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Date:   Wed, 18 Mar 2020 08:51:05 +0000
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Qian Cai <cai@lca.pw>
Cc:     akpm@linux-foundation.org, elver@google.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/kmemleak: silence KCSAN splats in checksum
Message-ID: <20200318085105.GQ3005@mbp>
References: <20200317182754.2180-1-cai@lca.pw>
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On Tue, Mar 17, 2020 at 02:27:54PM -0400, Qian Cai wrote:
> Even if KCSAN is disabled for kmemleak, update_checksum() could still
> call crc32() (which is outside of kmemleak.c) to dereference
> object->pointer. Thus, the value of object->pointer could be accessed
> concurrently as noticed by KCSAN,
> 
>  BUG: KCSAN: data-race in crc32_le_base / do_raw_spin_lock
> 
>  write to 0xffffb0ea683a7d50 of 4 bytes by task 23575 on cpu 12:
>   do_raw_spin_lock+0x114/0x200
>   debug_spin_lock_after at kernel/locking/spinlock_debug.c:91
>   (inlined by) do_raw_spin_lock at kernel/locking/spinlock_debug.c:115
>   _raw_spin_lock+0x40/0x50
>   __handle_mm_fault+0xa9e/0xd00
>   handle_mm_fault+0xfc/0x2f0
>   do_page_fault+0x263/0x6f9
>   page_fault+0x34/0x40
> 
>  read to 0xffffb0ea683a7d50 of 4 bytes by task 839 on cpu 60:
>   crc32_le_base+0x67/0x350
>   crc32_le_base+0x67/0x350:
>   crc32_body at lib/crc32.c:106
>   (inlined by) crc32_le_generic at lib/crc32.c:179
>   (inlined by) crc32_le at lib/crc32.c:197
>   kmemleak_scan+0x528/0xd90
>   update_checksum at mm/kmemleak.c:1172
>   (inlined by) kmemleak_scan at mm/kmemleak.c:1497
>   kmemleak_scan_thread+0xcc/0xfa
>   kthread+0x1e0/0x200
>   ret_from_fork+0x27/0x50
> 
> If a shattered value was returned due to a data race, it will be
> corrected in the next scan. Thus, let KCSAN ignore all reads in the
> region to silence KCSAN in case the write side is non-atomic.
> 
> Suggested-by: Marco Elver <elver@google.com>
> Signed-off-by: Qian Cai <cai@lca.pw>

Acked-by: Catalin Marinas <catalin.marinas@arm.com>