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

[Miklos Szeredi <miklos@xxxxxxxxxx>]

On Fri, Dec 13, 2019 at 11:38 AM Dan Carpenter <dan.carpenter@xxxxxxxxxx> 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.

Thanks,
Miklos