Received-SPF: pass (google.com: domain of linux-kernel+bounces-3303-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Feedback-ID: i56a14606:Fastmail
User-Agent: Cyrus-JMAP/3.9.0-alpha0-1283-g327e3ec917-fm-20231207.002-g327e3ec9
Precedence: bulk
MIME-Version: 1.0
Message-Id: <b091f9d3-14e9-4f2c-bf98-9a207cef412a@app.fastmail.com>
In-Reply-To: <3fb66648-5581-4371-b15e-23e52e6469ba@t-8ch.de>
References: <202312171924.4FozI5FG-lkp@intel.com>
 <3fb66648-5581-4371-b15e-23e52e6469ba@t-8ch.de>
Date: Mon, 18 Dec 2023 10:12:03 +0000
From: "Arnd Bergmann" <arnd@arndb.de>
To: =?UTF-8?Q?Thomas_Wei=C3=9Fschuh?= <linux@weissschuh.net>,
 "kernel test robot" <lkp@intel.com>
Cc: oe-kbuild-all@lists.linux.dev, linux-kernel@vger.kernel.org,
 "Jakub Kicinski" <kuba@kernel.org>, "Simon Horman" <horms@kernel.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
Content-Type: text/plain;charset=utf-8
Content-Transfer-Encoding: quoted-printable

On Sun, Dec 17, 2023, at 17:13, Thomas Wei=C3=9Fschuh 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/
>>=20
>> All errors (new ones prefixed by >>):
>>=20
>>    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 '__compileti=
me_assert_810' declared with attribute error: BUILD_BUG_ON failed: skb_e=
xt_total_length() > 255

I tried to count the actual number of bytes and got to a worst case of 2=
00
bytes (for 64-bit machines), but this may have been wrong. I can think o=
f 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 entrie=
s.
>
> 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