linux-kernel - Re: [PATCH v6 32/33] arm64: irq-gic: Replace unreachable() with -EINVAL

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Message-ID: <9217c85d-e39e-55b1-36c3-603d0c6203fd@huawei.com>
Date:   Fri, 24 Jun 2022 09:24:54 +0800
From:   Chen Zhongjin <chenzhongjin@...wei.com>
To:     Marc Zyngier <maz@...nel.org>
CC:     <linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>,
        <linuxppc-dev@...ts.ozlabs.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kbuild@...r.kernel.org>, <live-patching@...r.kernel.org>,
        <jpoimboe@...nel.org>, <peterz@...radead.org>,
        <catalin.marinas@....com>, <will@...nel.org>,
        <masahiroy@...nel.org>, <michal.lkml@...kovi.net>,
        <ndesaulniers@...gle.com>, <mark.rutland@....com>,
        <pasha.tatashin@...een.com>, <broonie@...nel.org>,
        <rmk+kernel@...linux.org.uk>, <madvenka@...ux.microsoft.com>,
        <christophe.leroy@...roup.eu>, <daniel.thompson@...aro.org>
Subject: Re: [PATCH v6 32/33] arm64: irq-gic: Replace unreachable() with
 -EINVAL

Hi,

Thanks for your review and patient.

On 2022/6/23 16:13, Marc Zyngier wrote:
> On 2022-06-23 02:49, Chen Zhongjin wrote:
>> Using unreachable() at default of switch generates an extra branch at
>> end of the function, and compiler won't generate a ret to close this
>> branch because it knows it's unreachable.
>>
>> If there's no instruction in this branch, compiler will generate a NOP,
>> And it will confuse objtool to warn this NOP as a fall through branch.
>>
>> In fact these branches are actually unreachable, so we can replace
>> unreachable() with returning a -EINVAL value.
>>
>> Signed-off-by: Chen Zhongjin <chenzhongjin@...wei.com>
>> ---
>>  arch/arm64/kvm/hyp/vgic-v3-sr.c | 7 +++----
>>  drivers/irqchip/irq-gic-v3.c    | 2 +-
>>  2 files changed, 4 insertions(+), 5 deletions(-)
> 
> Basic courtesy would have been to Cc the maintainers of this code.
> 
Sorry for that.

I'll cc everyone next time.

>>
>> diff --git a/arch/arm64/kvm/hyp/vgic-v3-sr.c b/arch/arm64/kvm/hyp/vgic-v3-sr.c
>> index 4fb419f7b8b6..f3cee92c3038 100644
>> --- a/arch/arm64/kvm/hyp/vgic-v3-sr.c
>> +++ b/arch/arm64/kvm/hyp/vgic-v3-sr.c
>> @@ -6,7 +6,6 @@
>>
>>  #include <hyp/adjust_pc.h>
>>
>> -#include <linux/compiler.h>
>>  #include <linux/irqchip/arm-gic-v3.h>
>>  #include <linux/kvm_host.h>
>>
>> @@ -55,7 +54,7 @@ static u64 __gic_v3_get_lr(unsigned int lr)
>>          return read_gicreg(ICH_LR15_EL2);
>>      }
>>
>> -    unreachable();
>> +    return -EINVAL;
> 
> NAK. That's absolutely *wrong*, and will hide future bugs.
> Nothing checks for -EINVAL, and we *never* expect to
> reach this, hence the perfectly valid annotation.
> 
> If something needs fixing, it probably is your tooling.
> 
>         M.

You are right.

Essentially, this is because objtool does not anticipate that the compiler will
generate additional instructions when marking unreachable instructions.

I'll fix this problem or add a specific check for this state.

Best,
Chen