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

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Date:   Mon, 25 Jun 2018 15:32:35 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Stafford Horne <shorne@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        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 15:24, Stafford Horne a écrit :
> On Mon, Jun 25, 2018 at 02:57:13PM +0200, Christophe LEROY wrote:
>>
>>
>> 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()' ?
> 
> Hi Christophe,
> 
> Here were are writing multiple strings into a buffer from back to front.  We are
> copying exactly strlen() bytes at a time to avoid the nul terminator being
> copied into the buffer.
> 
> I don't doubt we could use strcpy() but I was trying to keep the change small.

Ok, fair enough.

Reviewed-by: Christophe Leroy <christophe.leroy@....fr>

> 
> -Stafford
> 
>>>
>>> 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) = '/';
>>>    	}
>>>