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Date:   Sun, 24 May 2020 15:13:26 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Dmitry Golovin <dima@...ovin.in>,
        clang-built-linux@...glegroups.com,
        Ard Biesheuvel <ardb@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] x86/boot: Add .text.startup to setup.ld

On 2020-05-24, Arvind Sankar wrote:
>gcc puts the main function into .text.startup when compiled with -Os (or
>-O2). This results in arch/x86/boot/main.c having a .text.startup
>section which is currently not included explicitly in the linker script
>setup.ld in the same directory.
>
>The BFD linker places this orphan section immediately after .text, so
>this still works. However, LLD git, since [1], is choosing to place it
>immediately after the .bstext section instead (this is the first code
>section). This plays havoc with the section layout that setup.elf
>requires to create the setup header, for eg on 64-bit:
>
>    LD      arch/x86/boot/setup.elf
>  ld.lld: error: section .text.startup file range overlaps with .header
>  >>> .text.startup range is [0x200040, 0x2001FE]
>  >>> .header range is [0x2001EF, 0x20026B]
>
>  ld.lld: error: section .header file range overlaps with .bsdata
>  >>> .header range is [0x2001EF, 0x20026B]
>  >>> .bsdata range is [0x2001FF, 0x200398]
>
>  ld.lld: error: section .bsdata file range overlaps with .entrytext
>  >>> .bsdata range is [0x2001FF, 0x200398]
>  >>> .entrytext range is [0x20026C, 0x2002D3]
>
>  ld.lld: error: section .text.startup virtual address range overlaps
>  with .header
>  >>> .text.startup range is [0x40, 0x1FE]
>  >>> .header range is [0x1EF, 0x26B]
>
>  ld.lld: error: section .header virtual address range overlaps with
>  .bsdata
>  >>> .header range is [0x1EF, 0x26B]
>  >>> .bsdata range is [0x1FF, 0x398]
>
>  ld.lld: error: section .bsdata virtual address range overlaps with
>  .entrytext
>  >>> .bsdata range is [0x1FF, 0x398]
>  >>> .entrytext range is [0x26C, 0x2D3]
>
>  ld.lld: error: section .text.startup load address range overlaps with
>  .header
>  >>> .text.startup range is [0x40, 0x1FE]
>  >>> .header range is [0x1EF, 0x26B]
>
>  ld.lld: error: section .header load address range overlaps with
>  .bsdata
>  >>> .header range is [0x1EF, 0x26B]
>  >>> .bsdata range is [0x1FF, 0x398]
>
>  ld.lld: error: section .bsdata load address range overlaps with
>  .entrytext
>  >>> .bsdata range is [0x1FF, 0x398]
>  >>> .entrytext range is [0x26C, 0x2D3]
>
>Explicitly pull .text.startup into the .text output section to avoid
>this.
>
>[1] https://reviews.llvm.org/D75225
>
>Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
>Reviewed-by: Fangrui Song <maskray@...gle.com>
>---
> arch/x86/boot/setup.ld | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/arch/x86/boot/setup.ld b/arch/x86/boot/setup.ld
>index 24c95522f231..ed60abcdb089 100644
>--- a/arch/x86/boot/setup.ld
>+++ b/arch/x86/boot/setup.ld
>@@ -20,7 +20,7 @@ SECTIONS
> 	.initdata	: { *(.initdata) }
> 	__end_init = .;
>
>-	.text		: { *(.text) }
>+	.text		: { *(.text.startup) *(.text) }
> 	.text32		: { *(.text32) }
>
> 	. = ALIGN(16);
>-- 
>2.26.2

Should .text.startup* be used instead? If -ffunction-sections is used,

// a.c
int main() {}

gcc -O2 a.c                     # .text.startup
gcc -Os a.c                     # .text.startup

gcc -O2 -ffunction-sections a.c # .text.startup.main
gcc -Os -ffunction-sections a.c # .text.startup.main

-----

In case anyone wants to CC a GCC dev for the citation that 
  main compiles to `.text.startup` in -Os or -O2 mode, I have a small request
  that `.text.startup.` probably makes more sense. See

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95095

I made an llvm change recently https://reviews.llvm.org/D79600

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