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 <[email protected]>
>Reviewed-by: Fangrui Song <[email protected]>
>---
> 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 <[email protected]>
-----
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
