On Fri, 2015-08-28 at 11:35 +0800, Weilong Chen wrote:
> Thanks for reply.
>
> > On Wed, 2015-08-26 at 19:12 -0700, Eric Dumazet wrote:
> >> On Thu, 2015-08-27 at 08:56 +0800, [email protected] wrote:
> >>> From: Weilong Chen <[email protected]>
> >>>
> >>> When try to merge several skbs to prior one, if the frag_list is
> >>> used and the the last one is a small packet, once the condition
> >>> "len <= skb_tailroom(to)" is satisfied, we will get a wrong
> >>> packet!
> >>> This patch just check frag_lists before the condtion to prevent
> >>> this from happening.
> >>>
> >>> Signed-off-by: Weilong Chen <[email protected]>
> >>> ---
> >>> net/core/skbuff.c | 6 +++---
> >>> 1 file changed, 3 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> >>> index 8a725cc..d08edcb 100644
> >>> --- a/net/core/skbuff.c
> >>> +++ b/net/core/skbuff.c
> >>> @@ -4133,6 +4133,9 @@ bool skb_try_coalesce(struct sk_buff *to, struct
> >>> sk_buff *from,
> >>> if (skb_cloned(to))
> >>> return false;
> >>>
> >>> + if (skb_has_frag_list(to) || skb_has_frag_list(from))
> >>> + return false;
> >>> +
> >>> if (len <= skb_tailroom(to)) {
> >>> if (len)
> >>> BUG_ON(skb_copy_bits(from, 0, skb_put(to, len),
> >>> len));
> >>> @@ -4140,9 +4143,6 @@ bool skb_try_coalesce(struct sk_buff *to, struct
> >>> sk_buff *from,
> >>> return true;
> >>> }
> >>>
> >>> - if (skb_has_frag_list(to) || skb_has_frag_list(from))
> >>> - return false;
> >>> -
> >>> if (skb_headlen(from) != 0) {
> >>> struct page *page;
> >>> unsigned int offset;
> >>
> >> Sigh.
> >>
> >> No idea what problem you tried to solve.
> >>
> >> This patch is not needed.
> >>
> >> If (len <= skb_tailroom()), then it is obviously correct to copy_bits()
> >> the bytes.
> >>
> >> Hints :
> >>
> >> - If @to has a fraglist, then skb_tailroom(to) is 0 so the copy can not
> >> be done.
>
> How to make sure it?
> In my test, @to has a fraglist, but skb_tailroom(to) is not 0!
> The test is about tipc, the function tipc_buf_append will merge 3 skbs
> to one:
> packet 1: len = 1420 skb_tailroom = 190
> packet 2: len = 1420
> packet 2 will be add to 1' fraglist, but not update skb_tailroom
> packet 3: len = 60
> Here's the error!
Then fix tipc, or make sure you read and understood the code.
By definition, if one skb has something in fraglist, it is non linear.
If you read skb_tailroom(), you'll see this function returns 0 if skb is
non linear.
include/linux/skbuff.h-/**
include/linux/skbuff.h: * skb_tailroom - bytes at buffer end
include/linux/skbuff.h- * @skb: buffer to check
include/linux/skbuff.h- *
include/linux/skbuff.h- * Return the number of bytes of free space at the
tail of an sk_buff
include/linux/skbuff.h- */
include/linux/skbuff.h:static inline int skb_tailroom(const struct sk_buff *skb)
include/linux/skbuff.h-{
include/linux/skbuff.h- return skb_is_nonlinear(skb) ? 0 : skb->end - skb->tail;
include/linux/skbuff.h-}
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