[PATCH] skb_put: remove not needed check for skb linearity

[PATCH] skb_put: remove not needed check for skb linearity

[Paulius Zaleckas]

It is safe to call skb_put() on packets containing fragments.

Actually I have a case where I allocate packet header with some
extra headroom and then I dynamically add data as frag_list. After
adding frags I have to add more data to header and skb_put()
just BUG's on me :)

And we will save couple instructions for CPU.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@gmail.com>
---

 include/linux/skbuff.h |    1 -
 net/core/skbuff.c      |    1 -
 2 files changed, 0 insertions(+), 2 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 124f90c..194e9fa 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1108,7 +1108,6 @@ extern unsigned char *skb_put(struct sk_buff *skb, unsigned int len);
 static inline unsigned char *__skb_put(struct sk_buff *skb, unsigned int len)
 {
 	unsigned char *tmp = skb_tail_pointer(skb);
-	SKB_LINEAR_ASSERT(skb);
 	skb->tail += len;
 	skb->len  += len;
 	return tmp;
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 93c4e06..ea1ca61 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1011,7 +1011,6 @@ EXPORT_SYMBOL(skb_pad);
 unsigned char *skb_put(struct sk_buff *skb, unsigned int len)
 {
 	unsigned char *tmp = skb_tail_pointer(skb);
-	SKB_LINEAR_ASSERT(skb);
 	skb->tail += len;
 	skb->len  += len;
 	if (unlikely(skb->tail > skb->end))

Re: [PATCH] skb_put: remove not needed check for skb linearity

[David Miller]

From: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Date: Tue, 30 Mar 2010 16:01:31 +0300

> It is safe to call skb_put() on packets containing fragments.
> 
> Actually I have a case where I allocate packet header with some
> extra headroom and then I dynamically add data as frag_list. After
> adding frags I have to add more data to header and skb_put()
> just BUG's on me :)
> 
> And we will save couple instructions for CPU.
> 
> Signed-off-by: Paulius Zaleckas <paulius.zaleckas@gmail.com>

No, you really cannot do this, that check is very much intentional and
needs to be there.  Otherwise we will allow violations of the
semantics of the SKB data area.

Once you put even one byte of non-linear data into the skb, all data
must be "put" to the end of that non-linear area, without exception.

You can't just stuff arbitrary things "in-between" the linear and the
non-linear stuff.

You'll need to find a way to construct your SKBs properly such that
the entirety of the linear area is constructed before you start
adding non-linear elements.