[PATCH v2 2/2] fs: open_by_handle_at() support for decoding "explicit connectable" file handles

[Amir Goldstein <amir73il@gmail.com>]

Teach open_by_handle_at(2) about the type format of "explicit connectable"
file handles that were created using the AT_HANDLE_CONNECTABLE flag to
name_to_handle_at(2).

When decoding an "explicit connectable" file handles, name_to_handle_at(2)
should fail if it cannot open a "connected" fd with known path, which is
accessible (to capable user) from mount fd path.

Note that this does not check if the path is accessible to the calling
user, just that it is accessible wrt the mount namesapce, so if there
is no "connected" alias, or if parts of the path are hidden in the
mount namespace, open_by_handle_at(2) will return -ESTALE.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/fhandle.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/fs/fhandle.c b/fs/fhandle.c
index 6c87f1764235..68e59141f67b 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -247,7 +247,13 @@ static int vfs_dentry_acceptable(void *context, struct dentry *dentry)
 
 	if (!(ctx->flags & HANDLE_CHECK_SUBTREE) || d == root)
 		retval = 1;
-	WARN_ON_ONCE(d != root && d != root->d_sb->s_root);
+	/*
+	 * exportfs_decode_fh_raw() does not call acceptable() callback with
+	 * a disconnected directory dentry, so we should have reached either
+	 * mount fd directory or sb root.
+	 */
+	if (ctx->fh_flags & EXPORT_FH_DIR_ONLY)
+		WARN_ON_ONCE(d != root && d != root->d_sb->s_root);
 	dput(d);
 	return retval;
 }
@@ -346,10 +352,19 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
 		retval = -EINVAL;
 		goto out_path;
 	}
-	if (f_handle.handle_flags) {
+	if (f_handle.handle_flags & ~EXPORT_FH_USER_FLAGS) {
 		retval = -EINVAL;
 		goto out_path;
 	}
+	/*
+	 * If handle was encoded with AT_HANDLE_CONNECTABLE, verify that we
+	 * are decoding an fd with connected path, which is accessible from
+	 * the mount fd path.
+	 */
+	ctx.fh_flags |= f_handle.handle_flags;
+	if (ctx.fh_flags & EXPORT_FH_CONNECTABLE)
+		ctx.flags |= HANDLE_CHECK_SUBTREE;
+
 	handle = kmalloc(struct_size(handle, f_handle, f_handle.handle_bytes),
 			 GFP_KERNEL);
 	if (!handle) {
-- 
2.34.1