linux-kernel - Re: [PATCH v2 13/14] x86: runtime_const used for KASAN_SHADOW

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Message-ID: <2bhtxd4vrkyx26wczp25adpg6wqr6pq4j63ez42zjoxbl7inn6@4mnmpat7uve5>
Date: Wed, 26 Feb 2025 18:03:07 +0100
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: Andrey Konovalov <andreyknvl@...il.com>
CC: Florian Mayer <fmayer@...gle.com>, Vitaly Buka <vitalybuka@...gle.com>,
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Subject: Re: [PATCH v2 13/14] x86: runtime_const used for KASAN_SHADOW_END

On 2025-02-26 at 16:24:28 +0100, Andrey Konovalov wrote:
>On Wed, Feb 26, 2025 at 12:53 PM Maciej Wieczor-Retman
><maciej.wieczor-retman@...el.com> wrote:
>>
>> After adding
>>         kasan_params += hwasan-instrument-with-calls=0
>> to Makefile.kasan just under
>>         kasan_params += hwasan-mapping-offset=$(KASAN_SHADOW_OFFSET)
>> inline works properly in x86. I looked into assembly and before there were just
>> calls to __hwasan_load/store. After adding the the
>> hwasan-instrument-with-calls=0 I can see no calls and the KASAN offset is now
>> inlined, plus all functions that were previously instrumented now have the
>> kasan_check_range inlined in them.
>>
>> My LLVM investigation lead me to
>>         bool shouldInstrumentWithCalls(const Triple &TargetTriple) {
>>           return optOr(ClInstrumentWithCalls, TargetTriple.getArch() == Triple::x86_64);
>>         }
>> which I assume defaults to "1" on x86? So even with inline mode it doesn't care
>> and still does an outline version.
>
>Ah, indeed. Weird discrepancy between x86 and arm.
>
>Florian, Vitaly, do you recall why this was implemented like this?
>
>To account for this, let's then set hwasan-instrument-with-calls=0
>when CONFIG_KASAN_INLINE is enabled. And also please add a comment
>explaining why this is done.

Sure, will do :)

>
>[...]
>
>> >What do you mean by "The alignment doesn't fit the shadow memory size"?
>>
>> Maybe that's the wrong way to put it. I meant that KASAN_SHADOW_END and
>> KASAN_SHADOW_END aren't aligned to the size of shadow memory.
>
>I see. And the negative side-effect of this would be that we'll need
>extra page table entries to describe the shadow region?

I think so, yes. But I guess it's not a big issue, and anyway right now I'm not
sure how to change it so other necessary parts don't break :b

>
>[...]
>
>> I think this was a false alarm, sorry. I asked Kirill about turning
>> pgtable_l5_enabled() into a runtime_const value but it turns out it's already
>> patched by alternative code during boot. I just saw a bunch more stuff there
>> because I was looking at the assembly output and the code isn't patched there
>> yet.
>
>Great!

-- 
Kind regards
Maciej Wieczór-Retman