linux-kernel - Re: include/linux/compiler_types.h:397:45: error: call to '__compiletime_assert_810' declared with attribute error: BUILD_BUG_ON failed: skb_ext_total

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Date: Mon, 18 Dec 2023 10:12:03 +0000
From: "Arnd Bergmann" <arnd@...db.de>
To: Thomas Weißschuh <linux@...ssschuh.net>,
 "kernel test robot" <lkp@...el.com>
Cc: oe-kbuild-all@...ts.linux.dev, linux-kernel@...r.kernel.org,
 "Jakub Kicinski" <kuba@...nel.org>, "Simon Horman" <horms@...nel.org>
Subject: Re: include/linux/compiler_types.h:397:45: error: call to
 '__compiletime_assert_810' declared with attribute error: BUILD_BUG_ON
 failed: skb_ext_total_length() > 255

On Sun, Dec 17, 2023, at 17:13, Thomas Weißschuh wrote:
> +Cc Arnd who was taking care of CFLAGS_GCOV in the past.
>
> On 2023-12-17 19:39:34+0800, kernel test robot wrote:
>> | Closes: https://lore.kernel.org/oe-kbuild-all/202312171924.4FozI5FG-lkp@intel.com/
>> 
>> All errors (new ones prefixed by >>):
>> 
>>    In file included from <command-line>:
>>    In function 'skb_extensions_init',
>>        inlined from 'skb_init' at net/core/skbuff.c:4848:2:
>> >> include/linux/compiler_types.h:397:45: error: call to '__compiletime_assert_810' declared with attribute error: BUILD_BUG_ON failed: skb_ext_total_length() > 255

I tried to count the actual number of bytes and got to a worst case of 200
bytes (for 64-bit machines), but this may have been wrong. I can think of two
possible things that may have caused the problem:

a) there is an actual overflow but gcc fails to realize it without GCOV
b) the compile-time calculation goes wrong and is no longer a constant
   value, so the assertion fails to evaluate

We can probably elinminate a) if you can show that raising the limit does
not avoid the problem.

> [..]
>
> This seems to be a compiler bug/configuration issue.
>
> When I remove the entry for SKB_EXT_MCTP from skb_ext_type_len then the
> error goes away. However this entry works the same as all other entries.
>
> Also dropping -fno-tree-loop-im *or* -fprofile-arcs from CFLAGS_GCOV
> makes the code compile as-is.
>
> Or switching to a 64bit build...

The -fno-tree-loop-im option would likely stop the loop from getting
unrolled, which is how the skb_ext_total_length() return code is no
longer constant.

Does manually unrolling this loop avoid the problem?

       Arnd