Subject: Re: [PATCH v3] net: linearizing skb when downgrade gso_size

From: Fred Li <[email-masked]>
Date: Thu, 18 Jul 2024 15:42:56 +0800

> 
> > Linearizing skb when downgrade gso_size because it may
> > trigger the BUG_ON when segment skb as described in [1].
> > 
> > v3 changes:
> >   linearize skb if having frag_list as Willem de Bruijn suggested[2].
> > 
> > [1] https://lore.kernel.org/all/[email-masked]/
> > [2] https://lore.kernel.org/all/[email-masked]/
> > 
> > Signed-off-by: Fred Li <[email-masked]>
> 
> A fix needs a Fixed tag.
> 
> This might be the original commit that introduced gso_size adjustment,
> commit 6578171a7ff0c ("bpf: add bpf_skb_change_proto helper")
> 

Yes, this is the original commit, but it's already fixed by commit 
364745fbe981a (bpf: Do not change gso_size during bpf_skb_change_proto())

Another commit 2be7e212d5419 (bpf: add bpf_skb_adjust_room helper) introduced
gso_size too.

> Unless support for frag_list came later.
> 
> > ---
> >  net/core/filter.c | 16 ++++++++++++----
> >  1 file changed, 12 insertions(+), 4 deletions(-)
> > 
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index df4578219e82..70919b532d68 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -3525,13 +3525,21 @@ static int bpf_skb_net_grow(struct sk_buff *skb, u32 off, u32 len_diff,
> >  	if (skb_is_gso(skb)) {
> >  		struct skb_shared_info *shinfo = skb_shinfo(skb);
> >  
> > -		/* Due to header grow, MSS needs to be downgraded. */
> > -		if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO))
> > -			skb_decrease_gso_size(shinfo, len_diff);
> > -
> >  		/* Header must be checked, and gso_segs recomputed. */
> >  		shinfo->gso_type |= gso_type;
> >  		shinfo->gso_segs = 0;
> > +
> > +		/* Due to header grow, MSS needs to be downgraded.
> > +		 * There is BUG_ON When segment the frag_list with
> > +		 * head_frag true so linearize skb after downgrade
> > +		 * the MSS.
> > +		 */
> 
> Super tiny nit: no capitalization of When in the middle of a sentence.

Thanks, i'will fix.

> 
> > +		if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO)) {
> > +			skb_decrease_gso_size(shinfo, len_diff);
> > +			if (shinfo->frag_list)
> > +				return skb_linearize(skb);
> 
> I previously asked whether it was safe to call pskb_expand_head from
> within a BPF external function. There are actually plenty of existing
> examples of this, so this is fine.
> 
> > +		}
> > +
> >  	}
> >  
> >  	return 0;
> > -- 
> > 2.33.0
> >