[PATCH 01/10] fs: add is_mapped_mnt() helper

[Christian Brauner <brauner@kernel.org>]

From: Christian Brauner <christian.brauner@ubuntu.com>

Multiple places open-code the same check to determine whether a given
mount is idmapped. Introduce a simple helper function that can be used
instead. This allows us to get rid of the fragile open-coding. We will
later change the check that is used to determine whether a given mount
is idmapped. Introducing a helper allows us to do this in a single
place instead of doing it for multiple places.

Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
CC: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 fs/cachefiles/bind.c |  2 +-
 fs/ecryptfs/main.c   |  2 +-
 fs/namespace.c       |  2 +-
 fs/nfsd/export.c     |  2 +-
 fs/overlayfs/super.c |  2 +-
 fs/proc_namespace.c  |  2 +-
 include/linux/fs.h   | 14 ++++++++++++++
 7 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/fs/cachefiles/bind.c b/fs/cachefiles/bind.c
index d463d89f5db8..8130142d89c2 100644
--- a/fs/cachefiles/bind.c
+++ b/fs/cachefiles/bind.c
@@ -117,7 +117,7 @@ static int cachefiles_daemon_add_cache(struct cachefiles_cache *cache)
 	root = path.dentry;
 
 	ret = -EINVAL;
-	if (mnt_user_ns(path.mnt) != &init_user_ns) {
+	if (is_mapped_mnt(path.mnt)) {
 		pr_warn("File cache on idmapped mounts not supported");
 		goto error_unsupported;
 	}
diff --git a/fs/ecryptfs/main.c b/fs/ecryptfs/main.c
index d66bbd2df191..331ac3a59515 100644
--- a/fs/ecryptfs/main.c
+++ b/fs/ecryptfs/main.c
@@ -537,7 +537,7 @@ static struct dentry *ecryptfs_mount(struct file_system_type *fs_type, int flags
 		goto out_free;
 	}
 
-	if (mnt_user_ns(path.mnt) != &init_user_ns) {
+	if (is_mapped_mnt(path.mnt)) {
 		rc = -EINVAL;
 		printk(KERN_ERR "Mounting on idmapped mounts currently disallowed\n");
 		goto out_free;
diff --git a/fs/namespace.c b/fs/namespace.c
index 659a8f39c61a..7d7b80b375a4 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3936,7 +3936,7 @@ static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
 	 * mapping. It makes things simpler and callers can just create
 	 * another bind-mount they can idmap if they want to.
 	 */
-	if (mnt_user_ns(m) != &init_user_ns)
+	if (is_mapped_mnt(m))
 		return -EPERM;
 
 	/* The underlying filesystem doesn't support idmapped mounts yet. */
diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
index 9421dae22737..292bde9e1eb3 100644
--- a/fs/nfsd/export.c
+++ b/fs/nfsd/export.c
@@ -427,7 +427,7 @@ static int check_export(struct path *path, int *flags, unsigned char *uuid)
 		return -EINVAL;
 	}
 
-	if (mnt_user_ns(path->mnt) != &init_user_ns) {
+	if (is_mapped_mnt(path->mnt)) {
 		dprintk("exp_export: export of idmapped mounts not yet supported.\n");
 		return -EINVAL;
 	}
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 265181c110ae..113575fc6155 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -873,7 +873,7 @@ static int ovl_mount_dir_noesc(const char *name, struct path *path)
 		pr_err("filesystem on '%s' not supported\n", name);
 		goto out_put;
 	}
-	if (mnt_user_ns(path->mnt) != &init_user_ns) {
+	if (is_mapped_mnt(path->mnt)) {
 		pr_err("idmapped layers are currently not supported\n");
 		goto out_put;
 	}
diff --git a/fs/proc_namespace.c b/fs/proc_namespace.c
index 392ef5162655..788c687bb052 100644
--- a/fs/proc_namespace.c
+++ b/fs/proc_namespace.c
@@ -80,7 +80,7 @@ static void show_mnt_opts(struct seq_file *m, struct vfsmount *mnt)
 			seq_puts(m, fs_infop->str);
 	}
 
-	if (mnt_user_ns(mnt) != &init_user_ns)
+	if (is_mapped_mnt(mnt))
 		seq_puts(m, ",idmapped");
 }
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 1cb616fc1105..192242476b2b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2725,6 +2725,20 @@ static inline struct user_namespace *file_mnt_user_ns(struct file *file)
 {
 	return mnt_user_ns(file->f_path.mnt);
 }
+
+/**
+ * is_mapped_mnt - check whether a mount is mapped
+ * @mnt: the mount to check
+ *
+ * If @mnt has an idmapping attached to it @mnt is mapped.
+ *
+ * Return: true if mount is mapped, false if not.
+ */
+static inline bool is_mapped_mnt(const struct vfsmount *mnt)
+{
+	return mnt_user_ns(mnt) != &init_user_ns;
+}
+
 extern long vfs_truncate(const struct path *, loff_t);
 int do_truncate(struct user_namespace *, struct dentry *, loff_t start,
 		unsigned int time_attrs, struct file *filp);
-- 
2.30.2