linux-kernel - Re: [PATCH v1] futex: Add compat_sys_futex

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Date:   Fri, 1 Dec 2023 01:39:36 -0500
From:   Wei Gao <wegao@...e.com>
To:     André Almeida <andrealmeid@...lia.com>
Cc:     tglx@...utronix.de, dvhart@...radead.org,
        linux-kernel@...r.kernel.org, dave@...olabs.net, mingo@...hat.com,
        peterz@...radead.org, Arnd Bergmann <arnd@...nel.org>
Subject: Re: [PATCH v1] futex: Add compat_sys_futex_waitv for 32bit
 compatibility

On Wed, Nov 29, 2023 at 03:56:12PM -0300, André Almeida wrote:
> Hi Wei,
> 
> Em 27/11/2023 09:15, Wei Gao escreveu:
> > On Thu, Nov 23, 2023 at 01:09:55PM -0300, André Almeida wrote:
> > > [+CC Arnd]
> > > 
> > > Hi Wei,
> > > 
> > > Em 23/11/2023 02:31, Wei Gao escreveu:
> > > > From: wei gao <wegao@...e.com>
> > > > 
> > > > Current implementation lead LTP test case futex_waitv failed when compiled with
> > > > -m32. This patch add new compat_sys_futex_waitv to handle m32 mode syscall.
> > > > 
> > > > The failure reason is futex_waitv in m32 mode will deliver kernel with struct
> > > > old_timespec32 timeout, but this struct type can not directly used by current
> > > > sys_futex_waitv implementation.
> > > > 
> > > > The new function copy main logic of current sys_futex_waitv, just update parameter
> > > > type from "struct __kernel_timespec __user *" to "struct old_timespec32 __user *,"
> > > > and use get_old_timespec32 within the new function to get timeout value.
> > > > 
> > > 
> > > From, what I recall, we don't want to add new syscalls with old_timespec32,
> > > giving that they will have a limited lifetime. Instead, userspace should be
> > > able to come up with a 64-bit timespec implementation for -m32.
> > > 
> > > Thanks,
> > > 	André
> > 
> > Just a comment, I have checked the glibc latest code but do not see any implemention(*.c) on
> > futex_waitv syscall. So normally you have to do syscall directly with __NR_futex_waitv from
> > userspace. So i guess glibc-side can not covert this struct correctly currently. Correct me if
> > any misunderstanding.
> > 
> 
> futex() has no syscall wrappers in glibc. Userspace needs to figure out
> everything by themselves, including which struct they should use, and I
> don't think that glibc does any conversion. If you create manually a
> timespec64 that works in -m32, and pass this to sycall(__NR_futex_waitv,
> ..., &timeout, ...), it should work correctly. You can read more about how
> glibc is planning to deal with this at [1]. Please let me know if now it's
> more clear :)
> 
> [1] https://sourceware.org/glibc/wiki/Y2038ProofnessDesign

Thanks a lot for your detail explaination and good learning link, it's more clear to me now : )

> 
> > Thanks
> > Wei Gao