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

On Tue, May 19, 2020 at 10:24 PM Adhemerval Zanella
[off-list ref] wrote:

On 19/05/2020 16:54, Arnd Bergmann wrote:

quoted

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.

quoted

- 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.

quoted

- 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.

quoted

- 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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