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
>
