Re: [PATCH v2 0/3] pidfs: implement file handle support

[Amir Goldstein]

On Wed, Nov 13, 2024 at 7:01 PM Erin Shepherd <erin.shepherd@xxxxxx> wrote:
>
> Since the introduction of pidfs, we have had 64-bit process identifiers
> that will not be reused for the entire uptime of the system. This greatly
> facilitates process tracking in userspace.
>
> There are two limitations at present:
>
>  * These identifiers are currently only exposed to processes on 64-bit
>    systems. On 32-bit systems, inode space is also limited to 32 bits and
>    therefore is subject to the same reuse issues.
>  * There is no way to go from one of these unique identifiers to a pid or
>    pidfd.
>
> This patch implements fh_export and fh_to_dentry which enables userspace to
> convert PIDs to and from PID file handles. A process can convert a pidfd into
> a file handle using name_to_handle_at, store it (in memory, on disk, or
> elsewhere) and then convert it back into a pidfd suing open_by_handle_at.
>
> To support us going from a file handle to a pidfd, we have to store a pid
> inside the file handle. To ensure file handles are invariant and can move
> between pid namespaces, we stash a pid from the initial namespace inside
> the file handle.
>
>   (There has been some discussion as to whether or not it is OK to include
>   the PID in the initial pid namespace, but so far there hasn't been any
>   conclusive reason given as to why this would be a bad idea)

IIUC, this is already exposed as st_ino on a 64bit arch?
If that is the case, then there is certainly no new info leak in this patch.

>
> Signed-off-by: Erin Shepherd <erin.shepherd@xxxxxx>
> ---
> Changes in v2:
> - Permit filesystems to opt out of CAP_DAC_READ_SEARCH
> - Inline find_pid_ns/get_pid logic; remove unnecessary put_pid
> - Squash fh_export & fh_to_dentry into one commit

Not sure why you did that.
It was pretty nice as separate commits if you ask me. Whatever.

Thanks,
Amir.