linux-kernel - Re: [PATCH v6 03/18] kasan: sw_tags: Use arithmetic shift for shadow computation

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Message-ID: <zxn3unp5foytltq6xpzqmsuijaargjuigcu47fdvlhpgyvwfj4@zwcnws34hjfw>
Date: Mon, 17 Nov 2025 18:27:30 +0000
From: Maciej Wieczór-Retman <m.wieczorretman@...me>
To: Alexander Potapenko <glider@...gle.com>
Cc: xin@...or.com, peterz@...radead.org, kaleshsingh@...gle.com, kbingham@...nel.org, akpm@...ux-foundation.org, nathan@...nel.org, ryabinin.a.a@...il.com, dave.hansen@...ux.intel.com, bp@...en8.de, morbo@...gle.com, jeremy.linton@....com, smostafa@...gle.com, kees@...nel.org, baohua@...nel.org, vbabka@...e.cz, justinstitt@...gle.com, wangkefeng.wang@...wei.com, leitao@...ian.org, jan.kiszka@...mens.com, fujita.tomonori@...il.com, hpa@...or.com, urezki@...il.com, ubizjak@...il.com, ada.coupriediaz@....com, nick.desaulniers+lkml@...il.com, ojeda@...nel.org, brgerst@...il.com, elver@...gle.com, pankaj.gupta@....com, mark.rutland@....com, trintaeoitogc@...il.com, jpoimboe@...nel.org, thuth@...hat.com, pasha.tatashin@...een.com, dvyukov@...gle.com, jhubbard@...dia.com, catalin.marinas@....com, yeoreum.yun@....com, mhocko@...e.com, lorenzo.stoakes@...cle.com, samuel.holland@...ive.com, vincenzo.frascino@....com, bigeasy@...utronix.de, surenb@...gle.com, ardb@...nel.org,
	Liam.Howlett@...cle.com, nicolas.schier@...ux.dev, ziy@...dia.com, kas@...nel.org, tglx@...utronix.de, mingo@...hat.com, broonie@...nel.org, corbet@....net, andreyknvl@...il.com, maciej.wieczor-retman@...el.com, david@...hat.com, maz@...nel.org, rppt@...nel.org, will@...nel.org, luto@...nel.org, kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, x86@...nel.org, linux-kbuild@...r.kernel.org, linux-mm@...ck.org, llvm@...ts.linux.dev, linux-doc@...r.kernel.org
Subject: Re: [PATCH v6 03/18] kasan: sw_tags: Use arithmetic shift for shadow computation

On 2025-11-11 at 10:39:12 +0100, Alexander Potapenko wrote:
>> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
>> index b00849ea8ffd..952ade776e51 100644
>> --- a/include/linux/kasan.h
>> +++ b/include/linux/kasan.h
>> @@ -61,8 +61,14 @@ int kasan_populate_early_shadow(const void *shadow_start,
>>  #ifndef kasan_mem_to_shadow
>>  static inline void *kasan_mem_to_shadow(const void *addr)
>>  {
>> -       return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
>> -               + KASAN_SHADOW_OFFSET;
>> +       void *scaled;
>> +
>> +       if (IS_ENABLED(CONFIG_KASAN_GENERIC))
>> +               scaled = (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT);
>> +       else
>> +               scaled = (void *)((long)addr >> KASAN_SHADOW_SCALE_SHIFT);
>> +
>> +       return KASAN_SHADOW_OFFSET + scaled;
>>  }
>>  #endif
>
>As Marco pointed out, this part is reverted in Patch 17. Any reason to do that?

I hope I was able to answer that in my reply to Marco