[bug report] fs: Teach path_connected to handle nfs filesystems with multiple roots.

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

Hello Eric W. Biederman,

The patch 95dd77580ccd: "fs: Teach path_connected to handle nfs
filesystems with multiple roots." from Mar 14, 2018, leads to the
following static checker warning:

	fs/nfs/super.c:2634 nfs_fs_mount_common()
	error: potential NULL dereference 'server'.

fs/nfs/super.c
  2598          if (server->flags & NFS_MOUNT_UNSHARED)
  2599                  compare_super = NULL;
  2600  
  2601          /* -o noac implies -o sync */
  2602          if (server->flags & NFS_MOUNT_NOAC)
  2603                  sb_mntdata.mntflags |= SB_SYNCHRONOUS;
  2604  
  2605          if (mount_info->cloned != NULL && mount_info->cloned->sb != NULL)
  2606                  if (mount_info->cloned->sb->s_flags & SB_SYNCHRONOUS)
  2607                          sb_mntdata.mntflags |= SB_SYNCHRONOUS;
  2608  
  2609          /* Get a superblock - note that we may end up sharing one that already exists */
  2610          s = sget(nfs_mod->nfs_fs, compare_super, nfs_set_super, flags, &sb_mntdata);
  2611          if (IS_ERR(s)) {
  2612                  mntroot = ERR_CAST(s);
  2613                  goto out_err_nosb;
  2614          }
  2615  
  2616          if (s->s_fs_info != server) {
  2617                  nfs_free_server(server);
  2618                  server = NULL;
                        ^^^^^^^^^^^^^
We set server to NULL here.  Smatch and I don't know the value of
s->s_root at this point, but maybe it can't be NULL here?  If so, that's
fine then.

  2619          } else {
  2620                  error = super_setup_bdi_name(s, "%u:%u", MAJOR(server->s_dev),
  2621                                               MINOR(server->s_dev));
  2622                  if (error) {
  2623                          mntroot = ERR_PTR(error);
  2624                          goto error_splat_super;
  2625                  }
  2626                  s->s_bdi->ra_pages = server->rpages * NFS_MAX_READAHEAD;
  2627                  server->super = s;
  2628          }
  2629  
  2630          if (!s->s_root) {
  2631                  /* initial superblock/root creation */
  2632                  mount_info->fill_super(s, mount_info);
  2633                  nfs_get_cache_cookie(s, mount_info->parsed, mount_info->cloned);
  2634                  if (!(server->flags & NFS_MOUNT_UNSHARED))
                              ^^^^^^^^^^^^^
Potential NULL dereference.

  2635                          s->s_iflags |= SB_I_MULTIROOT;
  2636          }

regards,
dan carpenter