[PATCH 08/14] arm64: simplify access_ok()

[Ard Biesheuvel]

On Tue, 15 Feb 2022 at 10:13, Arnd Bergmann <arnd at kernel.org> wrote:
>
> On Tue, Feb 15, 2022 at 9:17 AM Ard Biesheuvel <ardb at kernel.org> wrote:
> > On Mon, 14 Feb 2022 at 17:37, Arnd Bergmann <arnd at kernel.org> wrote:
> > > From: Arnd Bergmann <arnd at arndb.de>
> > >
> >
> > With set_fs() out of the picture, wouldn't it be sufficient to check
> > that bit #55 is clear? (the bit that selects between TTBR0 and TTBR1)
> > That would also remove the need to strip the tag from the address.
> >
> > Something like
> >
> >     asm goto("tbnz  %0, #55, %2     \n"
> >              "tbnz  %1, #55, %2     \n"
> >              :: "r"(addr), "r"(addr + size - 1) :: notok);
> >     return 1;
> > notok:
> >     return 0;
> >
> > with an additional sanity check on the size which the compiler could
> > eliminate for compile-time constant values.
>
> That should work, but I don't see it as a clear enough advantage to
> have a custom implementation. For the constant-size case, it probably
> isn't better than a compiler-scheduled comparison against a
> constant limit, but it does hurt maintainability when the next person
> wants to change the behavior of access_ok() globally.
>

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);

> If we want to get into micro-optimizing uaccess, I think a better target
> would be a CONFIG_CC_HAS_ASM_GOTO_OUTPUT version
> of __get_user()/__put_user as we have on x86 and powerpc.
>
>          Arnd