pidfs: allow bind-mounts

Allow bind-mounting pidfds. Similar to nsfs let's allow bind-mounts for
pidfds. This allows pidfds to be safely recovered and checked for
process recycling.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

Author: brauner

fs/namespace.c

@@ -32,6 +32,7 @@
 #include <linux/fs_context.h>
 #include <linux/shmem_fs.h>
 #include <linux/mnt_idmapping.h>
+#include <linux/pidfs.h>
 #include <linux/nospec.h>
 
 #include "pnode.h"
@@ -2732,8 +2733,13 @@ static struct mount *__do_loopback(struct path *old_path, int recurse)
 	if (IS_MNT_UNBINDABLE(old))
 		return mnt;
 
-	if (!check_mnt(old) && old_path->dentry->d_op != &ns_dentry_operations)
-		return mnt;
+	if (!check_mnt(old)) {
+		const struct dentry_operations *d_op = old_path->dentry->d_op;
+
+		if (d_op != &ns_dentry_operations &&
+		    d_op != &pidfs_dentry_operations)
+			return mnt;
+	}
 
 	if (!recurse && has_locked_children(old, old_path->dentry))
 		return mnt;

fs/pidfs.c

@@ -510,7 +510,7 @@ static char *pidfs_dname(struct dentry *dentry, char *buffer, int buflen)
 	return dynamic_dname(buffer, buflen, "anon_inode:[pidfd]");
 }
 
-static const struct dentry_operations pidfs_dentry_operations = {
+const struct dentry_operations pidfs_dentry_operations = {
 	.d_delete	= always_delete_dentry,
 	.d_dname	= pidfs_dname,
 	.d_prune	= stashed_dentry_prune,

include/linux/pidfs.h

@@ -6,5 +6,6 @@ struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags);
 void __init pidfs_init(void);
 void pidfs_add_pid(struct pid *pid);
 void pidfs_remove_pid(struct pid *pid);
+extern const struct dentry_operations pidfs_dentry_operations;
 
 #endif /* _LINUX_PID_FS_H */