[PATCH] sysctl: simplify handling of permanently empty directories

[Thomas Weißschuh]

---
 fs/proc/proc_sysctl.c  | 76 +++++++++++++++++++-----------------------
 include/linux/sysctl.h | 13 ++------
 2 files changed, 36 insertions(+), 53 deletions(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index c92e9b972ada..c4d6d09b0e68 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -17,6 +17,7 @@
 #include <linux/bpf-cgroup.h>
 #include <linux/mount.h>
 #include <linux/kmemleak.h>
+#include <linux/cleanup.h>
 #include "internal.h"
 
 #define list_for_each_table_entry(entry, header)	\
@@ -29,32 +30,6 @@ static const struct inode_operations proc_sys_inode_operations;
 static const struct file_operations proc_sys_dir_file_operations;
 static const struct inode_operations proc_sys_dir_operations;
 
-/* Support for permanently empty directories */
-static const struct ctl_table sysctl_mount_point[] = {
-	{ }
-};
-
-/**
- * register_sysctl_mount_point() - registers a sysctl mount point
- * @path: path for the mount point
- *
- * Used to create a permanently empty directory to serve as mount point.
- * There are some subtle but important permission checks this allows in the
- * case of unprivileged mounts.
- */
-struct ctl_table_header *register_sysctl_mount_point(const char *path)
-{
-	return register_sysctl(path, sysctl_mount_point);
-}
-EXPORT_SYMBOL(register_sysctl_mount_point);
-
-#define sysctl_is_perm_empty_ctl_header(hptr)		\
-	(hptr->type == SYSCTL_TABLE_TYPE_PERMANENTLY_EMPTY)
-#define sysctl_set_perm_empty_ctl_header(hptr)		\
-	(hptr->type = SYSCTL_TABLE_TYPE_PERMANENTLY_EMPTY)
-#define sysctl_clear_perm_empty_ctl_header(hptr)		\
-	(hptr->type = SYSCTL_TABLE_TYPE_DEFAULT)
-
 void proc_sys_poll_notify(struct ctl_table_poll *poll)
 {
 	if (!poll)
@@ -199,8 +174,6 @@ static void init_header(struct ctl_table_header *head,
 	head->set = set;
 	head->parent = NULL;
 	head->node = node;
-	if (table == sysctl_mount_point)
-		sysctl_set_perm_empty_ctl_header(head);
 	INIT_HLIST_HEAD(&head->inodes);
 	if (node) {
 		const struct ctl_table *entry;
@@ -228,17 +201,9 @@ static int insert_header(struct ctl_dir *dir, struct ctl_table_header *header)
 
 
 	/* Is this a permanently empty directory? */
-	if (sysctl_is_perm_empty_ctl_header(dir_h))
+	if (dir->permanently_empty)
 		return -EROFS;
 
-	/* Am I creating a permanently empty directory? */
-	if (header->ctl_table_size > 0 &&
-	    sysctl_is_perm_empty_ctl_header(header)) {
-		if (!RB_EMPTY_ROOT(&dir->root))
-			return -EINVAL;
-		sysctl_set_perm_empty_ctl_header(dir_h);
-	}
-
 	dir_h->nreg++;
 	header->parent = dir;
 	err = insert_links(header);
@@ -254,8 +219,6 @@ static int insert_header(struct ctl_dir *dir, struct ctl_table_header *header)
 	erase_header(header);
 	put_links(header);
 fail_links:
-	if (header->ctl_table == sysctl_mount_point)
-		sysctl_clear_perm_empty_ctl_header(dir_h);
 	header->parent = NULL;
 	drop_sysctl_table(dir_h);
 	return err;
@@ -442,6 +405,7 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
 		struct ctl_table_header *head, const struct ctl_table *table)
 {
 	struct ctl_table_root *root = head->root;
+	struct ctl_dir *ctl_dir;
 	struct inode *inode;
 	struct proc_inode *ei;
 
@@ -475,7 +439,9 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
 		inode->i_mode |= S_IFDIR;
 		inode->i_op = &proc_sys_dir_operations;
 		inode->i_fop = &proc_sys_dir_file_operations;
-		if (sysctl_is_perm_empty_ctl_header(head))
+
+		ctl_dir = container_of(head, struct ctl_dir, header);
+		if (ctl_dir->permanently_empty)
 			make_empty_dir_inode(inode);
 	}
 
@@ -1214,8 +1180,7 @@ static bool get_links(struct ctl_dir *dir,
 	struct ctl_table_header *tmp_head;
 	const struct ctl_table *entry, *link;
 
-	if (header->ctl_table_size == 0 ||
-	    sysctl_is_perm_empty_ctl_header(header))
+	if (header->ctl_table_size == 0 || dir->permanently_empty)
 		return true;
 
 	/* Are there links available for every entry in table? */
@@ -1536,6 +1501,33 @@ void unregister_sysctl_table(struct ctl_table_header * header)
 }
 EXPORT_SYMBOL(unregister_sysctl_table);
 
+/**
+ * register_sysctl_mount_point() - registers a sysctl mount point
+ * @path: path for the mount point
+ *
+ * Used to create a permanently empty directory to serve as mount point.
+ * There are some subtle but important permission checks this allows in the
+ * case of unprivileged mounts.
+ */
+struct ctl_table_header *register_sysctl_mount_point(const char *path)
+{
+	struct ctl_dir *dir = sysctl_mkdir_p(&sysctl_table_root.default_set.dir, path);
+
+	if (IS_ERR(dir))
+		return NULL;
+
+	guard(spinlock)(&sysctl_lock);
+
+	if (!RB_EMPTY_ROOT(&dir->root)) {
+		drop_sysctl_table(&dir->header);
+		return NULL;
+	}
+
+	dir->permanently_empty = true;
+	return &dir->header;
+}
+EXPORT_SYMBOL(register_sysctl_mount_point);
+
 void setup_sysctl_set(struct ctl_table_set *set,
 	struct ctl_table_root *root,
 	int (*is_seen)(struct ctl_table_set *))
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 7c96d5abafc7..329e68d484ed 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -177,23 +177,14 @@ struct ctl_table_header {
 	struct ctl_dir *parent;
 	struct ctl_node *node;
 	struct hlist_head inodes; /* head for proc_inode->sysctl_inodes */
-	/**
-	 * enum type - Enumeration to differentiate between ctl target types
-	 * @SYSCTL_TABLE_TYPE_DEFAULT: ctl target with no special considerations
-	 * @SYSCTL_TABLE_TYPE_PERMANENTLY_EMPTY: Used to identify a permanently
-	 *                                       empty directory target to serve
-	 *                                       as mount point.
-	 */
-	enum {
-		SYSCTL_TABLE_TYPE_DEFAULT,
-		SYSCTL_TABLE_TYPE_PERMANENTLY_EMPTY
-	} type;
 };
 
 struct ctl_dir {
 	/* Header must be at the start of ctl_dir */
 	struct ctl_table_header header;
 	struct rb_root root;
+	/* Permanently empty directory target to serve as mount point. */
+	bool permanently_empty;
 };
 
 struct ctl_table_set {

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-- 
2.43.0


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