Messages in this thread | | | From | Arnd Bergmann <> | Date | Tue, 19 May 2020 22:31:23 +0200 | Subject | Re: clock_gettime64 vdso bug on 32-bit arm, rpi-4 |
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On Tue, May 19, 2020 at 10:24 PM Adhemerval Zanella <adhemerval.zanella@linaro.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. > > > > - 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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