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: 5d21d0a65b57 ("net: generalize calculation of skb extensions length")
Cc: <[email protected]>
Signed-off-by: Thomas Weißschuh <[email protected]>
---
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 <[email protected]>
On Mon, Dec 18, 2023, at 17:06, Thomas Weißschuh wrote:
> 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: 5d21d0a65b57 ("net: generalize calculation of skb extensions
> length")
> Cc: <[email protected]>
> Signed-off-by: Thomas Weißschuh <[email protected]>
Acked-by: Arnd Bergmann <[email protected]>
> ---
> 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
The way I would write this is
BUILD_BUG_ON(!IS_ENABLED(CONFIG_KCOV_INSTRUMENT_ALL) &&
skb_ext_total_length() > 255);
but of course the effect is the same.
Arnd
Hello:
This patch was applied to netdev/net.git (main)
by Paolo Abeni <[email protected]>:
On Mon, 18 Dec 2023 18:06:54 +0100 you wrote:
> 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.
>
> [...]
Here is the summary with links:
- net: avoid build bug in skb extension length calculation
https://git.kernel.org/netdev/net/c/d6e5794b06c0
You are awesome, thank you!
--
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