modifying struct sk_buff

DISCUSSION Sushant Sharma 2 participants 4 messages Jun 4, 2004

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Sushant Sharma

author

2004-06-04 00:22 UTC #0 lore raw

Hi
I want to add a new member (say uint32_t) in the
struct sk_buff{...}
in the file include/linux/skbuff.h.

If i add this new member and I want to assign it
some value when ever function
struct sk_buff *alloc_skb(unsigned int *size,* int gfp_mask) in file
net/core/skbuff.c
is called
Do I need to allocate memory for this member
( ie add sizeof(_new-member_) to *size* while doing kmalloc() ) or I should not worry abt that and I can assign some value to this _new-member_ every time this function is called.

Thanks
Sushant

Bart Trojanowski

2004-06-04 12:37 UTC #1 lore raw

Quoted 4 lines · Sushant Sharma

Hi
I want to add a new member (say uint32_t) in the
struct sk_buff{...}
in the file include/linux/skbuff.h.

<snip>

Quoted 2 lines

Do I need to allocate memory for this member
( ie add sizeof(_new-member_) to *size* while doing kmalloc() )

Hi Sushant,

I think you are confusing the allocation of 'data' with the allocation of 'skb'.

If you add the uint32_t to struct sk_buff you don't have to modify alloc_skb. The skbuff_head_cache is informed what size the sk_buff structure is in skb_init().

-Bart

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Sharma Sushant

author

2004-06-04 18:50 UTC #2 lore raw

Hi Bart
thanks for replying.
I was thinking functions calling the alloc_skb will not consider the size of new variable which I will add, thats why I thought of adding the size my self while allocating space.

I am using kernel 2.6.3 and here are few line from skbuff.h

Quoted 8 lines

/*
* This is the control buffer. It is free to use for every
* layer. Please put your private variables there. If you
* want to keep them across layers you have to do a skb_clone()
* first. This is owned by whoever has the skb queued ATM. */
uint32_t skBufId; /* by sushant */
char cb[48];

skBufId is the variable i added and I want to assign unique value to this id whenever alloc_skb is called.
Do you think it will be fine to modify alloc_skb(..)
and just assigning a unique value to this new variable which I added or will there be some side effects of this.

Thanks
Sushant

Quoted 23 lines · Bart Trojanowski

Quoted 4 lines · Sushant Sharma
Hi
I want to add a new member (say uint32_t) in the
struct sk_buff{...}
in the file include/linux/skbuff.h.
<snip>
Quoted 2 lines
Do I need to allocate memory for this member
( ie add sizeof(_new-member_) to *size* while doing kmalloc() )
Hi Sushant,
I think you are confusing the allocation of 'data' with the allocation of 'skb'.
If you add the uint32_t to struct sk_buff you don't have to modify alloc_skb. The skbuff_head_cache is informed what size the sk_buff structure is in skb_init().
-Bart
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Bart Trojanowski

2004-06-04 20:05 UTC #3 lore raw

Quoted 8 lines · Sharma Sushant

Quoted 2 lines
uint32_t skBufId; /* by sushant */
char cb[48];
skBufId is the variable i added and I want to assign unique value to this id whenever alloc_skb is called.
Do you think it will be fine to modify alloc_skb(..)
and just assigning a unique value to this new variable which I added or will there be some side effects of this.

Looks like you can start using skBufId anytime after memset(skb, ...) call. I didn't look at the version of the sources you are using. In 2.4.7-rc2 it looks fine.

-Bart

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