linux-kernel - Re: [RFC 0/4] futex2: Add new futex interface

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Date:   Thu, 25 Jun 2020 08:48:29 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     André Almeida <andrealmeid@...labora.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>, krisman@...labora.com,
        Collabora kernel ML <kernel@...labora.com>,
        Darren Hart <dvhart@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, pgriffais@...vesoftware.com,
        Florian Weimer <fweimer@...hat.com>,
        GNU C Library <libc-alpha@...rceware.org>,
        malteskarupke@....de, Linux API <linux-api@...r.kernel.org>
Subject: Re: [RFC 0/4] futex2: Add new futex interface

On Fri, Jun 12, 2020 at 8:51 PM André Almeida <andrealmeid@...labora.com> wrote:

> - The proposed interface uses ktime_t type for absolute timeout, and I
>   assumed that it should use values in a nsec resolution. If this is true,
>   we have some problems with i386 ABI, please check out the
>   COMPAT_32BIT_TIME implementation in patch 1 for more details. I
>   haven't added a time64 implementation yet, until this is clarified.

ktime_t is not part of the uapi headers, and has always been considered
an implementation detail of the kernel so far. I would argue it should
stay that way. The most sensible alternatives would be to either use
a "__u64 *timeout" argument for a relative timeout, or a
"struct __kernel_timespec *timeout" for an absolute timeout.

old_time32_t also makes no sense for multiple reasons:

- It's another kernel internal type and not part of the uapi headers
- your time32 call has different calling conventions from your time64
  version, not just a different type.
- there should be no need to add syscalls that are known to be buggy
  when there is a replacement type that does not have that bug.

> - Is expected to have a x32 ABI implementation as well? In the case of
>   wait and wake, we could use the same as x86_64 ABI. However, for the
>   waitv (aka wait on multiple futexes) we would need a proper x32 entry
>   since we are dealing with 32bit pointers.

For new syscalls, I'd actually recommend not having a separate
entry point, but just checking 'if (in_compat_syscall())' inside of the
implementation to pick one behavior vs the other when accessing
the user pointers. This keeps the implementation simpler and
avoids assigning a new x32 syscall number that would be different
from all the other architectures.

      Arnd