Re: [PATCH 0/4] pidfs: implement file handle support

[Erin Shepherd]

On 12/11/2024 14:57, Jeff Layton wrote:
> On Tue, 2024-11-12 at 14:10 +0100, Christian Brauner wrote:
> We should really just move to storing 64-bit inode numbers internally
> on 32-bit machines. That would at least make statx() give you all 64
> bits on 32-bit host.

I think that would be ideal from the perspective of exposing it to
userspace.
It does leave the question of going back from inode to pidfd unsolved
though.I like the name_to_handle_at/open_by_handle_at approach because
it neatly solves both sides of the problem with APIs we already have and
understand

> Hmm... I guess pid namespaces don't have a convenient 64-bit ID like
> mount namespaces do? In that case, stashing the pid from init_ns is
> probably the next best thing.

Not that I could identify, no; so stashing the PID seemed like the most
pragmatic
approach.

I'm not 100% sure it should be a documented property of the file handle
format; I
somewhat think that everything after the PID inode should be opaque to
userspace
and subject to change in the future (to the point I considered xoring it
with a
magic constant to make it less obvious to userspace/make it more obvious
that its
not to be relied upon; but that to my knowledge is not something that
the kernel
has done elsewhere).

- Erin