linux-kernel - Re: [PATCH 08/14] arm64: simplify access

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Message-ID: <YguCysZGYYWUhAPk@FVFF77S0Q05N>
Date:   Tue, 15 Feb 2022 10:39:06 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
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Subject: Re: [PATCH 08/14] arm64: simplify access_ok()

On Tue, Feb 15, 2022 at 10:39:46AM +0100, Arnd Bergmann wrote:
> On Tue, Feb 15, 2022 at 10:21 AM Ard Biesheuvel <ardb@...nel.org> wrote:
> > On Tue, 15 Feb 2022 at 10:13, Arnd Bergmann <arnd@...nel.org> wrote:
> >
> > arm64 also has this leading up to the range check, and I think we'd no
> > longer need it:
> >
> >     if (IS_ENABLED(CONFIG_ARM64_TAGGED_ADDR_ABI) &&
> >         (current->flags & PF_KTHREAD || test_thread_flag(TIF_TAGGED_ADDR)))
> >             addr = untagged_addr(addr);
> 
> I suspect the expensive part here is checking the two flags, as untagged_addr()
> seems to always just add a sbfx instruction. Would this work?
> 
> #ifdef CONFIG_ARM64_TAGGED_ADDR_ABI
> #define access_ok(ptr, size) __access_ok(untagged_addr(ptr), (size))
> #else // the else path is the default, this can be left out.
> #define access_ok(ptr, size) __access_ok((ptr), (size))
> #endif

This would be an ABI change, e.g. for tasks without TIF_TAGGED_ADDR.

I don't think we should change this as part of this series.

Thanks,
Mark.