linux-kernel - Re: [RFC PATCH v2 01/10] lib: vdso: ensure all arches have 32bit fallback

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Date:   Thu, 2 Jan 2020 12:29:12 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Andy Lutomirski <luto@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [RFC PATCH v2 01/10] lib: vdso: ensure all arches have 32bit fallback

On Mon, Dec 30, 2019 at 1:27 PM Arnd Bergmann <arnd@...db.de> wrote:
> On Mon, Dec 23, 2019 at 3:31 PM Christophe Leroy <christophe.leroy@....fr> wrote:
> > +static __always_inline
> > +long clock_getres32_fallback(clockid_t _clkid, struct old_timespec32 *_ts)
> > +{
> > +       struct __kernel_timespec ts;
> > +       int ret = clock_getres_fallback(clock, &ts);
> > +
> > +       if (likely(!ret && _ts)) {
> > +               _ts->tv_sec = ts.tv_sec;
> > +               _ts->tv_nsec = ts.tv_nsec;
> > +       }
> > +       return ret;
> > +}
>
> Please change these to call __NR_clock_gettime and __NR_clock_getres_time
> instead of __NR_clock_gettime64/__NR_clock_getres_time64 for multiple reasons.
>
> - When doing migration between containers, the vdso may get copied into
>   an application running on a kernel that does not support the time64
>   variants, and then the fallback fails.
>
> - When CONFIG_COMPAT_32BIT_TIME is disabled, the time32 syscalls
>   return -ENOSYS, and the vdso version should have the exact same behavior
>   to avoid surprises. In particular an application that checks clock_gettime()
>   to see if the time32 are in part of the kernel would get an incorrect result
>   here.
>
> arch/arm64/include/asm/vdso/compat_gettimeofday.h already does this,
> I think you can just copy the implementation or find a way to share it.

There was a related discussion on this after a vdso regression on mips,
and I suggested to drop the time32 functions completely from the
vdso when CONFIG_COMPAT_32BIT_TIME is disabled, such as

diff --git a/arch/powerpc/kernel/vdso32/vdso32.lds.S
b/arch/powerpc/kernel/vdso32/vdso32.lds.S
index 00c025ba4a92..605f259fa24c 100644
--- a/arch/powerpc/kernel/vdso32/vdso32.lds.S
+++ b/arch/powerpc/kernel/vdso32/vdso32.lds.S
@@ -145,10 +145,12 @@ VERSION

                __kernel_get_syscall_map;
 #ifndef CONFIG_PPC_BOOK3S_601
+#ifdef CONFIG_COMPAT_32BIT_TIME
                __kernel_gettimeofday;
                __kernel_clock_gettime;
                __kernel_clock_getres;
                __kernel_time;
+#endif
                __kernel_get_tbfreq;
 #endif
                __kernel_sync_dicache;

Any opinions on this? If everyone agrees with that approach, I can
send a cross-architecture patch to do this everywhere. It's probably
best though if Christophe adds that to his series as it touches a lot
of the same files and I would prefer to avoid conflicting changes.

       Arnd