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Subject[PATCH 4.0 02/58] sysctl: Allow creating permanently empty directories that serve as mountpoints.
Date
4.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: "Eric W. Biederman" <ebiederm@xmission.com>

commit f9bd6733d3f11e24f3949becf277507d422ee1eb upstream.

Add a magic sysctl table sysctl_mount_point that when used to
create a directory forces that directory to be permanently empty.

Update the code to use make_empty_dir_inode when accessing permanently
empty directories.

Update the code to not allow adding to permanently empty directories.

Update /proc/sys/fs/binfmt_misc to be a permanently empty directory.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/proc/proc_sysctl.c | 37 +++++++++++++++++++++++++++++++++++++
include/linux/sysctl.h | 3 +++
kernel/sysctl.c | 8 +-------
3 files changed, 41 insertions(+), 7 deletions(-)

--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -19,6 +19,28 @@ static const struct inode_operations pro
static const struct file_operations proc_sys_dir_file_operations;
static const struct inode_operations proc_sys_dir_operations;

+/* Support for permanently empty directories */
+
+struct ctl_table sysctl_mount_point[] = {
+ { }
+};
+
+static bool is_empty_dir(struct ctl_table_header *head)
+{
+ return head->ctl_table[0].child == sysctl_mount_point;
+}
+
+static void set_empty_dir(struct ctl_dir *dir)
+{
+ dir->header.ctl_table[0].child = sysctl_mount_point;
+}
+
+static void clear_empty_dir(struct ctl_dir *dir)
+
+{
+ dir->header.ctl_table[0].child = NULL;
+}
+
void proc_sys_poll_notify(struct ctl_table_poll *poll)
{
if (!poll)
@@ -187,6 +209,17 @@ static int insert_header(struct ctl_dir
struct ctl_table *entry;
int err;

+ /* Is this a permanently empty directory? */
+ if (is_empty_dir(&dir->header))
+ return -EROFS;
+
+ /* Am I creating a permanently empty directory? */
+ if (header->ctl_table == sysctl_mount_point) {
+ if (!RB_EMPTY_ROOT(&dir->root))
+ return -EINVAL;
+ set_empty_dir(dir);
+ }
+
dir->header.nreg++;
header->parent = dir;
err = insert_links(header);
@@ -202,6 +235,8 @@ fail:
erase_header(header);
put_links(header);
fail_links:
+ if (header->ctl_table == sysctl_mount_point)
+ clear_empty_dir(dir);
header->parent = NULL;
drop_sysctl_table(&dir->header);
return err;
@@ -419,6 +454,8 @@ static struct inode *proc_sys_make_inode
inode->i_mode |= S_IFDIR;
inode->i_op = &proc_sys_dir_operations;
inode->i_fop = &proc_sys_dir_file_operations;
+ if (is_empty_dir(head))
+ make_empty_dir_inode(inode);
}
out:
return inode;
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -188,6 +188,9 @@ struct ctl_table_header *register_sysctl
void unregister_sysctl_table(struct ctl_table_header * table);

extern int sysctl_init(void);
+
+extern struct ctl_table sysctl_mount_point[];
+
#else /* CONFIG_SYSCTL */
static inline struct ctl_table_header *register_sysctl_table(struct ctl_table * table)
{
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1510,12 +1510,6 @@ static struct ctl_table vm_table[] = {
{ }
};

-#if defined(CONFIG_BINFMT_MISC) || defined(CONFIG_BINFMT_MISC_MODULE)
-static struct ctl_table binfmt_misc_table[] = {
- { }
-};
-#endif
-
static struct ctl_table fs_table[] = {
{
.procname = "inode-nr",
@@ -1669,7 +1663,7 @@ static struct ctl_table fs_table[] = {
{
.procname = "binfmt_misc",
.mode = 0555,
- .child = binfmt_misc_table,
+ .child = sysctl_mount_point,
},
#endif
{



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