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

-Stafford

> >
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Eric Biggers <ebiggers3@gmail.com>
> > Signed-off-by: Stafford Horne <shorne@gmail.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) = '/';
> > }
> >

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