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6c537b845c
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.
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260604-netcons_fix_before_move-v3-2-ab055b3a6aa5@debian.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>