linux-kernel - Re: [PATCH 08/10] x86: remove address space overrides using set

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Message-ID: <CAHk-=wjxeN+KrCB2TyC5s2RWhz-dWWO8vbBwWcCiKb0+8ipayw@mail.gmail.com>
Date:   Thu, 27 Aug 2020 11:15:12 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 08/10] x86: remove address space overrides using set_fs()

On Thu, Aug 27, 2020 at 8:00 AM Christoph Hellwig <hch@....de> wrote:
>
>  SYM_FUNC_START(__get_user_2)
>         add $1,%_ASM_AX
>         jc bad_get_user

This no longer makes sense, and

> -       mov PER_CPU_VAR(current_task), %_ASM_DX
> -       cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
> +       LOAD_TASK_SIZE_MAX
> +       cmp %_ASM_DX,%_ASM_AX

This should be

        LOAD_TASK_SIZE_MAX_MINUS_N(1)
        cmp %_ASM_DX,%_ASM_AX

instead (and then because we no longer modify _ASM_AX, we'd also
remove the offset on the access).

>  SYM_FUNC_START(__put_user_2)
> -       ENTER
> -       mov TASK_addr_limit(%_ASM_BX),%_ASM_BX
> +       LOAD_TASK_SIZE_MAX
>         sub $1,%_ASM_BX

It's even more obvious here. We load a constant and then immediately
do a "sub $1" on that value.

It's not a huge deal, you don't have to respin the series for this, I
just wanted to point it out so that people are aware of it and if I
forget somebody else will hopefully remember that "we should fix that
too".

                   Linus