From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Date: Wed, 8 Jan 2020 16:59:02 -0500
Subject: net: introduce skb_list_walk_safe for skb segment walking
Patch-mainline: v5.6-rc1
Git-commit: dcfea72e79b0aa7a057c8f6024169d86a1bbc84b
References: bsc#1169021 jsc#SLE-12250

As part of the continual effort to remove direct usage of skb->next and
skb->prev, this patch adds a helper for iterating through the
singly-linked variant of skb lists, which are used for lists of GSO
packet. The name "skb_list_..." has been chosen to match the existing
function, "kfree_skb_list, which also operates on these singly-linked
lists, and the "..._walk_safe" part is the same idiom as elsewhere in
the kernel.

This patch removes the helper from wireguard and puts it into
linux/skbuff.h, while making it a bit more robust for general usage. In
particular, parenthesis are added around the macro argument usage, and it
now accounts for trying to iterate through an already-null skb pointer,
which will simply run the iteration zero times. This latter enhancement
means it can be used to replace both do { ... } while and while (...)
open-coded idioms.

This should take care of these three possible usages, which match all
current methods of iterations.

skb_list_walk_safe(segs, skb, next) { ... }
skb_list_walk_safe(skb, skb, next) { ... }
skb_list_walk_safe(segs, skb, segs) { ... }

Gcc appears to generate efficient code for each of these.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Nicolai Stange <nstange@suse.de>
---
 drivers/net/wireguard/device.h |    8 --------
 include/linux/skbuff.h         |    5 +++++
 2 files changed, 5 insertions(+), 8 deletions(-)

--- a/drivers/net/wireguard/device.h
+++ b/drivers/net/wireguard/device.h
@@ -62,12 +62,4 @@ struct wg_device {
 int wg_device_init(void);
 void wg_device_uninit(void);
 
-/* Later after the dust settles, this can be moved into include/linux/skbuff.h,
- * where virtually all code that deals with GSO segs can benefit, around ~30
- * drivers as of writing.
- */
-#define skb_list_walk_safe(first, skb, next)                                   \
-	for (skb = first, next = skb->next; skb;                               \
-	     skb = next, next = skb ? skb->next : NULL)
-
 #endif /* _WG_DEVICE_H */
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1472,6 +1472,11 @@ static inline void skb_mark_not_on_list(
 	skb->next = NULL;
 }
 
+/* Iterate through singly-linked GSO fragments of an skb. */
+#define skb_list_walk_safe(first, skb, next)                                   \
+	for ((skb) = (first), (next) = (skb) ? (skb)->next : NULL; (skb);      \
+	     (skb) = (next), (next) = (skb) ? (skb)->next : NULL)
+
 static inline void skb_list_del_init(struct sk_buff *skb)
 {
 	__list_del_entry(&skb->list);