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GCC inserts branch profiling > counters into the skb_ext_total_length() loop and, combined with > -fno-tree-loop-im (which disables loop invariant motion), cannot > constant-fold the result. > BUILD_BUG_ON requires a compile-time constant and fails. > > The issue manifests in kernels with 5+ SKB extension types enabled > (e.g., after addition of SKB_EXT_CAN, SKB_EXT_PSP). With 4 extensions > GCC can still unroll and fold the loop despite GCOV instrumentation; > with 5+ it gives up. > > Mark skb_ext_total_length() with __no_profile to prevent GCOV from > inserting counters into this function. Without counters the loop is > "clean" and GCC can constant-fold it even with -fno-tree-loop-im active. > This allows BUILD_BUG_ON to work correctly while keeping GCOV profiling > for the rest of the kernel. > > This also removes the CONFIG_KCOV_INSTRUMENT_ALL preprocessor guard > introduced by d6e5794b06c0, as __no_profile handles both GCOV and KCOV > instrumentation at the root cause level rather than just disabling the > check. > > Fixes: 5d21d0a65b57 ("net: generalize calculation of skb extensions length") > Fixes: d6e5794b06c0 ("net: avoid build bug in skb extension length calculation") > > Signed-off-by: Konstantin Khorenko > Reviewed-by: Thomas Weißschuh > --- > net/core/skbuff.c | 4 +--- > 1 file changed, 1 insertion(+), 3 deletions(-) > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c > index 0e217041958a..47c7f0ab6e84 100644 > --- a/net/core/skbuff.c > +++ b/net/core/skbuff.c > @@ -5145,7 +5145,7 @@ static const u8 skb_ext_type_len[] = { > #endif > }; > > -static __always_inline unsigned int skb_ext_total_length(void) > +static __always_inline __no_profile unsigned int skb_ext_total_length(void) > { > unsigned int l = SKB_EXT_CHUNKSIZEOF(struct skb_ext); > int i; > @@ -5159,9 +5159,7 @@ 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(),