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

[Christophe LEROY]

Le 25/06/2018 Ã 15:24, Stafford Horne a ÃcritÂ:
> On Mon, Jun 25, 2018 at 02:57:13PM +0200, Christophe LEROY wrote:
> > Le 25/06/2018 Ã 14:45, Stafford Horne a ÃcritÂ:
> > > When compiling with GCC 9.0.0 I am seeing the following warning:
> > >
> > >       In function âfill_kobj_pathâ,
> > > 	inlined from âkobject_get_pathâ at lib/kobject.c:155:2:
> > >       lib/kobject.c:128:3: warning: âstrncpyâ 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 âkobject_get_pathâ:
> > >       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@xxxxxx>

> -Stafford
>
> > > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > Cc: Arnd Bergmann <arnd@xxxxxxxx>
> > > Cc: Eric Biggers <ebiggers3@xxxxxxxxx>
> > > Signed-off-by: Stafford Horne <shorne@xxxxxxxxx>
> > > ---
> > >    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) = '/';
> > >    	}