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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Tue, 15 Feb 2022 10:39:46 +0100
Message-ID: <CAK8P3a0NTqK_m7q909d8FN6is8k4_u3zeckC9XOrjEi7kqSvmg@mail.gmail.com>
Subject: Re: [PATCH 08/14] arm64: simplify access_ok()
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Christoph Hellwig <hch@lst.de>,
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On Tue, Feb 15, 2022 at 10:21 AM Ard Biesheuvel <ardb@kernel.org> wrote:
> On Tue, 15 Feb 2022 at 10:13, Arnd Bergmann <arnd@kernel.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

       Arnd