[PATCH net 2/3] netconsole: do not dequeue pooled skbs that cannot satisfy len

[Breno Leitao <leitao@debian.org>]

find_skb() falls back to np->skb_pool when the GFP_ATOMIC alloc_skb()
fails. The pool is refilled by refill_skbs(), which always allocates
buffers of MAX_SKB_SIZE (ethhdr + iphdr + udphdr + MAX_UDP_CHUNK ==
1502 bytes).

netconsole, however, computes the requested length dynamically as

        total_len + np->dev->needed_tailroom

If the egress device declares a non-zero needed_tailroom (e.g. some
tunnel or hardware accelerator devices), the required length can exceed
MAX_SKB_SIZE. The pooled skb is then handed back to the caller, which
immediately performs skb_put(skb, len), trips the tail > end check, and
triggers skb_over_panic().

Leave the normal alloc_skb(len, GFP_ATOMIC) path untouched -- the slab
allocator can still satisfy oversized requests when memory is available,
so senders to devices with non-zero needed_tailroom keep working in the
common case. Only the pool fallback is gated: when alloc_skb() failed
and len exceeds the pool buffer size, skip the skb_dequeue() instead of
burning a pre-allocated skb on a request that would later trip
skb_over_panic(). Reserving pool entries for requests they can actually
satisfy also keeps the panic path, which depends on the pool being
primed, intact.

When that drop happens, emit a rate-limited net_warn() so the user
notices that netconsole is unable to push messages on the egress device.
The warn is skipped under in_nmi() for the same reason schedule_work()
is: printk machinery taken by net_warn_ratelimited() is not NMI-safe and
would risk recursing into the same nbcon console we are servicing.

MAX_SKB_SIZE / MAX_UDP_CHUNK were private to net/core/netpoll.c. Move
them to include/linux/netpoll.h so netconsole can reference the same
definition that refill_skbs() uses, keeping the two in sync by
construction. The header now pulls in <linux/ip.h> and <linux/udp.h>
explicitly so MAX_SKB_SIZE remains self-contained for any future user.

Fixes: 954fba027405 ("netpoll: fix netpoll_send_udp() bugs")
Signed-off-by: Breno Leitao <leitao@debian.org>
---
 drivers/net/netconsole.c |  7 ++++++-
 include/linux/netpoll.h  | 16 ++++++++++++++++
 net/core/netpoll.c       |  7 -------
 3 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 699bdfa1fb45..a3dcbe713a0b 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -1680,8 +1680,13 @@ static struct sk_buff *find_skb(struct netpoll *np, int len, int reserve)
 repeat:
 
 	skb = alloc_skb(len, GFP_ATOMIC);
-	if (!skb)
+	if (!skb) {
+		/* The pool is refilled with MAX_SKB_SIZE buffers */
+		if (WARN_ON_ONCE(len > MAX_SKB_SIZE))
+			return NULL;
+
 		skb = netcons_skb_pop(np);
+	}
 
 	if (!skb) {
 		if (++count < 10) {
diff --git a/include/linux/netpoll.h b/include/linux/netpoll.h
index e4b8f1f91e54..88f7daa8560e 100644
--- a/include/linux/netpoll.h
+++ b/include/linux/netpoll.h
@@ -13,12 +13,28 @@
 #include <linux/rcupdate.h>
 #include <linux/list.h>
 #include <linux/refcount.h>
+#include <linux/ip.h>
+#include <linux/udp.h>
 
 union inet_addr {
 	__be32		ip;
 	struct in6_addr	in6;
 };
 
+/*
+ * Maximum payload netpoll's preallocated skb pool can carry. Keep this in
+ * sync with the buffer size used by refill_skbs() in net/core/netpoll.c;
+ * callers (e.g. netconsole) use it to detect requests the pool can never
+ * satisfy and avoid dequeuing a pooled skb that would later trip
+ * skb_over_panic() in skb_put().
+ */
+#define MAX_UDP_CHUNK	1460
+#define MAX_SKB_SIZE						\
+	(sizeof(struct ethhdr) +				\
+	 sizeof(struct iphdr) +					\
+	 sizeof(struct udphdr) +				\
+	 MAX_UDP_CHUNK)
+
 struct netpoll {
 	struct net_device *dev;
 	netdevice_tracker dev_tracker;
diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index b3fe59445f2d..229dde818ab3 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -41,16 +41,9 @@
  * message gets out even in extreme OOM situations.
  */
 
-#define MAX_UDP_CHUNK 1460
 #define MAX_SKBS 32
 #define USEC_PER_POLL	50
 
-#define MAX_SKB_SIZE							\
-	(sizeof(struct ethhdr) +					\
-	 sizeof(struct iphdr) +						\
-	 sizeof(struct udphdr) +					\
-	 MAX_UDP_CHUNK)
-
 static unsigned int carrier_timeout = 4;
 module_param(carrier_timeout, uint, 0644);
 

-- 
2.51.0