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

quoted

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/ (local)
[2] https://lore.kernel.org/all/668d5cf1ec330_1c18c32947@willemb.c.googlers.com.notmuch/ (local)

Signed-off-by: Fred Li <redacted>

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.

quoted

---
 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.

quoted

+		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.

quoted

+		}
+
 	}
 
 	return 0;
-- 
2.33.0