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

[Arvind Sankar <nivedita@alum.mit.edu>]

On Sun, May 24, 2020 at 03:13:26PM -0700, Fangrui Song wrote:
> 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@alum.mit.edu>
> >Reviewed-by: Fangrui Song <maskray@google.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

It's probably unlikely we'll use function-sections on the setup code,
but *(.text.*) might be more future-proof, since gcc/clang might grow
the ability to stick code into .text.hot or .text.unlikely etc
automatically.

> 
> -----
> 
> 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