fs: Teach path_connected to handle nfs filesystems with multiple roots.

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

It is not pretty but I believe in practice the code is fine.

!server implies sget returned a preexisting superblock.

!s->s_root implies !(s->s_flags & SB_BORN)

sget will not return a superblock without SB_BORN being set. Further it is mount_fs our ``caller'' (via way of whatever calls nfs_mount_common) that sets SB_BORN when .mount succeeds.

Eric