linux-kernel - Re: [PATCH v4 0/3] Fix bugs and performance of kstack offset randomisation

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Message-ID: <785a7d90-babe-43af-93a6-c220c1877bcf@arm.com>
Date: Tue, 20 Jan 2026 16:32:17 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Kees Cook <kees@...nel.org>, Dave Hansen <dave.hansen@...el.com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Huacai Chen <chenhuacai@...nel.org>,
 Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Michael Ellerman <mpe@...erman.id.au>, Paul Walmsley <pjw@...nel.org>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Alexander Gordeev <agordeev@...ux.ibm.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>, Arnd Bergmann
 <arnd@...db.de>, Mark Rutland <mark.rutland@....com>,
 "Jason A. Donenfeld" <Jason@...c4.com>, Ard Biesheuvel <ardb@...nel.org>,
 Jeremy Linton <jeremy.linton@....com>,
 David Laight <david.laight.linux@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 loongarch@...ts.linux.dev, linuxppc-dev@...ts.ozlabs.org,
 linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH v4 0/3] Fix bugs and performance of kstack offset
 randomisation

On 19/01/2026 16:44, Kees Cook wrote:
> 
> 
> On January 19, 2026 8:00:00 AM PST, Dave Hansen <dave.hansen@...el.com> wrote:
>> On 1/19/26 05:01, Ryan Roberts wrote:
>>> x86 (AWS Sapphire Rapids):
>>> +-----------------+--------------+-------------+---------------+
>>> | Benchmark       | Result Class |   v6.18-rc5 | per-task-prng |
>>> |                 |              | rndstack-on |               |
>>> |                 |              |             |               |
>>> +=================+==============+=============+===============+
>>> | syscall/getpid  | mean (ns)    |  (R) 13.32% |     (R) 4.60% |
>>> |                 | p99 (ns)     |  (R) 13.38% |    (R) 18.08% |
>>> |                 | p99.9 (ns)   |      16.26% |    (R) 19.38% |
>>
>> Like you noted, this is surprising. This would be a good thing to make
>> sure it goes in very early after -rc1 and gets plenty of wide testing.
> 
> Right, we are pretty late in the dev cycle (rc6). It would be prudent to get this into -next after the coming rc1 (1 month from now).
> 
> On the other hand, the changes are pretty "binary" in the sense that mistakes should be VERY visible right away. Would it be better to take this into -next immediately instead?

I don't think this question was really addressed to me, but I'll give my opinion
anyway; I agree it's pretty binary - it will either work or it will explode.
I've tested on arm64 and x86_64 so I have high confidence that it works. If you
get it into -next ASAP it has 3 weeks to soak before the merge window opens
right? (Linus said he would do an -rc8 this cycle). That feels like enough time
to me. But it's your tree ;-)

Thanks,
Ryan

> 
>> But I don't see any problems with the approach, and the move to common
>> code looks like a big win as well:
> 
> Agreed; I think it's looking great.
>