netdev - [PATCH] net: avoid build bug in skb extension length calculation

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Message-Id: <20231218-net-skbuff-build-bug-v1-1-eefc2fb0a7d3@weissschuh.net>
Date: Mon, 18 Dec 2023 18:06:54 +0100
From: Thomas Weißschuh <linux@...ssschuh.net>
To: "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 kernel test robot <lkp@...el.com>, Arnd Bergmann <arnd@...db.de>, 
 stable@...r.kernel.org, 
 Thomas Weißschuh <linux@...ssschuh.net>
Subject: [PATCH] net: avoid build bug in skb extension length calculation

GCC seems to incorrectly fail to evaluate skb_ext_total_length() at
compile time under certain conditions.

The issue even occurs if all values in skb_ext_type_len[] are "0",
ruling out the possibility of an actual overflow.

As the patch has been in mainline since v6.6 without triggering the
problem it seems to be a very uncommon occurrence.

As the issue only occurs when -fno-tree-loop-im is specified as part of
CFLAGS_GCOV, disable the BUILD_BUG_ON() only when building with coverage
reporting enabled.

Reported-by: kernel test robot <lkp@...el.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312171924.4FozI5FG-lkp@intel.com/
Suggested-by: Arnd Bergmann <arnd@...db.de>
Link: https://lore.kernel.org/lkml/487cfd35-fe68-416f-9bfd-6bb417f98304@app.fastmail.com/
Fixes: 5d21d0a65b57 ("net: generalize calculation of skb extensions length")
Cc:  <stable@...r.kernel.org>
Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
---
 net/core/skbuff.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 83af8aaeb893..94cc40a6f797 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4825,7 +4825,9 @@ static __always_inline unsigned int skb_ext_total_length(void)
 static void skb_extensions_init(void)
 {
 	BUILD_BUG_ON(SKB_EXT_NUM >= 8);
+#if !IS_ENABLED(CONFIG_KCOV_INSTRUMENT_ALL)
 	BUILD_BUG_ON(skb_ext_total_length() > 255);
+#endif

 	skbuff_ext_cache = kmem_cache_create("skbuff_ext_cache",
 					     SKB_EXT_ALIGN_VALUE * skb_ext_total_length(),

---
base-commit: ceb6a6f023fd3e8b07761ed900352ef574010bcb
change-id: 20231218-net-skbuff-build-bug-4a7c1103d0a6

Best regards,
-- 
Thomas Weißschuh <linux@...ssschuh.net>