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

[Fangrui Song]

On 2020-05-24, Arvind Sankar wrote:
> 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@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
>
> 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.

*(.text.*) looks good to me. When you send PATCH v2, feel free to add

(There is indeed no guarantee that future clang FDO will not place it .text.unknown. :)

Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>

> > -----
> >
> > 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