[PATCH 2/4] pidfs: implement file handle export support

[Erin Shepherd]

On 64-bit platforms, userspace can read the pidfd's inode in order to
get a never-repeated PID identifier. On 32-bit platforms this identifier
is not exposed, as inodes are limited to 32 bits. Instead expose the
identifier via export_fh, which makes it available to userspace via
name_to_handle_at

Signed-off-by: Erin Shepherd <erin.shepherd@xxxxxx>
---
 fs/pidfs.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/fs/pidfs.c b/fs/pidfs.c
index 80675b6bf884..c8e7e9011550 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/anon_inodes.h>
+#include <linux/exportfs.h>
 #include <linux/file.h>
 #include <linux/fs.h>
 #include <linux/magic.h>
@@ -347,6 +348,25 @@ 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,
+			   struct inode *parent)
+{
+	struct pid *pid = inode->i_private;
+
+	if (*max_len < 2) {
+		*max_len = 2;
+		return FILEID_INVALID;
+	}
+
+	*max_len = 2;
+	*(u64 *)fh = pid->ino;
+	return FILEID_KERNFS;
+}
+
+static const struct export_operations pidfs_export_operations = {
+	.encode_fh = pidfs_encode_fh,
+};
+
 static int pidfs_init_inode(struct inode *inode, void *data)
 {
 	inode->i_private = data;
@@ -382,6 +402,7 @@ static int pidfs_init_fs_context(struct fs_context *fc)
 		return -ENOMEM;
 
 	ctx->ops = &pidfs_sops;
+	ctx->eops = &pidfs_export_operations;
 	ctx->dops = &pidfs_dentry_operations;
 	fc->s_fs_info = (void *)&pidfs_stashed_ops;
 	return 0;
-- 
2.46.1