linux-kernel - Re: [patch V6 01/11] rseq: Add fields and constants for time slice extension

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <5d2f428d-4fdc-486d-90e1-474f3ee9f54f@efficios.com>
Date: Mon, 19 Jan 2026 11:30:53 +0100
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Florian Weimer <fweimer@...hat.com>, Thomas Gleixner <tglx@...nel.org>,
 LKML <linux-kernel@...r.kernel.org>, "Paul E. McKenney"
 <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
 Jonathan Corbet <corbet@....net>,
 Prakash Sangappa <prakash.sangappa@...cle.com>,
 Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
 K Prateek Nayak <kprateek.nayak@....com>,
 Steven Rostedt <rostedt@...dmis.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
 Randy Dunlap <rdunlap@...radead.org>, Ron Geva <rongevarg@...il.com>,
 Waiman Long <longman@...hat.com>, "carlos@...hat.com" <carlos@...hat.com>,
 Michael Jeanson <mjeanson@...icios.com>
Subject: Re: [patch V6 01/11] rseq: Add fields and constants for time slice
 extension

On 2026-01-19 05:21, Peter Zijlstra wrote:
> On Sat, Jan 17, 2026 at 05:16:16PM +0100, Mathieu Desnoyers wrote:
> 
>> My main concern is about the overhead of added system calls at thread
>> creation. I recall that doing an additional rseq system call at thread
>> creation was analyzed thoroughly for performance regressions at the
>> libc level. I would not want to start requiring libc to issue a
>> handful of additional prctl system calls per thread creation for no good
>> reason.
> 
> A wee something like so?
> 
> That would allow registering rseq with RSEQ_FLAG_SLICE_EXT_DEFAULT_ON
> set and if all the stars align, it will then have it on at the end.

That's a very good step in the right direction. I just wonder how
userspace is expected to learn that it runs on a kernel which
accepts the RSEQ_FLAG_SLICE_EXT_DEFAULT_ON flag ?

I think it could expect it when getauxval for AT_RSEQ_FEATURE_SIZE
includes the slice ext field. This gives us a cheap way to know
from userspace whether this new flag is supported or not.

One nit below:

[...]
> -	if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION))
> +	if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION)) {
>   		rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
> +		if (rseq_slice_extension_enabled() &&
> +		    flags & RSEQ_FLAG_SLICE_EXT_DEFAULT_ON)

I think you want to surround flags & RSEQ_FLAG_SLICE_EXT_DEFAULT_ON with
parentheses () to have the expected operator priority.

Thanks!

Mathieu

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