linux-kernel - Re: [PATCHv6 22/36] x86/vdso: Add offsets page in vvar

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Date:   Fri, 16 Aug 2019 21:20:59 +0100
From:   Dmitry Safonov <0x7f454c46@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Safonov <dima@...sta.com>
Cc:     linux-kernel@...r.kernel.org, Andrei Vagin <avagin@...nvz.org>,
        Adrian Reber <adrian@...as.de>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jeff Dike <jdike@...toit.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        linux-api@...r.kernel.org, x86@...nel.org,
        Andrei Vagin <avagin@...il.com>
Subject: Re: [PATCHv6 22/36] x86/vdso: Add offsets page in vvar

Hi Thomas,

On 8/15/19 8:21 PM, Thomas Gleixner wrote:
> On Thu, 15 Aug 2019, Dmitry Safonov wrote:
>> ---
>>  arch/Kconfig                          |  5 +++
>>  arch/x86/Kconfig                      |  1 +
>>  arch/x86/entry/vdso/vdso-layout.lds.S |  9 ++++-
>>  arch/x86/entry/vdso/vdso2c.c          |  3 ++
>>  arch/x86/entry/vdso/vma.c             | 12 +++++++
>>  arch/x86/include/asm/vdso.h           |  1 +
>>  init/Kconfig                          |  1 +
>>  lib/vdso/gettimeofday.c               | 47 +++++++++++++++++++++++++++
> 
> This needs to be split into the generic lib/vdso part and then x86 making
> use of it.

Ok

>> +#ifdef CONFIG_TIME_NS
> 
> This should be COMPILE_WITH_TIME_NS and not CONFIG_TIME_NS
> 
>> +extern u8 timens_page
>> +	__attribute__((visibility("hidden")));
>> +
>> +notrace static __always_inline void clk_to_ns(clockid_t clk, struct __kernel_timespec *ts)
> 
> This needs notrace because?

Heh, well it's alive from the time it was in arch/x86.
I believe, functions there had it since commit 23adec554a76 ("x86: add
notrace annotations to vsyscall"). Probably, lib/vdso is compiled
without mcount in the Makefile somewhere.
Will drop.

[..]
>> +	/*
>> +	 * The kernel allows to set a negative offset only if the current clock
>> +	 * value in a namespace is positive, so the result tv_sec can't be
>> +	 * negative here.
>> +	 */
>> +	ts->tv_nsec += offset64->tv_nsec;
>> +	ts->tv_sec += offset64->tv_sec;
>> +	if (ts->tv_nsec >= NSEC_PER_SEC) {
>> +		ts->tv_nsec -= NSEC_PER_SEC;
>> +		ts->tv_sec++;
>> +	}
>> +	if (ts->tv_nsec < 0) {
>> +		ts->tv_nsec += NSEC_PER_SEC;
>> +		ts->tv_sec--;
>> +	}
> 
> That's broken for 32bit user space on 64bit hosts. On LE due to
> misalignment and on BE because 32bit will read always 0.

Ugh, will look into that.

Thanks,
          Dmitry