[PATCH 1/2] quota: Add mountpath based quota support

[Sascha Hauer <s.hauer@pengutronix.de>]

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 | 77 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)

diff --git a/fs/quota/quota.c b/fs/quota/quota.c
index 6d16b2be5ac4..9ac09e128686 100644
--- a/fs/quota/quota.c
+++ b/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 +969,79 @@ 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)
+{
+	uint cmds, type;
+	struct super_block *sb = NULL;
+	struct path path, *pathp = NULL;
+	struct path mountpath;
+	bool excl = false, thawed = false;
+	int ret;
+
+	cmds = cmd >> SUBCMDSHIFT;
+	type = cmd & SUBCMDMASK;
+
+	if (type >= MAXQUOTAS)
+		return -EINVAL;
+
+	if (!mountpoint)
+		return -ENODEV;
+
+	/*
+	 * Path for quotaon has to be resolved before grabbing superblock
+	 * because that gets s_umount sem which is also possibly needed by path
+	 * resolution (think about autofs) and thus deadlocks could arise.
+	 */
+	if (cmds == Q_QUOTAON) {
+		ret = user_path_at(AT_FDCWD, addr,
+				   LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT, &path);
+		if (ret)
+			pathp = ERR_PTR(ret);
+		else
+			pathp = &path;
+	}
+
+	ret = user_path_at(AT_FDCWD, mountpoint,
+			     LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT, &mountpath);
+	if (ret)
+		goto out;
+
+	if (quotactl_cmd_onoff(cmds)) {
+		excl = true;
+		thawed = true;
+	} else if (quotactl_cmd_write(cmds)) {
+		thawed = true;
+	}
+
+	if (thawed) {
+		ret = mnt_want_write(mountpath.mnt);
+		if (ret)
+			goto out1;
+	}
+
+	sb = mountpath.dentry->d_inode->i_sb;
+
+	if (excl)
+		down_write(&sb->s_umount);
+	else
+		down_read(&sb->s_umount);
+
+	ret = do_quotactl(sb, type, cmds, id, addr, pathp);
+
+	if (excl)
+		up_write(&sb->s_umount);
+	else
+		up_read(&sb->s_umount);
+
+	if (thawed)
+		mnt_drop_write(mountpath.mnt);
+out1:
+	path_put(&mountpath);
+
+out:
+	if (pathp && !IS_ERR(pathp))
+		path_put(pathp);
+	return ret;
+}
-- 
2.20.1