linux-kernel - Re: [PATCH 2/4] mm: introduce external memory hinting API

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Message-ID: <CAKOZuev5k3EquMd-6VbvruahjjtxQzRhUVo2ttgVyk+yYz9aOA@mail.gmail.com>
Date:   Mon, 13 Jan 2020 11:27:03 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Minchan Kim <minchan@...nel.org>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux API <linux-api@...r.kernel.org>, oleksandr@...hat.com,
        Suren Baghdasaryan <surenb@...gle.com>,
        Tim Murray <timmurray@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Dias <joaodias@...gle.com>
Subject: Re: [PATCH 2/4] mm: introduce external memory hinting API

On Mon, Jan 13, 2020 at 11:10 AM Christian Brauner
<christian.brauner@...ntu.com> wrote:
> This does not
> affect the permission checking you're performing here.

Pidfds-as-capabilities sounds like a good change. Can you clarify what
you mean here though? Do you mean that in order to perform some
process-directed operation X on process Y, the pidfd passed to X must
have been opened with PIDFD_CAP_X *and* the process *using* the pidfds
must be able to perform operation X on process Y? Or do pidfds in this
model "carry" permissions in the same way that an ordinary file
descriptor "carries" the ability to write to a file if it was opened
with O_WRONLY even if the FD is passed to a process that couldn't
otherwise write to that file? Right now, pidfds are identity-only and
always rely on the caller's permissions. I like the capability bit
model because it makes pidfds more consistent with other file
descriptors and enabled delegation of capabilities across the system.