exportfs: be careful to only return expected errors.

When nfsd calls fh_to_dentry, it expect ESTALE or ENOMEM as errors.
In particular it can be tempting to return ENOENT, but this is not
handled well by nfsd.

Rather than requiring strict adherence to error code code filesystems,
treat all unexpected error codes the same as ESTALE.  This is safest.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/exportfs/expfs.c

@@ -428,10 +428,10 @@ struct dentry *exportfs_decode_fh(struct vfsmount *mnt, struct fid *fid,
 	if (!nop || !nop->fh_to_dentry)
 		return ERR_PTR(-ESTALE);
 	result = nop->fh_to_dentry(mnt->mnt_sb, fid, fh_len, fileid_type);
-	if (!result)
-		result = ERR_PTR(-ESTALE);
-	if (IS_ERR(result))
-		return result;
+	if (PTR_ERR(result) == -ENOMEM)
+		return ERR_CAST(result);
+	if (IS_ERR_OR_NULL(result))
+		return ERR_PTR(-ESTALE);
 
 	if (d_is_dir(result)) {
 		/*
@@ -541,6 +541,8 @@ struct dentry *exportfs_decode_fh(struct vfsmount *mnt, struct fid *fid,
 
  err_result:
 	dput(result);
+	if (err != -ENOMEM)
+		err = -ESTALE;
 	return ERR_PTR(err);
 }
 EXPORT_SYMBOL_GPL(exportfs_decode_fh);

include/linux/exportfs.h

@@ -157,12 +157,13 @@ struct fid {
  *    @fh_to_dentry is given a &struct super_block (@sb) and a file handle
  *    fragment (@fh, @fh_len). It should return a &struct dentry which refers
  *    to the same file that the file handle fragment refers to.  If it cannot,
- *    it should return a %NULL pointer if the file was found but no acceptable
- *    &dentries were available, or an %ERR_PTR error code indicating why it
- *    couldn't be found (e.g. %ENOENT or %ENOMEM).  Any suitable dentry can be
- *    returned including, if necessary, a new dentry created with d_alloc_root.
- *    The caller can then find any other extant dentries by following the
- *    d_alias links.
+ *    it should return a %NULL pointer if the file cannot be found, or an
+ *    %ERR_PTR error code of %ENOMEM if a memory allocation failure occurred.
+ *    Any other error code is treated like %NULL, and will cause an %ESTALE error
+ *    for callers of exportfs_decode_fh().
+ *    Any suitable dentry can be returned including, if necessary, a new dentry
+ *    created with d_alloc_root.  The caller can then find any other extant
+ *    dentries by following the d_alias links.
  *
  * fh_to_parent:
  *    Same as @fh_to_dentry, except that it returns a pointer to the parent