[bug report] VFS: add common error checks to lookup_one_qstr_excl()

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

Hello NeilBrown,

Commit 22d9d5e93d0e ("VFS: add common error checks to
lookup_one_qstr_excl()") from Feb 7, 2025 (linux-next), leads to the
following Smatch static checker warning:

	fs/namei.c:1696 lookup_one_qstr_excl()
	error: 'dentry' dereferencing possible ERR_PTR()

fs/namei.c
  1671  struct dentry *lookup_one_qstr_excl(const struct qstr *name,
  1672                                      struct dentry *base,
  1673                                      unsigned int flags)
  1674  {
  1675          struct dentry *dentry = lookup_dcache(name, base, flags);
  1676          struct dentry *old;
  1677          struct inode *dir = base->d_inode;
  1678  
  1679          if (dentry)
  1680                  goto found;

It looks like lookup_dcache() can return both error pointers and NULL.

  1681  
  1682          /* Don't create child dentry for a dead directory. */
  1683          if (unlikely(IS_DEADDIR(dir)))
  1684                  return ERR_PTR(-ENOENT);
  1685  
  1686          dentry = d_alloc(base, name);
  1687          if (unlikely(!dentry))
  1688                  return ERR_PTR(-ENOMEM);
  1689  
  1690          old = dir->i_op->lookup(dir, dentry, flags);
  1691          if (unlikely(old)) {
  1692                  dput(dentry);
  1693                  dentry = old;
  1694          }
  1695  found:
  1696          if (d_is_negative(dentry) && !(flags & LOOKUP_CREATE)) {
                                  ^^^^^^
Unchecked dereference.

  1697                  dput(dentry);
  1698                  return ERR_PTR(-ENOENT);
  1699          }
  1700          if (d_is_positive(dentry) && (flags & LOOKUP_EXCL)) {
  1701                  dput(dentry);
  1702                  return ERR_PTR(-EEXIST);
  1703          }
  1704          return dentry;
  1705  }

regards,
dan carpenter