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References: <20230925110023.1796789-1-masahiroy@kernel.org> <CAMj1kXHM_dRNVeMuwiPyniz4qUh8JT6mmX2R2VdhBMLCjFA8WQ@mail.gmail.com>
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From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Tue, 26 Sep 2023 23:26:48 +0900
Message-ID: <CAK7LNAR5bjfRShbbPsoAriGR5=fG85yzBWfvBFaaK_=kd5U_LA@mail.gmail.com>
Subject: Re: [PATCH] ARM: fix get_user() broken with veneer
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     linux-arm-kernel@lists.infradead.org,
        Russell King <linux@armlinux.org.uk>,
        linux-kernel@vger.kernel.org, linux-kbuild@vger.kernel.org,
        Nick Desaulniers <ndesaulniers@google.com>
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On Tue, Sep 26, 2023 at 8:37=E2=80=AFPM Ard Biesheuvel <ardb@kernel.org> wr=
ote:
>
> Hello Masahiro,
>
> Thanks for getting to the bottom of this.
>
> The patch looks correct to me, but could we please drop this huge
> commit log text? The AAPCS simply states that a BL instruction might
> clobber IP, so any inline asm that contains a BL should have IP in its
> clobber list. In our case, the use of PIC veneers is what might cause
> this, so we should mention that as well. But your entire journey to
> get to that conclusion is not something that deserves to be included
> in the git commit log forever, imho.
>
> So with the commit log trimmed:
>
> Reviewed-by: Ard Biesheuvel <ardb@kernel.org>


Is this commit log OK?


=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D>8=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
The 32-bit ARM kernel stops working if the kernel grows to the point
where veneers for __get_user_* are created.

AAPCS32 [1] states, "Register r12 (IP) may be used by a linker as a
scratch register between a routine and any subroutine it calls. It
can also be used within a routine to hold intermediate values between
subroutine calls."

However, the bl instruction buried within the inline asm is unpredictable
for compilers; hence, "ip" must be added to the clobber list.

This becomes critical when veneers for __get_user_* are created because
veneers use the ip register since commit 02e541db0540 ("ARM: 8323/1:
force linker to use PIC veneers").

[1]: https://github.com/ARM-software/abi-aa/blob/2023Q1/aapcs32/aapcs32.rst
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=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D>8=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D


--=20
Best Regards
Masahiro Yamada