linux-kernel - Re: [PATCH v2 2/2] kobject: Fix -Wstringop-truncation warning

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Date:   Mon, 25 Jun 2018 14:57:13 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Stafford Horne <shorne@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Greg KH <gregkh@...uxfoundation.org>, arnd@...db.de,
        Eric Biggers <ebiggers3@...il.com>,
        linux-crypto@...r.kernel.org
Subject: Re: [PATCH v2 2/2] kobject: Fix -Wstringop-truncation warning



Le 25/06/2018 à 14:45, Stafford Horne a écrit :
> When compiling with GCC 9.0.0 I am seeing the following warning:
> 
>      In function ‘fill_kobj_path’,
> 	inlined from ‘kobject_get_path’ at lib/kobject.c:155:2:
>      lib/kobject.c:128:3: warning: ‘strncpy’ output truncated before terminating nul copying as many bytes from a string as its length [-Wstringop-truncation]
>         strncpy(path + length, kobject_name(parent), cur);
>         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>      lib/kobject.c: In function ‘kobject_get_path’:
>      lib/kobject.c:125:13: note: length computed here
>         int cur = strlen(kobject_name(parent));
> 		 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> This is not really an issue since the buffer we are writing to is
> pre-zero'd and we have already allocated the buffer based on the
> calculated strlen size and accounted for the terminating '\0'.
> Just use memcpy() instead.

If we are already sure the destination is big enough, why not just do a 
strcpy() and drop the 'cur = strlen()' ?

Christophe

> 
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Eric Biggers <ebiggers3@...il.com>
> Signed-off-by: Stafford Horne <shorne@...il.com>
> ---
>   lib/kobject.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/kobject.c b/lib/kobject.c
> index 18989b5b3b56..e876957743c8 100644
> --- a/lib/kobject.c
> +++ b/lib/kobject.c
> @@ -125,7 +125,7 @@ static void fill_kobj_path(struct kobject *kobj, char *path, int length)
>   		int cur = strlen(kobject_name(parent));
>   		/* back up enough to print this name with '/' */
>   		length -= cur;
> -		strncpy(path + length, kobject_name(parent), cur);
> +		memcpy(path + length, kobject_name(parent), cur);
>   		*(path + --length) = '/';
>   	}
>   
>