[PATCH pve-kernel] cherry-pick fix for broken UDP offloading

["Fabian Grünbichler" <f.gruenbichler@proxmox.com>]

this affected - among other things - tailscale's userspace wireguard
implementation:

https://github.com/tailscale/tailscale/issues/19777

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
---
 ...ndling-checksum-in-__udp_gso_segment.patch | 71 +++++++++++++++++++
 1 file changed, 72 insertions(+)
 create mode 100644 patches/kernel/0053-udp-gso-Fix-handling-checksum-in-__udp_gso_segment.patch

diff --git a/patches/kernel/0053-udp-gso-Fix-handling-checksum-in-__udp_gso_segment.patch b/patches/kernel/0053-udp-gso-Fix-handling-checksum-in-__udp_gso_segment.patch
new file mode 100644
index 0000000..c5ab651
--- /dev/null
+++ b/patches/kernel/0053-udp-gso-Fix-handling-checksum-in-__udp_gso_segment.patch
@@ -0,0 +1,72 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Alice Mikityanska <alice@isovalent.com>
+Date: Mon, 18 May 2026 09:22:49 +0300
+Subject: [PATCH] udp: gso: Fix handling checksum in __udp_gso_segment
+
+The cited commit started using msslen for uh->len, but still uses newlen
+to adjust uh->check. Although the checksum is ignored in most cases due
+to the hardware offload, __udp_gso_segment attempts to maintain the
+correct one. Fix uh->check and adjust it by the right value.
+
+Additionally, after the fix, newlen becomes assigned and unused before
+the loop. The code can be simplified a bit if mss adjustment is dropped,
+so that newlen becomes equal to msslen before the loop, and msslen can
+be also dropped, saving a few lines of code.
+
+This brings us back to one variable, drops an unneeded arithmetic for
+mss, and fixes the UDP checksum.
+
+Fixes: b10b446ce7ad ("udp: gso: Use single MSS length in UDP header for GSO_PARTIAL")
+Signed-off-by: Alice Mikityanska <alice@isovalent.com>
+Reviewed-by: Willem de Bruijn <willemb@google.com>
+Signed-off-by: Gal Pressman <gal@nvidia.com>
+Link: https://patch.msgid.link/20260518062250.3019914-2-gal@nvidia.com
+Signed-off-by: Jakub Kicinski <kuba@kernel.org>
+(cherry picked from commit 5f17ae0f595aeb560155ce98edbe44d3eacc7e40)
+Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
+---
+ net/ipv4/udp_offload.c | 13 ++-----------
+ 1 file changed, 2 insertions(+), 11 deletions(-)
+
+diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
+index 6b1654c1ad4ac0b7c611c338d75632f1974f842b..e831234326c414a1004609656ef2207d087d3346 100644
+--- a/net/ipv4/udp_offload.c
++++ b/net/ipv4/udp_offload.c
+@@ -483,11 +483,11 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
+ 	struct sock *sk = gso_skb->sk;
+ 	unsigned int sum_truesize = 0;
+ 	struct sk_buff *segs, *seg;
+-	__be16 newlen, msslen;
+ 	struct udphdr *uh;
+ 	unsigned int mss;
+ 	bool copy_dtor;
+ 	__sum16 check;
++	__be16 newlen;
+ 	int ret = 0;
+ 
+ 	mss = skb_shinfo(gso_skb)->gso_size;
+@@ -556,15 +556,6 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
+ 		return segs;
+ 	}
+ 
+-	msslen = htons(sizeof(*uh) + mss);
+-
+-	/* GSO partial and frag_list segmentation only requires splitting
+-	 * the frame into an MSS multiple and possibly a remainder, both
+-	 * cases return a GSO skb. So update the mss now.
+-	 */
+-	if (skb_is_gso(segs))
+-		mss *= skb_shinfo(segs)->gso_segs;
+-
+ 	seg = segs;
+ 	uh = udp_hdr(seg);
+ 
+@@ -587,7 +578,7 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
+ 		if (!seg->next)
+ 			break;
+ 
+-		uh->len = msslen;
++		uh->len = newlen;
+ 		uh->check = check;
+ 
+ 		if (seg->ip_summed == CHECKSUM_PARTIAL)
-- 
2.47.3