On Tue, 15 Feb 2022 at 10:13, Arnd Bergmann [off-list ref] wrote:

On Tue, Feb 15, 2022 at 9:17 AM Ard Biesheuvel [off-list ref] wrote:

quoted

On Mon, 14 Feb 2022 at 17:37, Arnd Bergmann [off-list ref] wrote:

quoted

From: Arnd Bergmann <arnd@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