Re: [PATCH 06/19] vfs: Allow fsinfo() to look up a mount object by ID [ver #16]

[Christian Brauner]

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.