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 <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Suggested-by: Arnd Bergmann <[email protected]>
Link: https://lore.kernel.org/lkml/[email protected]/
Fixes: 5d21d0a ("net: generalize calculation of skb extensions length")
Cc: <[email protected]>
Signed-off-by: Thomas Weißschuh <[email protected]>
Acked-by: Arnd Bergmann <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: t-8ch

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(),