Re: [PATCH] ARM: decompressor: fix BSS size calculation for LLVM ld.lld

From: Guillaume Tucker <guillaume.tucker@collabora.com>
To: Ard Biesheuvel <ardb@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Russell King <linux@armlinux.org.uk>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Fangrui Song <maskray@google.com>
Subject: Re: [PATCH] ARM: decompressor: fix BSS size calculation for LLVM ld.lld
Date: Sat, 6 Feb 2021 13:16:02 +0000	[thread overview]
Message-ID: <b68e267f-ec86-4ae4-7785-7fc9162a7c2b@collabora.com> (raw)
In-Reply-To: <CAMj1kXHKOEJokzHH_Bf0bq-ATmJ4u-yZUN_tAYJvTX4c_trMXw@mail.gmail.com>

On 05/02/2021 18:11, Ard Biesheuvel wrote:
> On Fri, 5 Feb 2021 at 19:00, Nick Desaulniers <ndesaulniers@google.com> wrote:
>>
>> On Fri, Feb 5, 2021 at 12:52 AM Ard Biesheuvel <ardb@kernel.org> wrote:
>>>
>>> The LLVM ld.lld linker uses a different symbol type for __bss_start,
>>> resulting in the calculation of KBSS_SZ to be thrown off. Up until now,
>>> this has gone unnoticed as it only affects the appended DTB case, but
>>> pending changes for ARM in the way the decompressed kernel is cleaned
>>> from the caches has uncovered this problem.
>>>
>>> On a ld.lld build:
>>>
>>>   $ nm vmlinux |grep bss_
>>>   c1c22034 D __bss_start
>>>   c1c86e98 B __bss_stop
>>>
>>> resulting in
>>>
>>
>> $ readelf -s arch/arm/boot/compressed/vmlinux | grep bss_size
>>
>>>   433: c1c86e98     0 NOTYPE  GLOBAL DEFAULT  ABS _kernel_bss_size
>>>
>>> which is obviously incorrect, and may cause the cache clean to access
>>> unmapped memory, or cause the size calculation to wrap, resulting in no
>>> cache clean to be performed at all.
>>>
>>> Fix this by updating the sed regex to take D type symbols into account.
>>>
>>> Cc: Nick Desaulniers <ndesaulniers@google.com>
>>> Cc: Nathan Chancellor <nathan@kernel.org>
>>> Cc: Guillaume Tucker <guillaume.tucker@collabora.com>
>>> Link: https://lore.kernel.org/linux-arm-kernel/6c65bcef-d4e7-25fa-43cf-2c435bb61bb9@collabora.com/
>>> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
>>
>>
>> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
>> Tested-by: Nick Desaulniers <ndesaulniers@google.com>
>>
>> Thanks for debugging+fixing this, and Guillaume for the report.  It's
>> nice to see a fix so early; thinking back to last year before KernelCI
>> integration, we probably would have only noticed when CrOS went to
>> upgrade their rk3288 platform devices.
>>
>> Some other tags that might be nice to apply:
>>
>> Cc: stable@kernel.org
>> Fixes: 429f7a062e3b ("ARM: decompressor: fix BSS size calculation")
>> Reported-by: Guillaume Tucker <guillaume.tucker@collabora.com>
>> Reported-by: "kernelci.org bot" <bot@kernelci.org>
>>
> 
> Thanks. I'll add these tags and drop this patch into the patch system

Thanks!  You may even add:

  Tested-by: "kernelci.org bot" <bot@kernelci.org>

See some results here, showing it's all booting fine:

  https://staging.kernelci.org/test/job/ardb/branch/for-kernelci/kernel/v5.11-rc6-146-g923ca344043a/plan/baseline/

with this revision from your for-kernelci branch:

  https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/commit/?id=923ca344043a19067c21f46bd8649f35f61ce920

Best wishes,
Guillaume

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