On Tue, Feb 18, 2020 at 05:05:43PM +0000, David Howells wrote:

Allow the fsinfo() syscall to look up a mount object by ID rather than by
pathname.  This is necessary as there can be multiple mounts stacked up at
the same pathname and there's no way to look through them otherwise.

This is done by passing FSINFO_FLAGS_QUERY_MOUNT to fsinfo() in the
parameters and then passing the mount ID as a string to fsinfo() in place
of the filename:

	struct fsinfo_params params = {
		.flags	 = FSINFO_FLAGS_QUERY_MOUNT,
		.request = FSINFO_ATTR_IDS,
	};

	ret = fsinfo(AT_FDCWD, "21", &params, buffer, sizeof(buffer));

The caller is only permitted to query a mount object if the root directory
of that mount connects directly to the current chroot if dfd == AT_FDCWD[*]
or the directory specified by dfd otherwise.  Note that this is not
available to the pathwalk of any other syscall.

[*] This needs to be something other than AT_FDCWD, perhaps AT_FDROOT.

Sounds like it should accept LOOKUP_BENEATH.