Re: [PATCH v3] net: linearizing skb when downgrade gso_size
From: Willem de Bruijn
Date: Wed Jul 17 2024 - 12:32:36 EST
Fred Li wrote:
> 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@xxxxxxxxx/
> [2] https://lore.kernel.org/all/668d5cf1ec330_1c18c32947@xxxxxxxxxxxxxxxxxxxxxx.notmuch/
>
> Signed-off-by: Fred Li <dracodingfly@xxxxxxxxx>
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")
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.
> + 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
>