linux-kernel - Re: clock_gettime64 vdso bug on 32-bit arm, rpi-4

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Date:   Wed, 20 May 2020 16:41:29 +0100
From:   Szabolcs Nagy <szabolcs.nagy@....com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Adhemerval Zanella <adhemerval.zanella@...aro.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Will Deacon <will@...nel.org>, Rich Felker <dalias@...c.org>,
        Jack Schmidt <jack.schmidt@....edu>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>, nd@....com
Subject: Re: clock_gettime64 vdso bug on 32-bit arm, rpi-4

The 05/19/2020 22:31, Arnd Bergmann wrote:
> On Tue, May 19, 2020 at 10:24 PM Adhemerval Zanella
> <adhemerval.zanella@...aro.org> wrote:
> > 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?
> 
> Unclear.
> 
> > > - 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?
> 
> Mainline linux-5.6 should boot on that machine but might not have
> all the other features, so I think users tend to use the raspberry pi
> kernel sources for now.
> 
> > > - 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?
> 
> The reproducer is very simple without threads or signals,
> see the start of https://github.com/richfelker/musl-cross-make/issues/96
> 
> It does rely on calling into the musl wrapper, not the direct vdso
> call.
> 
> > > - 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.

note: i could not reproduce it in qemu-system with these configs:

qemu-system-aarch64 + arm64 kernel + compat vdso
qemu-system-aarch64 + kvm accel (on cortex-a72) + 32bit arm kernel
qemu-system-arm + cpu max + 32bit arm kernel

so i think it's something specific to that user's setup
(maybe rpi hw bug or gcc miscompiled the vdso or something
with that particular linux, i built my own linux 5.6 because
i did not know the exact kernel version where the bug was seen)

i don't have access to rpi (or other cortex-a53 where i
can install my own kernel) so this is as far as i got.

> > >
> > > - 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?
> 
> I hope we can figure out what is actually going on soon, there is probably
> no need to change glibc before we have.
> 
>           Arnd

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