[PATCH net] net: refine debug info in skb_checksum_help()

From: Eric Dumazet <edumazet@google.com>
To: "David S . Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	 Paolo Abeni <pabeni@redhat.com>
Cc: netdev@vger.kernel.org, eric.dumazet@gmail.com,
	 Eric Dumazet <edumazet@google.com>,
	Willem de Bruijn <willemb@google.com>
Subject: [PATCH net] net: refine debug info in skb_checksum_help()
Date: Fri,  6 Oct 2023 17:33:54 +0000	[thread overview]
Message-ID: <20231006173355.2254983-1-edumazet@google.com> (raw)

syzbot uses panic_on_warn.

This means that the skb_dump() I added in the blamed commit are
not even called.

Rewrite this so that we get the needed skb dump before syzbot crashes.

Fixes: eeee4b77dc52 ("net: add more debug info in skb_checksum_help()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Willem de Bruijn <willemb@google.com>
---
 net/core/dev.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 85df22f05c38b663f050410b9f7bcd32dd781951..5aaf5753d4e46c7c4b67b00daadeda9784708dfe 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3292,15 +3292,19 @@ int skb_checksum_help(struct sk_buff *skb)
 
 	offset = skb_checksum_start_offset(skb);
 	ret = -EINVAL;
-	if (WARN_ON_ONCE(offset >= skb_headlen(skb))) {
+	if (unlikely(offset >= skb_headlen(skb))) {
 		DO_ONCE_LITE(skb_dump, KERN_ERR, skb, false);
+		WARN_ONCE(true, "offset (%d) >= skb_headlen() (%u)\n",
+			  offset, skb_headlen(skb));
 		goto out;
 	}
 	csum = skb_checksum(skb, offset, skb->len - offset, 0);
 
 	offset += skb->csum_offset;
-	if (WARN_ON_ONCE(offset + sizeof(__sum16) > skb_headlen(skb))) {
+	if (unlikely(offset + sizeof(__sum16) > skb_headlen(skb))) {
 		DO_ONCE_LITE(skb_dump, KERN_ERR, skb, false);
+		WARN_ONCE(true, "offset+2 (%zu) > skb_headlen() (%u)\n",
+			  offset + sizeof(__sum16), skb_headlen(skb));
 		goto out;
 	}
 	ret = skb_ensure_writable(skb, offset + sizeof(__sum16));
-- 
2.42.0.609.gbb76f46606-goog

