Date:   Thu, 2 Mar 2023 12:58:57 -0500
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Suren Baghdasaryan <surenb@google.com>, tj@kernel.org,
        lizefan.x@bytedance.com, peterz@infradead.org, johunt@akamai.com,
        keescook@chromium.org, quic_sudaraja@quicinc.com,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] psi: remove 500ms min window size limitation for
 triggers
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On Thu, Mar 02, 2023 at 06:41:47PM +0100, Michal Hocko wrote:
> On Thu 02-03-23 08:13:54, Suren Baghdasaryan wrote:
> [...]
> > Let's roll this check without additional changes and then consolidate
> > the checking inside psi_trigger_create() in a separate patch. If
> > anybody objects to the late permission check we will just revert that
> > last change without affecting anything else.
> 
> Permissions checks at write time are problematic because userspace
> cannot drop privileges. Also I think it would be an antipattern for how
> we do this in general. 

The permissions can be checked against opener privileges through
file->f_cred. This allows dropping privileges, as well as passing the
fd to a trusted but unprivileged process to delegate trigger setup.

I agree with keeping it in open() for now. But it will matter when we
distinguish between privileged and unprivileged trigger parameters.