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

[Christian Brauner <brauner@kernel.org>]

On Thu, Nov 14, 2024 at 01:48:02PM +0100, Erin Shepherd wrote:
>    
> 
> On 14/11/2024 08:02, Amir Goldstein wrote:
> > On Wed, Nov 13, 2024 at 7:01 PM Erin Shepherd <erin.shepherd@e43.eu> 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.
> 
> pid.ino is exposed, but the init-ns pid isn't exposed anywhere to my knowledge.

I see what you mean. That might be an information leak. Not a very
interesting one, I think but I need to think about it.

> 
> >> Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
> >> ---
> >> 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.
> 
> I can revert that if you prefer. I squashed them because there was some churn
> when adding the init-ns-pid necessary to restore them, but I am happy to do
> things in two steps.
> 
> Do you prefer having the final handle format in the first step, or letting it
> evolve into final form over the series?
>