linux-kernel - Re: [PATCH] arm64: lds: move .got section out of .text

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Message-ID: <CAMj1kXGpbKgS8mNxVuAyPvT-vW0LWZOXgqsy5TvKhzJRs_rHkA@mail.gmail.com>
Date:   Fri, 28 Apr 2023 08:29:56 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Bill Wendling <morbo@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH] arm64: lds: move .got section out of .text

Hello Fangrui,

On Fri, 28 Apr 2023 at 06:05, Fangrui Song <maskray@...gle.com> wrote:
>
> Currently, the .got section is placed within the output section .text.
> However, when .got is non-empty, the SHF_WRITE flag is set when linked
> by lld. GNU ld recognizes .text as a special section and ignores the
> SHF_WRITE flag. By renaming .text, we can also get the SHF_WRITE flag.
>
> Conventionally, the .got section is placed just before .got.plt (which
> should be empty and omitted in the kernel). Therefore, we move the .got
> section to a conventional location (between .text and .data) and remove
> the unneeded `. = ALIGN(16)`.
>
> Signed-off-by: Fangrui Song <maskray@...gle.com>
> ---
>  arch/arm64/kernel/vmlinux.lds.S | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
>
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index b9202c2ee18e..2bcb3b30db41 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -181,18 +181,8 @@ SECTIONS
>                         KPROBES_TEXT
>                         HYPERVISOR_TEXT
>                         *(.gnu.warning)
> -               . = ALIGN(16);
> -               *(.got)                 /* Global offset table          */
>         }
>
> -       /*
> -        * Make sure that the .got.plt is either completely empty or it
> -        * contains only the lazy dispatch entries.
> -        */
> -       .got.plt : { *(.got.plt) }
> -       ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18,
> -              "Unexpected GOT/PLT entries detected!")
> -
>         . = ALIGN(SEGMENT_ALIGN);
>         _etext = .;                     /* End of text section */
>
> @@ -247,6 +237,16 @@ SECTIONS
>
>         . = ALIGN(SEGMENT_ALIGN);
>         __inittext_end = .;
> +
> +       .got : { *(.got) }

This is the .init region, which gets freed and unmapped after boot. If
the GOT is non-empty, it needs to remain mapped, so we cannot place it
here.

We have the same issue with the .rodata section, which incorporates
variables marked as __ro_after_init, which are not const qualified. So
given that .rodata is already emitted as WA, and we cannot do anything
about that, let's move the GOT in there.


> +       /*
> +        * Make sure that the .got.plt is either completely empty or it
> +        * contains only the lazy dispatch entries.
> +        */
> +       .got.plt : { *(.got.plt) }
> +       ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18,
> +              "Unexpected GOT/PLT entries detected!")
> +
>         __initdata_begin = .;
>
>         init_idmap_pg_dir = .;
> --
> 2.40.1.495.gc816e09b53d-goog
>