Date:   Thu, 2 Dec 2021 11:15:59 +0100
From:   Marco Elver <elver@...gle.com>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        joey.gouly@....com
Subject: Re: [PATCH v2] arm64: Enable KCSAN

On Thu, Dec 02, 2021 at 09:35AM +0800, Kefeng Wang wrote:
> 
> On 2021/12/1 19:53, Mark Rutland wrote:
> > Hi Kefeng,
> > 
> > On Mon, Nov 29, 2021 at 10:57:32PM +0800, Kefeng Wang wrote:
> > > This patch enables KCSAN for arm64, with updates to build rules
> > > to not use KCSAN for several incompatible compilation units.
> > > 
> > > Resent GCC version(at least GCC10) made outline-atomics as the
> > > default option(unlike Clang), which will cause linker errors
> > > for kernel/kcsan/core.o.
> > > 
> > > Disables the out-of-line atomics by no-outline-atomics to fix
> > > the linker errors.
> > > 
> > > Tested selftest and kcsan_test(built with GCC11 and Clang 13),
> > > and all passed.
> > Nice!
> > 
> > I think there are a few additional bits and pieces we'll need:
> > 
> > * Prior to clang 12.0.0, KCSAN would produce warnings with BTI, as I found in:
> > 
> >    https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=arm64/kcsan&id=2d67c39ae4f619ca94d9790e09186e77922fa826
> > 
> >    Since BTI is in defconfig, I think arm64's Kconfig should require a minimum
> >    of clang 12.0.0 to enable KCSAN.
> 
> I don't have different clang version to test,  when check KCSAN,
> 
> commit eb32f9f990d9 ("kcsan: Improve some Kconfig comments") saids,
> 
> 
>     The compiler instruments plain compound read-write operations
>     differently (++, --, +=, -=, |=, &=, etc.), which allows KCSAN to
>     distinguish them from other plain accesses. This is currently
>     supported by Clang 12 or later.
> 
> Should we add a  "depends on CLANG_VERSION >= 120000"

KCSAN works just fine with Clang 11. Clang 12 merely improves some
instrumentation, which is what this comment is about.

What Mark meant is that there's a specific issue with arm64 and BTI that
is fixed by Clang 12. Therefore, arm64's Kconfig will have to do

	select HAVE_ARCH_KCSAN if CC_IS_GCC || CLANG_VERSION >= 120000

> > 
> > * In the past clang did not have an attribute to suppress tsan instrumenation
> >    and would instrument noinstr regions. I'm not sure when clang gained the
> >    relevant attribute to supress this, but we will need to depend on this
> >    existing, either based on the clang version or with a test for the attribute.
> > 
> >    (If we're lucky, clang 12.0.0 is sufficient, and we solve BTI and this in one
> >    go).
> > 
> >    I *think* GCC always had an attribute, but I'm not certain.
> > 
> >    Marco, is there an existing dependency somewhere for this to work on x86? I
> >    thought there was an objtool pass to NOP this out, but I couldn't find it in
> >    mainline. If x86 is implicitly depending on a sufficiently recent version of
> >    clang, we add something to the common KCSAN Kconfig for ARCH_WANTS_NO_INSTR?
> > 
> > * There are some latent issues with some code (e.g. alternatives, patching, insn)
> >    code being instrumentable even though this is unsound, and depending on
> >    compiler choices this can happen to be fine or can result in boot-time
> >    failures (I saw lockups when I started trying to add KCSAN for arm64).
> > 
> >    While this isn't just a KCSAN problem, fixing that requires some fairly
> >    significant rework to a bunch of code, and until that's done we're on very
> >    shaky ground. So I'd like to make KCSAN depend on EXPERT for now.
> > 
> >    I had an initial stab at fixing some of that, e.g.
> > 
> >    https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/patching/rework
> >    Joey has started looking into this too.
> 
> Thanks for your information,  I don't know about this. As your say, we could
> add a depend on EXPERT
> 
> for now and more explanation into changlog.

So what I gather arm64's final select line may look like:

	select HAVE_ARCH_KCSAN if EXPERT && (CC_IS_GCC || CLANG_VERSION >= 120000)

> > 
> > * When I last tested, for simple boots I would get frequent KCSAN splats for a
> >    few common issues, and those drowned out all other reports.
> > 
> >    One case was manipulation of thread_info::flags, which Thomas Gleixner has
> >    queued some fixes at:
> > 
> >    https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/log/?h=core/entry
> >    There were some other common failures, e.g. accesses to task_struct::on_cpu,
> >    and I hadn't had the chance to investigate/fix those, beyond a (likely
> >    unsound) hack:
> > 
> >    https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=arm64/kcsan&id=4fe9d6c2ef85257d80291086e4514eaaebd3504e
> > 
> >    It would be good if we could identify the most frequent problems (e.g. those
> >    that will occur when just booting) before we enable this generally, to avoid
> >    everyone racing to report/fix those as soon as we enable the feature.
> > 
> >    When you tested, did KCSAN flag anything beyond the selftests?
> 
> Yes, there are some KCSAN reports, but this is not only exist on arm64, I
> saw  owner->on_cpu warning
> 
> on x86 too, eg, we also hack to disable it via data_race.
> 
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 7 PID: 2530 Comm: syz-executor.11 Not tainted 5.10.0+ #113
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.104/01/2014
> ==================================================================
> BUG: KCSAN: data-race in rwsem_spin_on_owner+0xf4/0x180
> 
> race at unknown origin, with read to 0xffff9767d3becfac of 4 bytes by task 18119 on cpu 0:
>  rwsem_spin_on_owner+0xf4/0x180
>  rwsem_optimistic_spin+0x48/0x480
>  rwsem_down_read_slowpath+0x4a0/0x670
>  down_read+0x69/0x190
>  process_vm_rw+0x41e/0x840
>  __x64_sys_process_vm_writev+0x76/0x90
>  do_syscall_64+0x37/0x50
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9

I think fixing data races is not a pre-requisite for arch-enablement.
Some are slowly being addressed (and others aren't -- syzbot has a list
of >200 data races that I try to moderate and fix some or forward those
that I think will get fixed). I expect the most frequent issues will be
the same on arm64 as they are on x86.

I actually have a "fix" for the data race in rwsem_spin_on_owner, that
also shows where the other racing access comes from... which reminds me:
https://lkml.kernel.org/r/20211202101238.33546-1-elver@google.com

Thanks,
-- Marco

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