Re: [PATCH 1/4] x86/boot: Add .text.startup to setup.ld

[Fangrui Song]

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@xxxxxxxxxxxx>
> Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>
> ---
> 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