
}
