Re: [PATCH 1/2] y2038: make CONFIG_64BIT_TIME unconditional
From: Lukasz Majewski
Date: Sat Apr 27 2019 - 11:06:26 EST
> 27.04.2019 Ð 00:46:53 +0200 Lukasz Majewski ÐÐÐÐÑÐÐ:
> > Hi Arnd,
> > > As Stepan Golosunov points out, we made a small mistake in the
> > > get_timespec64() function in the kernel. It was originally added
> > > under the assumption that CONFIG_64BIT_TIME would get enabled on
> > > all 32-bit and 64-bit architectures, but when I did the
> > > conversion, I only turned it on for 32-bit ones.
> > >
> > > The effect is that the get_timespec64() function never clears the
> > > upper half of the tv_nsec field for 32-bit tasks in compat mode.
> > > Clearing this is required for POSIX compliant behavior of
> > > functions that pass a 'timespec' structure with a 64-bit tv_sec
> > > and a 32-bit tv_nsec, plus uninitialized padding.
> > At least for my setup (32bit ARM with 64 bit time support) this
> > patch is not fixing anything.
> The patch is not supposed to fix anything on 32-bit architectures as
> in-kernel struct timespec64 has 32-bit tv_nsec there. Thus truncation
> should happen automatically. I also missed that fact when I was
> reading get_timespec64 code.
Yes. You are right. The tv_nsec is 32 bit.
> (I am wondering whether such trucation is undefined behaviour in C
According to  - Chapter 18.104.22.168 - Point 3 it is
> whether there should be sign-extension instead of zeroing-out for the
> in_compat_syscall() case though.)
What I've found is that "typically" the high order bits are discarded.
However, it is still "implementation dependent".
> > > The easiest fix for linux-5.1 is to just make the Kconfig symbol
> > > unconditional, as it was originally intended. As a follow-up,
> > > we should remove any #ifdef CONFIG_64BIT_TIME completely.
> > >
> > > Link:
> > > https://lore.kernel.org/lkml/20190422090710.bmxdhhankurhafxq@xxxxxxxxxxxxxxxxxxxxx/
> > > Cc: Lukasz Majewski <lukma@xxxxxxx> Cc: Stepan Golosunov
> > > <stepan@xxxxxxxxxxxxxxx> Signed-off-by: Arnd Bergmann
> > > <arnd@xxxxxxxx> ---
> > > Please apply this one as a bugfix for 5.1
> > > ---
> > > arch/Kconfig | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/arch/Kconfig b/arch/Kconfig
> > > index 33687dddd86a..9092e0ffe4d3 100644
> > > --- a/arch/Kconfig
> > > +++ b/arch/Kconfig
> > > @@ -764,7 +764,7 @@ config COMPAT_OLD_SIGACTION
> > > bool
> > >
> > > config 64BIT_TIME
> > > - def_bool ARCH_HAS_64BIT_TIME
> > > + def_bool y
> > > help
> > > This should be selected by all architectures that need
> > > to support new system calls with a 64-bit time_t. This is
> > > relevant on all 32-bit
 - http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
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