Re: [PATCH v1] futex: Add compat_sys_futex_waitv for 32bit compatibility

[André Almeida]

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@xxxxxxxx>
> > >
> > > 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
> Wei Gao