linux-kernel - Re: [PATCH v2 1/2] arm64/crash_core: Export KERNELPACMASK in vmcoreinfo

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Message-ID: <20200506123112.GF8043@willie-the-truck>
Date:   Wed, 6 May 2020 13:31:12 +0100
From:   Will Deacon <will@...nel.org>
To:     Amit Kachhap <amit.kachhap@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Bhupesh Sharma <bhsharma@...hat.com>,
        Vincenzo Frascino <Vincenzo.Frascino@....com>,
        James Morse <james.morse@....com>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v2 1/2] arm64/crash_core: Export KERNELPACMASK in
 vmcoreinfo

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?

Will