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

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Date:   Sat, 23 Jun 2018 11:50:08 +0900
From:   Stafford Horne <shorne@...il.com>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>, arnd@...db.de,
        linux-crypto@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] kobject: Fix -Wstringop-truncation warning

On Fri, Jun 22, 2018 at 07:31:49PM -0700, Eric Biggers wrote:
> On Sat, Jun 23, 2018 at 11:07:53AM +0900, Stafford Horne wrote:
> > 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 pointing out a bug that the strncpy limit is the source string not the
> > destination buffer remaining length.  Fix it.
> > 
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: Arnd Bergmann <arnd@...db.de>
> > 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..15338e5a96f2 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);
> > +		strncpy(path + length, kobject_name(parent), length);
> >  		*(path + --length) = '/';
> >  	}
> 
> It should be replaced with memcpy(), AFAICS; it wouldn't change the behavior but
> it would quiet the gcc warning.  Your proposed "fix" is heavily broken: notice
> that the code is building a string backwards (end to beginning).

Sorry about that, I didnt notice.  I will fix.

-Stafford