[REGRESSION] rseq: refactoring in v6.19 broke everyone on arm64 and tcmalloc everywhere

[Mathieu Desnoyers]

On 2026-04-23 13:38, Chris Kennelly wrote:
> On Thu, Apr 23, 2026 at 1:19 PM Thomas Gleixner <tglx at kernel.org> wrote:

[...]

>>
>>    3) The RO for userspace property has been enforced by RSEQ debugging
>>       mode since day one. If such a debug enabled kernel detects user
>>       space changing the field it kills the task/application.
> 
> The optimization in TCMalloc that you're describing has been available
> since September 2023:
> https://github.com/google/tcmalloc/commit/aaa4fbf6fcdce1b7f86fcadd659874645c75ddb9
> 
> I thought the RSEQ debug checks were added in December 2024:
> https://github.com/torvalds/linux/commit/7d5265ffcd8b41da5e09066360540d6e0716e9cd,
> but perhaps I misidentified the ones in question.

You are correct, I added the RSEQ field corruption validation under
debug config in Nov. 2024 when I noticed the world of pain we were
heading towards with incompatible tcmalloc vs glibc (and general) use
due to tcmalloc not respecting the ABI contract. RSEQ has been
upstreamed in 2018. So that's not exactly a day one enforcement.
The ABI contract was clear about this being an invalid use from
day one though.

[...]

>>    7) tcmalloc violates the ABI from day one and has since refused to
>>       address the problem despite being offered a kernel side rseq
>>       extension to solve it many years ago.
> 
> I know there was some discussion around a preemption notification
> scheme, rseq_sched_state; but I thought the discussion moved in favor
> of the timeslice extension interface that recently landed. Timeslice
> extension solves some use cases, but I'm not sure it addresses this
> one.

I have actively engaged with the tcmalloc developers to
understand their needs and figure out a proper solution for the
past ~3-4 years, without success.

I have done a POC branch extending rseq with a "reset a linked list of
userspace areas on preemption" back in 2024 which would have solved
tcmalloc's issues cleanly. I never posted it publicly because the
tcmalloc devs told me they could not justify spending time even trying
this out to their managers.

I still have that feature branch gathering dust somewhere.

Thanks,

Mathieu

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