merge fchmod() and fchmodat() guts, kill ancient broken kludge

The kludge in question is undocumented and doesn't work for 32bit
binaries on amd64, sparc64 and s390.  Passing (mode_t)-1 as
mode had (since 0.99.14v and contrary to behaviour of any
other Unix, prescriptions of POSIX, SuS and our own manpages)
was kinda-sorta no-op.  Note that any software relying on
that (and looking for examples shows none) would be visibly
broken on sparc64, where practically all userland is built
32bit.  No such complaints noticed...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/open.c

@@ -446,74 +446,52 @@ out:
 	return error;
 }
 
-SYSCALL_DEFINE2(fchmod, unsigned int, fd, mode_t, mode)
+static int chmod_common(struct path *path, umode_t mode)
 {
-	struct inode * inode;
-	struct dentry * dentry;
-	struct file * file;
-	int err = -EBADF;
+	struct inode *inode = path->dentry->d_inode;
 	struct iattr newattrs;
+	int error;
 
-	file = fget(fd);
-	if (!file)
-		goto out;
-
-	dentry = file->f_path.dentry;
-	inode = dentry->d_inode;
-
-	audit_inode(NULL, dentry);
-
-	err = mnt_want_write_file(file);
-	if (err)
-		goto out_putf;
+	error = mnt_want_write(path->mnt);
+	if (error)
+		return error;
 	mutex_lock(&inode->i_mutex);
-	err = security_path_chmod(dentry, file->f_vfsmnt, mode);
-	if (err)
+	error = security_path_chmod(path->dentry, path->mnt, mode);
+	if (error)
 		goto out_unlock;
-	if (mode == (mode_t) -1)
-		mode = inode->i_mode;
 	newattrs.ia_mode = (mode & S_IALLUGO) | (inode->i_mode & ~S_IALLUGO);
 	newattrs.ia_valid = ATTR_MODE | ATTR_CTIME;
-	err = notify_change(dentry, &newattrs);
+	error = notify_change(path->dentry, &newattrs);
 out_unlock:
 	mutex_unlock(&inode->i_mutex);
-	mnt_drop_write(file->f_path.mnt);
-out_putf:
-	fput(file);
-out:
+	mnt_drop_write(path->mnt);
+	return error;
+}
+
+SYSCALL_DEFINE2(fchmod, unsigned int, fd, mode_t, mode)
+{
+	struct file * file;
+	int err = -EBADF;
+
+	file = fget(fd);
+	if (file) {
+		audit_inode(NULL, file->f_path.dentry);
+		err = chmod_common(&file->f_path, mode);
+		fput(file);
+	}
 	return err;
 }
 
 SYSCALL_DEFINE3(fchmodat, int, dfd, const char __user *, filename, mode_t, mode)
 {
 	struct path path;
-	struct inode *inode;
 	int error;
-	struct iattr newattrs;
 
 	error = user_path_at(dfd, filename, LOOKUP_FOLLOW, &path);
-	if (error)
-		goto out;
-	inode = path.dentry->d_inode;
-
-	error = mnt_want_write(path.mnt);
-	if (error)
-		goto dput_and_out;
-	mutex_lock(&inode->i_mutex);
-	error = security_path_chmod(path.dentry, path.mnt, mode);
-	if (error)
-		goto out_unlock;
-	if (mode == (mode_t) -1)
-		mode = inode->i_mode;
-	newattrs.ia_mode = (mode & S_IALLUGO) | (inode->i_mode & ~S_IALLUGO);
-	newattrs.ia_valid = ATTR_MODE | ATTR_CTIME;
-	error = notify_change(path.dentry, &newattrs);
-out_unlock:
-	mutex_unlock(&inode->i_mutex);
-	mnt_drop_write(path.mnt);
-dput_and_out:
-	path_put(&path);
-out:
+	if (!error) {
+		error = chmod_common(&path, mode);
+		path_put(&path);
+	}
 	return error;
 }