Re: [PATCH v2] arm64: Enable KCSAN

[Marco Elver <elver@google.com>]

On Wed, Dec 01, 2021 at 11:53AM +0000, Mark Rutland wrote:
[...]
> * 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?

Apologies for the confusion w.r.t. attributes and which sanitizers on
which compilers respect which attributes. I think you may be confusing
things with KCOV (see 540540d06e9d9). I think the various 'select
ARCH_HAS_KCOV' may need adjusting, that is true.

But KCOV != KCSAN, and for KCSAN the story is different. Since the first
version of KCSAN in 5.8, we've had a working __no_kcsan (aka
__no_sanitize_thread) with all versions of Clang and GCC that support
KCSAN (see HAVE_KCSAN_COMPILER).

The recent discussion was for CONFIG_KCSAN_WEAK_MEMORY [1], because
Clang's no_sanitize("thread") would still instrument builtin atomics and
__tsan_func_{entry,exit}, which only that mode would start inserting
instrumentation for. That's not in mainline yet.

[1] https://lkml.kernel.org/r/20211130114433.2580590-1-elver@google.com

It is true that v1 and v2 of that series would have caused issues on
arm64, but after our discussion last week and tried a little harder and
v3 does the right thing for all architectures now and __no_kcsan will
disable all instrumentation (even for barriers).

So the attribute and noinstr story should not need anything else, and
the new KCSAN_WEAK_MEMORY has all right Kconfig dependencies in place
when it lands in mainline.

> * 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 take it you mean arm64 should do 'select HAVE_ARCH_KCSAN if EXPERT'. I
certainly don't mind, but usually folks interested in running debug
tools won't be stopped by a dependency on EXPERT. You could do 'select
HAVE_ARCH_KCSAN if BROKEN' which is more likely to prevent usage while
things are still likely to be broken on arm64.

Thanks,
-- Marco

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