linux-kernel - Re: [PATCH v2 20/22] selftests: futex: Test sys_futex

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Date:   Tue, 9 Nov 2021 09:52:58 -0300
From:   André Almeida <andrealmeid@...labora.com>
To:     Vasily Gorbik <gor@...ux.ibm.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        kernel@...labora.com, krisman@...labora.com,
        linux-api@...r.kernel.org, libc-alpha@...rceware.org,
        mtk.manpages@...il.com, Davidlohr Bueso <dave@...olabs.net>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v2 20/22] selftests: futex: Test sys_futex_waitv() timeout

Hi Vasily,

Às 08:18 de 09/11/21, Vasily Gorbik escreveu:
> On Thu, Sep 23, 2021 at 02:11:09PM -0300, André Almeida wrote:
>> Test if the futex_waitv timeout is working as expected, using the
>> supported clockid options.
> 
>> +	/* futex_waitv with CLOCK_MONOTONIC */
>> +	if (futex_get_abs_timeout(CLOCK_MONOTONIC, &to, timeout_ns))
>> +		return RET_FAIL;
>> +	res = futex_waitv(&waitv, 1, 0, &to, CLOCK_MONOTONIC);
>> +	test_timeout(res, &ret, "futex_waitv monotonic", ETIMEDOUT);
>> +
>> +	/* futex_waitv with CLOCK_REALTIME */
>> +	if (futex_get_abs_timeout(CLOCK_REALTIME, &to, timeout_ns))
>> +		return RET_FAIL;
>> +	res = futex_waitv(&waitv, 1, 0, &to, CLOCK_REALTIME);
>> +	test_timeout(res, &ret, "futex_waitv realtime", ETIMEDOUT);
> 
> Hi André,
> 
> when built with -m32 and run as compat this two futex_waitv calls hang
> on x86 and s390 (noticed while wiring up futex_waitv). The rest of the
> futex selftests pass. This suggests some common compat issue? Any ideas?

The issue is that futex_waitv() only accepts struct timespec that uses
64bit members. When using -m32, glibc will give you a 32bit timespec,
thus the timeout won't wort. Someday glibc will provide 64bit timespec
to 32bit userspace, given that this is affected by y2038 bug.

In previous submissions I added a workaround for that in the
selftest[0]. Search for "Y2038 section for 32-bit applications" in that
link. I'll submit something like that for futex_waitv() timeout test.

[0]
https://lore.kernel.org/lkml/20210709001328.329716-6-andrealmeid@collabora.com/