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Message-Id: <20240718074256.65274-1-dracodingfly@gmail.com>
Date: Thu, 18 Jul 2024 15:42:56 +0800
From: Fred Li <dracodingfly@...il.com>
To: willemdebruijn.kernel@...il.com
Cc: andrii@...nel.org,
ast@...nel.org,
bpf@...r.kernel.org,
daniel@...earbox.net,
dracodingfly@...il.com,
herbert@...dor.apana.org.au,
john.fastabend@...il.com,
kpsingh@...nel.org,
linux-kernel@...r.kernel.org,
netdev@...r.kernel.org,
pabeni@...hat.com,
song@...nel.org,
yonghong.song@...ux.dev
Subject: Re: [PATCH v3] net: linearizing skb when downgrade gso_size
>
> > 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/20240626065555.35460-2-dracodingfly@gmail.com/
> > [2] https://lore.kernel.org/all/668d5cf1ec330_1c18c32947@willemb.c.googlers.com.notmuch/
> >
> > Signed-off-by: Fred Li <dracodingfly@...il.com>
>
> 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
> >
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