linux-kernel - Re: [PATCH RESEND v4 0/7] futex: Create set_robust

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Message-Id: <e79698f8-c428-44dc-afac-8d918968d11d@app.fastmail.com>
Date: Wed, 18 Jun 2025 19:03:39 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Sebastian Andrzej Siewior" <bigeasy@...utronix.de>,
 André Almeida <andrealmeid@...lia.com>
Cc: "Thomas Gleixner" <tglx@...utronix.de>, "Ingo Molnar" <mingo@...hat.com>,
 "Peter Zijlstra" <peterz@...radead.org>,
 "Darren Hart" <dvhart@...radead.org>, "Davidlohr Bueso" <dave@...olabs.net>,
 shuah <shuah@...nel.org>, "Waiman Long" <longman@...hat.com>,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 linux-api@...r.kernel.org, kernel-dev@...lia.com
Subject: Re: [PATCH RESEND v4 0/7] futex: Create set_robust_list2

On Wed, Jun 18, 2025, at 18:56, Sebastian Andrzej Siewior wrote:
> On 2025-06-18 13:39:46 [-0300], André Almeida wrote:
>> 
>> Ops, I forgot to address them. I will do it for v5.
>> 
>> > - You say 64bit x86-64 does not have the problem due the compat syscall.
>> >    Arm64 has this problem. New arm64 do not provide arm32 facility. You
>> >    introduce the syscall here. Why not introduce the compat syscall
>> >    instead? I'm sorry if this has been answered somewhere below but this
>> >    was one question I had while I initially skimmed over the patches.
>> > 
>> 
>> The main target for this new syscall is Arm64, that can't handle 32 pointers
>> in the current syscall, so this new interface allows the robust list
>> mechanism to know if it needs to do 64 or 32 bit pointer arithmetic
>> operations to walk in the list.
>> 
>> Introducing a compat syscall won't fix this, giving that it only works in
>> x86-64. We need an entry point for Arm64 that can handle 32 bit pointers.
>
> I would need to dig into details to figure out why it won't work for
> arm64 and works only for x86-64. 
> There is the set_robust_list syscall as compat which sets
> ::compat_robust_list. And non-compat sets ::robust_list. The 32bit
> application on 64bit kernel should set ::compat_robust_list which what
> your syscall provides.
> That is why I don't understand the need for it so far. Maybe I am
> missing a detail.
> We have other architectures with 64 bit kernel and a possible 32bit
> userland such as mips, s390 or powerpc which would have the same issue
> then. Or there is something special about arm64 in this case which makes
> it unique.

x86 is the special case here, since it allows applications to
call both the 32-bit (compat) and 64-bit syscalls directly on
a 64-bit kernel. I think MIPS may do that as well, but the other
architectures only allow a process to call syscalls for its native
ABI, so the only way to call a compat syscall is from a 32-bit
task. On Arm and RISC-V it's also common to have CPUs that cannot
run 32-bit tasks at all, so even running your x86-32 emulator as
an arm32 or rv32 task won't work.

      Arnd