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Subject: Re: [PATCH v2 1/2] arm64/crash_core: Export KERNELPACMASK in
 vmcoreinfo
To:     Will Deacon <will@kernel.org>
Cc:     linux-arm-kernel@lists.infradead.org, kexec@lists.infradead.org,
        linux-kernel@vger.kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Bhupesh Sharma <bhsharma@redhat.com>,
        Vincenzo Frascino <Vincenzo.Frascino@arm.com>,
        James Morse <james.morse@arm.com>,
        Mark Rutland <mark.rutland@arm.com>
References: <1587968702-19996-1-git-send-email-amit.kachhap@arm.com>
 <20200504171741.GD1833@willie-the-truck>
 <bc5e6fc5-15f4-40bb-4466-816de5912893@arm.com>
 <20200506123112.GF8043@willie-the-truck>
From:   Amit Kachhap <amit.kachhap@arm.com>
Message-ID: <590176f0-802b-f71a-0c1e-00283808e1d8@arm.com>
Date:   Wed, 6 May 2020 18:34:20 +0530
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Hi,

On 5/6/20 6:01 PM, Will Deacon wrote:
> On Wed, May 06, 2020 at 05:32:56PM +0530, Amit Kachhap wrote:
>> On 5/4/20 10:47 PM, Will Deacon wrote:
>>> On Mon, Apr 27, 2020 at 11:55:01AM +0530, Amit Daniel Kachhap wrote:
>>>> diff --git a/arch/arm64/include/asm/compiler.h b/arch/arm64/include/asm/compiler.h
>>>> index eece20d..32d5900 100644
>>>> --- a/arch/arm64/include/asm/compiler.h
>>>> +++ b/arch/arm64/include/asm/compiler.h
>>>> @@ -19,6 +19,9 @@
>>>>    #define __builtin_return_address(val)					\
>>>>    	(void *)(ptrauth_clear_pac((unsigned long)__builtin_return_address(val)))
>>>> +#else  /* !CONFIG_ARM64_PTR_AUTH */
>>>> +#define	ptrauth_user_pac_mask()		0ULL
>>>> +#define	ptrauth_kernel_pac_mask()	0ULL
>>>
>>> This doesn't look quite right to me, since you still have to take into
>>> account the case where CONFIG_ARM64_PTR_AUTH=y but the feature is not
>>> available at runtime:
>>
>> Yes agree with you here. However the config gaurd is saving some extra
>> computation for __builtin_return_address. There are some compiler
>> support being added in __builtin_extract_return_address to mask the PAC.
>> Hopefully that will improve this code. In the meantime let it be like this.
> 
> Does the extra computation matter? Isn't it just a couple of instructions?

ok sure. I will push v3 as you suggested.

Thanks,
Amit

> 
> Will
>