Re: [bug report] ovl: make sure that real fid is 32bit aligned in memory

[Dan Carpenter <dan.carpenter@oracle.com>]

On Fri, Dec 13, 2019 at 01:22:10PM +0100, Miklos Szeredi wrote:
> On Fri, Dec 13, 2019 at 11:38 AM Dan Carpenter <dan.carpenter@oracle.com> wrote:
> >
> > Hello Amir Goldstein,
> >
> > This is a semi-automatic email about new static checker warnings.
> >
> > The patch cbe7fba8edfc: "ovl: make sure that real fid is 32bit
> > aligned in memory" from Nov 15, 2019, leads to the following Smatch
> > complaint:
> >
> >     fs/overlayfs/copy_up.c:338 ovl_set_origin()
> >      warn: variable dereferenced before check 'fh' (see line 337)
> >
> > fs/overlayfs/copy_up.c
> >    336           */
> >    337          err = ovl_check_setxattr(dentry, upper, OVL_XATTR_ORIGIN, fh->buf,
> >                                                                           ^^^^^^^
> > The patch adds an unconditional dereference
> 
> But in fact fh->buf is not a dereference:
> 
> struct ovl_fh {
>     u8 padding[3];    /* make sure fb.fid is 32bit aligned */
>     union {
>         struct ovl_fb fb;
>         u8 buf[0];
>     };
> } __packed;
> 
> Subsequent code will also not dereference fh->buf, because the
> supplied size is zero.

Ah yes.  Thanks.  Smatch got confused because the array is inside a
union.  Sorry.

regards,
dan carpenter