From: Sascha Hauer <s.hauer@pengutronix.de>
Date: January 22, 2021 at 07:17:45 PST
To: linux-fsdevel@vger.kernel.org
Cc: Richard Weinberger <richard@nod.at>, linux-mtd@lists.infradead.org, kernel@pengutronix.de, Jan Kara <jack@suse.com>, Sascha Hauer <s.hauer@pengutronix.de>
Subject: [PATCH 1/8] quota: Allow to pass mount path to quotactl

This patch introduces the Q_PATH flag to the quotactl cmd argument.
When given, the path given in the special argument to quotactl will
be the mount path where the filesystem is mounted, instead of a path
to the block device.
This is necessary for filesystems which do not have a block device as
backing store. Particularly this is done for upcoming UBIFS support.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
---
fs/quota/quota.c           | 66 ++++++++++++++++++++++++++++----------
include/uapi/linux/quota.h |  1 +
2 files changed, 50 insertions(+), 17 deletions(-)

diff --git a/fs/quota/quota.c b/fs/quota/quota.c
index 6d16b2be5ac4..f653b27a9a4e 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"
@@ -859,25 +860,10 @@ static bool quotactl_cmd_onoff(int cmd)
        (cmd == Q_XQUOTAON) || (cmd == Q_XQUOTAOFF);
}

-/*
- * look up a superblock on which quota ops will be performed
- * - use the name of a block device to find the superblock thereon
- */
-static struct super_block *quotactl_block(const char __user *special, int cmd)
+static struct super_block *quotactl_sb(dev_t dev, int cmd)
{
-#ifdef CONFIG_BLOCK
   struct super_block *sb;
-    struct filename *tmp = getname(special);
   bool excl = false, thawed = false;
-    int error;
-    dev_t dev;
-
-    if (IS_ERR(tmp))
-        return ERR_CAST(tmp);
-    error = lookup_bdev(tmp->name, &dev);
-    putname(tmp);
-    if (error)
-        return ERR_PTR(error);

   if (quotactl_cmd_onoff(cmd)) {
       excl = true;
@@ -901,12 +887,50 @@ static struct super_block *quotactl_block(const char __user *special, int cmd)
       goto retry;
   }
   return sb;
+}
+
+/*
+ * look up a superblock on which quota ops will be performed
+ * - use the name of a block device to find the superblock thereon
+ */
+static struct super_block *quotactl_block(const char __user *special, int cmd)
+{
+#ifdef CONFIG_BLOCK
+    struct filename *tmp = getname(special);
+    int error;
+    dev_t dev;

+    if (IS_ERR(tmp))
+        return ERR_CAST(tmp);
+    error = lookup_bdev(tmp->name, &dev);
+    putname(tmp);
+    if (error)
+        return ERR_PTR(error);
+
+    return quotactl_sb(dev, cmd);
#else
   return ERR_PTR(-ENODEV);
#endif
}

+static struct super_block *quotactl_path(const char __user *special, int cmd)
+{
+    struct super_block *sb;
+    struct path path;
+    int error;
+
+    error = user_path_at(AT_FDCWD, special, LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT,
+               &path);
+    if (error)
+        return ERR_PTR(error);
+
+    sb = quotactl_sb(path.mnt->mnt_sb->s_dev, cmd);
+
+    path_put(&path);
+
+    return sb;
+}
+
/*
 * This is the system call interface. This communicates with
 * the user-level programs. Currently this only supports diskquota
@@ -920,6 +944,7 @@ SYSCALL_DEFINE4(quotactl, unsigned int, cmd, const char __user *, special,
   struct super_block *sb = NULL;
   struct path path, *pathp = NULL;
   int ret;
+    bool q_path;

   cmds = cmd >> SUBCMDSHIFT;
   type = cmd & SUBCMDMASK;
@@ -927,6 +952,9 @@ SYSCALL_DEFINE4(quotactl, unsigned int, cmd, const char __user *, special,
   if (type >= MAXQUOTAS)
       return -EINVAL;

+    q_path = cmds & Q_PATH;
+    cmds &= ~Q_PATH;
+
   /*
    * As a special case Q_SYNC can be called without a specific device.
    * It will iterate all superblocks that have quota enabled and call
@@ -951,7 +979,11 @@ SYSCALL_DEFINE4(quotactl, unsigned int, cmd, const char __user *, special,
           pathp = &path;
   }

-    sb = quotactl_block(special, cmds);
+    if (q_path)
+        sb = quotactl_path(special, cmds);
+    else
+        sb = quotactl_block(special, cmds);
+
   if (IS_ERR(sb)) {
       ret = PTR_ERR(sb);
       goto out;
diff --git a/include/uapi/linux/quota.h b/include/uapi/linux/quota.h
index f17c9636a859..e1787c0df601 100644
--- a/include/uapi/linux/quota.h
+++ b/include/uapi/linux/quota.h
@@ -71,6 +71,7 @@
#define Q_GETQUOTA 0x800007    /* get user quota structure */
#define Q_SETQUOTA 0x800008    /* set user quota structure */
#define Q_GETNEXTQUOTA 0x800009    /* get disk limits and usage >= ID */
+#define Q_PATH     0x400000    /* quotactl special arg contains mount path */

/* Quota format type IDs */
#define    QFMT_VFS_OLD 1
--
2.20.1