[PATCH 4/4] pidfs: implement fh_to_dentry

[Erin Shepherd <erin.shepherd@xxxxxx>]

This enables userspace to use name_to_handle_at to recover a pidfd
to a process.

We stash the process' PID in the root pid namespace inside the handle,
and use that to recover the pid (validating that pid->ino matches the
value in the handle, i.e. that the pid has not been reused).

We use the root namespace in order to ensure that file handles can be
moved across namespaces; however, we validate that the PID exists in
the current namespace before returning the inode.

Signed-off-by: Erin Shepherd <erin.shepherd@xxxxxx>
---
 fs/pidfs.c | 50 +++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 43 insertions(+), 7 deletions(-)

diff --git a/fs/pidfs.c b/fs/pidfs.c
index c8e7e9011550..2d66610ef385 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -348,23 +348,59 @@ static const struct dentry_operations pidfs_dentry_operations = {
 	.d_prune	= stashed_dentry_prune,
 };
 
-static int pidfs_encode_fh(struct inode *inode, __u32 *fh, int *max_len,
+#define PIDFD_FID_LEN 3
+
+struct pidfd_fid {
+	u64 ino;
+	s32 pid;
+} __packed;
+
+static int pidfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
 			   struct inode *parent)
 {
 	struct pid *pid = inode->i_private;
-	
-	if (*max_len < 2) {
-		*max_len = 2;
+	struct pidfd_fid *fid = (struct pidfd_fid *)fh;
+
+	if (*max_len < PIDFD_FID_LEN) {
+		*max_len = PIDFD_FID_LEN;
 		return FILEID_INVALID;
 	}
 
-	*max_len = 2;
-	*(u64 *)fh = pid->ino;
-	return FILEID_KERNFS;
+	fid->ino = pid->ino;
+	fid->pid = pid_nr(pid);
+	*max_len = PIDFD_FID_LEN;
+	return FILEID_INO64_GEN;
+}
+
+static struct dentry *pidfs_fh_to_dentry(struct super_block *sb,
+					 struct fid *gen_fid,
+					 int fh_len, int fh_type)
+{
+	int ret;
+	struct path path;
+	struct pidfd_fid *fid = (struct pidfd_fid *)gen_fid;
+	struct pid *pid;
+
+	if (fh_type != FILEID_INO64_GEN || fh_len < PIDFD_FID_LEN)
+		return NULL;
+
+	pid = find_get_pid_ns(fid->pid, &init_pid_ns);
+	if (!pid || pid->ino != fid->ino || pid_vnr(pid) == 0) {
+		put_pid(pid);
+		return NULL;
+	}
+
+	ret = path_from_stashed(&pid->stashed, pidfs_mnt, pid, &path);
+	if (ret < 0)
+		return ERR_PTR(ret);
+
+	mntput(path.mnt);
+	return path.dentry;
 }
 
 static const struct export_operations pidfs_export_operations = {
 	.encode_fh = pidfs_encode_fh,
+	.fh_to_dentry = pidfs_fh_to_dentry,
 };
 
 static int pidfs_init_inode(struct inode *inode, void *data)
-- 
2.46.1