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To:     Arnd Bergmann <arnd@arndb.de>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Russell King - ARM Linux <linux@armlinux.org.uk>
Cc:     Will Deacon <will@kernel.org>, Rich Felker <dalias@libc.org>,
        Jack Schmidt <jack.schmidt@uky.edu>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Szabolcs Nagy <szabolcs.nagy@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Stephen Boyd <sboyd@kernel.org>
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Subject: Re: clock_gettime64 vdso bug on 32-bit arm, rpi-4
Message-ID: <0c2abcd1-7da8-2559-1e93-4c3bdd38dec1@linaro.org>
Date:   Tue, 19 May 2020 17:24:18 -0300
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On 19/05/2020 16:54, Arnd Bergmann wrote:
> Jack Schmidt reported a bug for the arm32 clock_gettimeofday64 vdso call last
> month: https://github.com/richfelker/musl-cross-make/issues/96 and
> https://github.com/raspberrypi/linux/issues/3579
> 
> As Will Deacon pointed out, this was never reported on the mailing list,
> so I'll try to summarize what we know, so this can hopefully be resolved soon.
> 
> - This happened reproducibly on Linux-5.6 on a 32-bit Raspberry Pi patched
>    kernel running on a 64-bit Raspberry Pi 4b (bcm2711) when calling
>    clock_gettime64(CLOCK_REALTIME)

Does it happen with other clocks as well?

> 
> - The kernel tree is at https://github.com/raspberrypi/linux/, but I could
>   see no relevant changes compared to a mainline kernel.

Is this bug reproducible with mainline kernel or mainline kernel can't be
booted on bcm2711?

> 
> - From the report, I see that the returned time value is larger than the
>   expected time, by 3.4 to 14.5 million seconds in four samples, my
>   guess is that a random number gets added in at some point.

What kind code are you using to reproduce it? It is threaded or issue
clock_gettime from signal handlers?

> 
> - From other sources, I found that the Raspberry Pi clocksource runs
>   at 54 MHz, with a mask value of 0xffffffffffffff. From these numbers
>   I would expect that reading a completely random hardware register
>   value would result in an offset up to 1.33 billion seconds, which is
>   around factor 100 more than the error we see, though similar.
> 
> - The test case calls the musl clock_gettime() function, which falls back to
>   the clock_gettime64() syscall on kernels prior to 5.5, or to the 32-bit
>   clock_gettime() prior to Linux-5.1. As reported in the bug, Linux-4.19 does
>   not show the bug.
> 
> - The behavior was not reproduced on the same user space in qemu,
>   though I cannot tell whether the exact same kernel binary was used.
> 
> - glibc-2.31 calls the same clock_gettime64() vdso function on arm to
>   implement clock_gettime(), but earlier versions did not. I have not
>   seen any reports of this bug, which could be explained by users
>   generally being on older versions.
> 
> - As far as I can tell, there are no reports of this bug from other users,
>   and so far nobody could reproduce it.
> 
> - The current musl git tree has been patched to not call clock_gettime64
>    on ARM because of this problem, so it cannot be used for reproducing it.

So should glibc follow musl and remove arm clock_gettime6y4 vDSO support
or this bug is localized to an specific kernel version running on an
specific hardware?

> 
> If anyone has other information that may help figure out what is going
> on, please share.
> 
>         Arnd
>