linux-kernel - Re: [PATCH v2 2/7] y2038: Introduce __ASSUME_64BIT

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Date:   Sun, 5 May 2019 22:46:23 +0200
From:   Lukasz Majewski <lukma@...x.de>
To:     Stepan Golosunov <stepan@...osunov.pp.ru>
Cc:     Joseph Myers <joseph@...esourcery.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, libc-alpha@...rceware.org,
        Arnd Bergmann <arnd@...db.de>, Paul Eggert <eggert@...ucla.edu>
Subject: Re: [PATCH v2 2/7] y2038: Introduce __ASSUME_64BIT_TIME define

On Sun, 5 May 2019 18:10:54 +0400
Stepan Golosunov <stepan@...osunov.pp.ru> wrote:

> 02.05.2019 в 15:04:18 +0000 Joseph Myers написал:
> > On Tue, 30 Apr 2019, Lukasz Majewski wrote:
> >   
> > >  - The need for explicit clearing padding when calling syscalls
> > > (as to be better safe than sorry in the future - there was related
> > >    discussion started by Stepan).  
> > 
> > This really isn't a difficult question.  What it comes down to is
> > whether the Linux kernel, in the first release version with these
> > syscalls (we don't care about old -rc versions; what matters is the
> > actual 5.1 release), ignores the padding.
> > 
> > If 5.1 *release* ignores the padding, that is part of the
> > kernel/userspace ABI, in accordance with the kernel principle of
> > not breaking userspace. Thus, it is something userspace can rely
> > on, now and in the future.
> > 
> > If 5.1 release does not ignore the padding, syscall presence does
> > not mean the padding is ignored by the kernel and so glibc needs to
> > clear padding. Of course, it needs to clear padding in a *copy* of
> > the value provided by the user unless the glibc API in question
> > requires the timespec value in question to be in writable memory.
> > 
> > So, which is (or will be) the case in 5.1 release?  Padding ignored
> > or not?  If more complicated (ignored for some architectures / ABIs
> > but not for others, or depending on whether compat syscalls are in
> > use), then say so - give a precise description of the exact
> > circumstances under which the padding around a 32-bit tv_nsec will
> > or will not be ignored by the kernel on input from userspace.  
> 
> In current linux git it looks like padding is correctly ignored in
> 32-bit kernels (because kernel itself has 32-bit tv_nsec there) but
> the code to clear it on compat syscalls in 64-bit kernels seems to be
> broken.
> 
> The patch to fix this is at
> 
> https://lore.kernel.org/lkml/20190429131951.471701-1-arnd@arndb.de/
> 
> but it doesn't seem like it has reached Linus yet.
> 

I hope that this patch will be pulled soon (before final cut) - for that
reason we can assume that the padding is ignored by the kernel and
hence do not explicitly clear it in glibc (as it was done in sent
patches)

> 
> (Hmm.  I think that old ipc and socketcall syscalls in 32-bit kernels
> are broken without that patch too.  They would try to read
> __kernel_timespec when callers are passing old_timespec32.)

Please correct me if I'm wrong, but this problem is related to x32
machines (and not to ARM 32 bit ones with Y2038).


Best regards,

Lukasz Majewski

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