quota: Add mountpath based quota support

Add syscall quotactl_path, a variant of quotactl which allows to specify
the mountpath instead of a path of to a block device.

The quotactl syscall expects a path to the mounted block device to
specify the filesystem to work on. This limits usage to filesystems
which actually have a block device. quotactl_path replaces the path
to the block device with a path where the filesystem is mounted at.

The global Q_SYNC command to sync all filesystems is not supported for
this new syscall, otherwise quotactl_path behaves like quotactl.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

fs/quota/quota.c

@@ -17,6 +17,7 @@
 #include <linux/capability.h>
 #include <linux/quotaops.h>
 #include <linux/types.h>
+#include <linux/mount.h>
 #include <linux/writeback.h>
 #include <linux/nospec.h>
 #include "compat.h"
@@ -968,3 +969,51 @@ SYSCALL_DEFINE4(quotactl, unsigned int, cmd, const char __user *, special,
 		path_put(pathp);
 	return ret;
 }
+
+SYSCALL_DEFINE4(quotactl_path, unsigned int, cmd, const char __user *,
+		mountpoint, qid_t, id, void __user *, addr)
+{
+	struct super_block *sb;
+	struct path mountpath;
+	unsigned int cmds = cmd >> SUBCMDSHIFT;
+	unsigned int type = cmd & SUBCMDMASK;
+	int ret;
+
+	if (type >= MAXQUOTAS)
+		return -EINVAL;
+
+	if (!mountpoint)
+		return -ENODEV;
+
+	ret = user_path_at(AT_FDCWD, mountpoint,
+			     LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT, &mountpath);
+	if (ret)
+		return ret;
+
+	sb = mountpath.dentry->d_inode->i_sb;
+
+	if (quotactl_cmd_write(cmds)) {
+		ret = mnt_want_write(mountpath.mnt);
+		if (ret)
+			goto out;
+	}
+
+	if (quotactl_cmd_onoff(cmds))
+		down_write(&sb->s_umount);
+	else
+		down_read(&sb->s_umount);
+
+	ret = do_quotactl(sb, type, cmds, id, addr, ERR_PTR(-EINVAL));
+
+	if (quotactl_cmd_onoff(cmds))
+		up_write(&sb->s_umount);
+	else
+		up_read(&sb->s_umount);
+
+	if (quotactl_cmd_write(cmds))
+		mnt_drop_write(mountpath.mnt);
+out:
+	path_put(&mountpath);
+
+	return ret;
+}