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Re: [PATCH 1/2] y2038: make CONFIG_64BIT_TIME unconditional
- From: Arnd Bergmann <arnd at arndb dot de>
- To: Lukasz Majewski <lukma at denx dot de>
- Cc: Stepan Golosunov <stepan at golosunov dot pp dot ru>, Thomas Gleixner <tglx at linutronix dot de>, Joseph Myers <joseph at codesourcery dot com>, GNU C Library <libc-alpha at sourceware dot org>, Linux API <linux-api at vger dot kernel dot org>, Linux Kernel Mailing List <linux-kernel at vger dot kernel dot org>, Deepa Dinamani <deepa dot kernel at gmail dot com>
- Date: Sat, 27 Apr 2019 22:47:17 +0200
- Subject: Re: [PATCH 1/2] y2038: make CONFIG_64BIT_TIME unconditional
- References: <firstname.lastname@example.org> <20190427004653.3cecd1cb@jawa> <email@example.com> <20190427170613.38bdfbbd@jawa>
On Sat, Apr 27, 2019 at 5:06 PM Lukasz Majewski <firstname.lastname@example.org> wrote:
> > 27.04.2019 в 00:46:53 +0200 Lukasz Majewski написал:
> > (I am wondering whether such trucation is undefined behaviour in C
> According to  - Chapter 188.8.131.52 - Point 3 it is
The kernel relies on the sane behavior for integer overflow in many
places already, and it is built with -fno-strict-overflow to also make
sure gcc doesn't optimize cases that would be undefined otherwise.
> > and
> > 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".
I think the question was whether we should use
kts.tv_nsec = (int)kts.tv_nsec;
kts.tv_nsec &= 0xfffffffful;
Both have a clearly defined meaning in the C dialect we use in the
kernel, but differ in the upper 32 bits for negative input values.
I would say that using sign-extension indeed makes more sense
here, but we don't need to change it for linux-5.1, since none of the
callers of get_timespec64() care -- any negative 32-bit tv_nsec
value results in -EINVAL, including the utimensat() syscall that
has two special cases outside of the 0...999999999 range.