linux-kernel - Re: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq

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Message-ID: <CANn89iKChc2Xf7fnJN0A7OfA7v=S0f6KruB91dKmEPVRhxQyPg@mail.gmail.com>
Date:   Tue, 13 Apr 2021 19:01:37 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        paulmck <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        Arjun Roy <arjunroy@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs()

On Tue, Apr 13, 2021 at 6:57 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Tue, Apr 13, 2021 at 6:54 PM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
> >
> > ----- On Apr 13, 2021, at 12:22 PM, Eric Dumazet eric.dumazet@...il.com wrote:
> >
> > > From: Eric Dumazet <edumazet@...gle.com>
> > >
> > > Commit ec9c82e03a74 ("rseq: uapi: Declare rseq_cs field as union,
> > > update includes") added regressions for our servers.
> > >
> > > Using copy_from_user() and clear_user() for 64bit values
> > > is suboptimal.
> > >
> > > We can use faster put_user() and get_user().
> > >
> > > 32bit arches can be changed to use the ptr32 field,
> > > since the padding field must always be zero.
> > >
> > > v2: added ideas from Peter and Mathieu about making this
> > >    generic, since my initial patch was only dealing with
> > >    64bit arches.
> >
> > Ah, now I remember the reason why reading and clearing the entire 64-bit
> > is important: it's because we don't want to allow user-space processes to
> > use this change in behavior to figure out whether they are running on a
> > 32-bit or in a 32-bit compat mode on a 64-bit kernel.
> >
> > So although I'm fine with making 64-bit kernels faster, we'll want to keep
> > updating the entire 64-bit ptr field on 32-bit kernels as well.
> >
> > Thanks,
> >
>
> So... back to V1 then ?

Or add more stuff as in :

#ifdef CONFIG_64BIT
+       return put_user(0UL, &t->rseq->rseq_cs.ptr64);
+#else
+       return put_user(0UL, &t->rseq->rseq_cs.ptr.ptr32) |
put_user(0UL, &t->rseq->rseq_cs.ptr.padding);
+#endif