[PATCH RFC 1/2] pidfs: allow bind-mounts

[Christian Brauner <brauner@xxxxxxxxxx>]

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.

Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
---
 fs/namespace.c        | 10 ++++++++--
 fs/pidfs.c            |  2 +-
 include/linux/pidfs.h |  1 +
 3 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 23e81c2a1e3fee7d97df2a84a69438a677933654..7baffa2ea582dacd0fa70959174fc2a47fb5de1f 100644
--- a/fs/namespace.c
+++ b/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;
diff --git a/fs/pidfs.c b/fs/pidfs.c
index c5a51c69acc86694152ff006743bbfa516a2c1f5..049352f973de38967ad35c6440b9480e7e2e2775 100644
--- a/fs/pidfs.c
+++ b/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,
diff --git a/include/linux/pidfs.h b/include/linux/pidfs.h
index df574d6708d423e2eb2f5154b4a0ead1765d5ad2..7c830d0dec9a7c098f65f98a9b9f605e9108da96 100644
--- a/include/linux/pidfs.h
+++ b/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 */

-- 
2.45.2