net: fix skb length accounting after generic XDP frag adjustment

net: fix skb length accounting after generic XDP frag adjustment

Generic XDP exposes non-linear skb fragments through an xdp_buff. If an
XDP program adjusts the fragment area, bpf_prog_run_generic_xdp() copies
xdp_frags_size back to skb->data_len but leaves skb->len containing the
old fragment contribution.

After a fragment shrink, this makes skb_headlen() larger than the actual
linear area. In the reproduced UDP receive path, __skb_datagram_iter()
copied 1024 bytes past the actual linear tail to userspace, starting at
struct skb_shared_info. The copied bytes included the affected skb's
nr_frags, xdp_frags_size and a kernel pointer from
skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same
amount and truncated at the end.

Subtract the old data_len before replacing it and add the new data_len
afterwards, keeping skb->len and skb->data_len synchronized.

A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by
1024 bytes from its fragment area. Before the fix, all 10 runs produced
corrupted payloads. After the fix, all 10 runs matched the expected
payload exactly.

Fixes: e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in generic mode")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/
Reviewed-by: Mohsin Bashir <hmohsin@meta.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Link: https://patch.msgid.link/20260804054040.613675-2-sun.jian.kdev@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>