linux-kernel - -Wfortify-source in kernel/printk/printk.c

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Message-ID: <20200130021648.GA32309@ubuntu-x2-xlarge-x86>
Date:   Wed, 29 Jan 2020 19:16:48 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: -Wfortify-source in kernel/printk/printk.c

Hi all,

After commit 6d485ff455e ("Improve static checks for sprintf and
__builtin___sprintf_chk") in clang [1], the following warning appears
when CONFIG_PRINTK is disabled (e.g. allnoconfig):

../kernel/printk/printk.c:2416:10: warning: 'sprintf' will always
overflow; destination buffer has size 0, but format string expands
to at least 33 [-Wfortify-source]
                        len = sprintf(text,
                              ^
1 warning generated.

Specifically referring to
https://elixir.bootlin.com/linux/v5.5/source/kernel/printk/printk.c#L2416.

It isn't wrong, given that when CONFIG_PRINTK is disabled, text's length
is 0 (LOG_LINE_MAX and PREFIX_MAX are both zero). How should this
warning be dealt this? I am not familiar enough with the printk code to
say myself.

[1]: https://github.com/llvm/llvm-project/commit/6d485ff455ea2b37fef9e06e426dae6c1241b231

Cheers,
Nathan