Re: [rcu] 2f08469563: BUG:kernel_reboot-without-warning_in_boot_stage

[Marco Elver]

On Tue, 19 May 2020 at 12:16, Marco Elver <elver@xxxxxxxxxx> wrote:
>
> On Mon, 18 May 2020 at 20:05, Marco Elver <elver@xxxxxxxxxx> wrote:
> >
> > On Mon, 18 May 2020, 'Nick Desaulniers' via kasan-dev wrote:
> >
> > > On Mon, May 18, 2020 at 7:34 AM Marco Elver <elver@xxxxxxxxxx> wrote:
> > > >
> > > > On Mon, 18 May 2020 at 14:44, Marco Elver <elver@xxxxxxxxxx> wrote:
> > > > >
> > > > > [+Cc clang-built-linux FYI]
> > > > >
> > > > > On Mon, 18 May 2020 at 12:11, Marco Elver <elver@xxxxxxxxxx> wrote:
> > > > > >
> > > > > > On Sun, 17 May 2020 at 05:47, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > > > > > >
> > > > > > > On Sun, May 17, 2020 at 09:17:32AM +0800, kernel test robot wrote:
> > > > > > > > Greeting,
> > > > > > > >
> > > > > > > > FYI, we noticed the following commit (built with clang-11):
> > > > > > > >
> > > > > > > > commit: 2f08469563550d15cb08a60898d3549720600eee ("rcu: Mark rcu_state.ncpus to detect concurrent writes")
> > > > > > > > https://git.kernel.org/cgit/linux/kernel/git/paulmck/linux-rcu.git dev.2020.05.14c
> > > > > > > >
> > > > > > > > in testcase: boot
> > > > > > > >
> > > > > > > > on test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 8G
> > > > > > > >
> > > > > > > > caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace):
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > > If you fix the issue, kindly add following tag
> > > > > > > > Reported-by: kernel test robot <rong.a.chen@xxxxxxxxx>
> > > > > > > >
> > > > > > > >
> > > > > > > > [    0.054943] BRK [0x05204000, 0x05204fff] PGTABLE
> > > > > > > > [    0.061181] BRK [0x05205000, 0x05205fff] PGTABLE
> > > > > > > > [    0.062403] BRK [0x05206000, 0x05206fff] PGTABLE
> > > > > > > > [    0.065200] RAMDISK: [mem 0x7a247000-0x7fffffff]
> > > > > > > > [    0.067344] ACPI: Early table checksum verification disabled
> > > > > > > > BUG: kernel reboot-without-warning in boot stage
> > > > > > >
> > > > > > > I am having some difficulty believing that this commit is at fault given
> > > > > > > that the .config does not list CONFIG_KCSAN=y, but CCing Marco Elver
> > > > > > > for his thoughts.  Especially given that I have never built with clang-11.
> > > > > > >
> > > > > > > But this does invoke ASSERT_EXCLUSIVE_WRITER() in early boot from
> > > > > > > rcu_init().  Might clang-11 have objections to early use of this macro?
> > > > > >
> > > > > > The macro is a noop without KCSAN. I think the bisection went wrong.
> > > > > >
> > > > > > I am able to reproduce a reboot-without-warning when building with
> > > > > > Clang 11 and the provided config. I did a bisect, starting with v5.6
> > > > > > (good), and found this:
> > > > > > - Since v5.6, first bad commit is
> > > > > > 20e2aa812620439d010a3f78ba4e05bc0b3e2861 (Merge tag
> > > > > > 'perf-urgent-2020-04-12' of
> > > > > > git://git.kernel.org/pub/scm/linux/kernel//git/tip/tip)
> > > > > > - The actual commit that introduced the problem is
> > > > > > 2b3b76b5ec67568da4bb475d3ce8a92ef494b5de (perf/x86/intel/uncore: Add
> > > > > > Ice Lake server uncore support) -- reverting it fixes the problem.
> > > >
> > > > Some more clues:
> > > >
> > > > 1. I should have noticed that this uses CONFIG_KASAN=y.
> > >
> > > Thanks for the report, testing, and bisection.  I don't see any
> > > smoking gun in the code.
> > > https://godbolt.org/z/qbK26r
> >
> > My guess is data layout and maybe some interaction with KASAN. I also
> > played around with leaving icx_mmio_uncores empty, meaning none of the
> > data it refers to end up in the data section (presumably because
> > optimized out), which resulted in making the bug disappear as well.
> >
> > > >
> > > > 2. Something about function icx_uncore_mmio_init(). Making it a noop
> > > > also makes the issue go away.
> > > >
> > > > 3. Leaving icx_uncore_mmio_init() a noop but removing the 'static'
> > > > from icx_mmio_uncores also presents the issue. So this seems to be
> > > > something about how/where icx_mmio_uncores is allocated.
> > >
> > > Can you share the disassembly of icx_uncore_mmio_init() in the given
> > > configuration?
> >
> > ffffffff8102c097 <icx_uncore_mmio_init>:
> > ffffffff8102c097:       e8 b4 52 bd 01          callq  ffffffff82c01350 <__fentry__>
> > ffffffff8102c09c:       48 c7 c7 e0 55 c3 83    mov    $0xffffffff83c355e0,%rdi
> > ffffffff8102c0a3:       e8 69 9a 3b 00          callq  ffffffff813e5b11 <__asan_store8>
> > ffffffff8102c0a8:       48 c7 05 2d 95 c0 02    movq   $0xffffffff83c388e0,0x2c0952d(%rip)        # ffffffff83c355e0 <uncore_mmio_uncores>
> > ffffffff8102c0af:       e0 88 c3 83
> > ffffffff8102c0b3:       c3                      retq
> >
> > The problem still happens if we add a __no_sanitize_address (or even
> > KASAN_SANITIZE := n) here. I think this function is a red herring: you
> > can make this function be empty, but as long as icx_mmio_uncores and its
> > dependencies are added to the data section somewhere, does the bug
> > appear.
>
> I also tried to bisect Clang/LLVM, and found that
> https://reviews.llvm.org/D78162 introduced the breaking change to
> Clang/LLVM. Reverting that change results in a bootable kernel *with*
> "perf/x86/intel/uncore: Add Ice Lake server uncore support" still
> applied.

I found that with Clang/LLVM change D78162, a bunch of memcpys are
optimized into just a bunch of loads/stores. It may turn out that this
is again a red herring, because the result is that more code is
generated, affecting layout. So in the end, the Clang/LLVM bisection
might just point at the first change that causes data layout to change
in a way that triggers the bug.

Thanks,
-- Marco