linux-kernel - Re: [PATCH] kallsyms: work around bogus -Wrestrict warning

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Message-Id: <20200108102602.43d4c5433eb495cdbf387e9b@kernel.org>
Date:   Wed, 8 Jan 2020 10:26:02 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Oleksandr Natalenko <oleksandr@...hat.com>, linux-mm@...ck.org,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Will Deacon <will@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Song Liu <songliubraving@...com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Marc Zyngier <maz@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kallsyms: work around bogus -Wrestrict warning

Hi Arnd,

On Tue,  7 Jan 2020 22:40:26 +0100
Arnd Bergmann <arnd@...db.de> wrote:

> gcc -O3 produces some really odd warnings for this file:
> 
> kernel/kallsyms.c: In function 'sprint_symbol':
> kernel/kallsyms.c:369:3: error: 'strcpy' source argument is the same as destination [-Werror=restrict]
>    strcpy(buffer, name);
>    ^~~~~~~~~~~~~~~~~~~~
> kernel/kallsyms.c: In function 'sprint_symbol_no_offset':
> kernel/kallsyms.c:369:3: error: 'strcpy' source argument is the same as destination [-Werror=restrict]
>    strcpy(buffer, name);
>    ^~~~~~~~~~~~~~~~~~~~
> kernel/kallsyms.c: In function 'sprint_backtrace':
> kernel/kallsyms.c:369:3: error: 'strcpy' source argument is the same as destination [-Werror=restrict]
>    strcpy(buffer, name);
>    ^~~~~~~~~~~~~~~~~~~~
> 
> This obviously cannot be since it is preceded by an 'if (name != buffer)'
> check.

Hmm, this looks like a bug in gcc.

> 
> Using sprintf() instead of strcpy() is a bit wasteful but is
> the best workaround I could come up with.
> 
> Fixes: mmtom ("init/Kconfig: enable -O3 for all arches")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  kernel/kallsyms.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index d812b90f4c86..726b8eeb223e 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -366,7 +366,7 @@ static int __sprint_symbol(char *buffer, unsigned long address,
>  		return sprintf(buffer, "0x%lx", address - symbol_offset);
>  
>  	if (name != buffer)
> -		strcpy(buffer, name);
> +		sprintf(buffer, "%s", name);

BTW, this seems not happen. kallsyms_lookup() (and it's subfunctions)
always stores the result into buffer unless name == NULL.
Maybe we can remove these 2 lines?
(and add a comment line for kallsyms_lookup() so that it guarantees the
 symbol name always stored in namebuf argument)

Thank you,


-- 
Masami Hiramatsu <mhiramat@...nel.org>