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

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Message-ID: <373117462.72486.1618333576943.JavaMail.zimbra@efficios.com>
Date:   Tue, 13 Apr 2021 13:06:16 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Eric Dumazet <edumazet@...gle.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 Apr 13, 2021, at 12:57 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 ?

In terms of behavior, yes. And it's probably the "easy" fix, but I hate that
it adds lots of preprocessor ifdefs into the rseq code.

But this would require auditing get_user()/put_user() for each architecture
supported by rseq to ensure they support 8-byte load/store. And it would become
an added burden on architecture maintainers wishing to add rseq support for their
architecture.

One alternative would be to implement rseq_get_user_u64 and rseq_put_user_u64
wrappers as static functions within rseq.c to hide the preprocessor ifdeffery
from the higher-level code. I try very hard to avoid mixing preprocessor ifdefs
with C code logic whenever I can.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com