linux-kernel - Re: [PATCH] arm64: kcsan: Fix kcsan test

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Date:   Tue, 26 Apr 2022 23:39:44 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Mark Rutland <mark.rutland@....com>
CC:     <elver@...gle.com>, <catalin.marinas@....com>, <will@...nel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: kcsan: Fix kcsan test_barrier fail and panic


On 2022/4/26 20:42, Mark Rutland wrote:
> Hi,
>
> On Tue, Apr 26, 2022 at 08:17:00AM +0000, Kefeng Wang wrote:
>> As "kcsan: Support detecting a subset of missing memory barriers"
>> introduced KCSAN_STRICT which make kcsan detects more missing memory
>> barrier, but arm64 don't have KCSAN instrumentation for barriers, so
>> the new selftest test_barrier() will fail, then panic.
>>
>> Let's prefix all barriers with __ on arm64, as asm-generic/barriers.h
>> defined the final instrumented version of these barriers, which will
>> fix the above issues.
>>
>> Fixes: dd03762ab608 ("arm64: Enable KCSAN")
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
> I don't think the Fixes tag is necessary given this is a new feature
> which depends upon EXPERT, and I'm worried it encourages backporting
> this into a kernel where it would be broken, so I'd prefer we drop that
> tag.
>
> IIUC when we originially looked at this the instrumentation wasn't safe
> and would violate noinstr requirements. Looking at linux/kcsan-checks.h,
> the comments block for __KCSAN_BARRIER_TO_SIGNAL_FENCE() say that it
> will respect __nokcsan, so it looks like that might be safe now.
>
> It looks like that's the case as of commit:
>    
>    bd3d5bd1a0ad3864 ("kcsan: Support WEAK_MEMORY with Clang where no objtool support exists")
>
> ... which requires clang 14.0.0+.
>
> That looks to have gone in concurrently with dd03762ab608, but is
> clearly a prerequisite, so as above I'd strongly prefer we drop the
> Fixes tag.

Sure, the "kcsan: Support detecting a subset of missing memory 
barriers"[1] and

dd03762ab608 "arm64: Enable KCSAN" are both merged in v5.17.  I will 
drop the Fixes tag.

> It would be good if we could note that explicitly in the commit message.
I will add some message into v2.
>
> Have you eyeballed the generated assembly to verify that this works as
> expected for __no_kcsan ?
Look good,  will recheck it.
>
> Thanks,
> Mark.
[1] 
https://patchwork.kernel.org/project/linux-mm/cover/20211130114433.2580590-1-elver@google.com/