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

[Chen Zhongjin]

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@xxxxxxxxxx>
>> ---
>>  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