Messages in this thread | | | Date | Sun, 24 May 2020 16:49:31 -0700 | From | Fangrui Song <> | Subject | Re: [PATCH 1/4] x86/boot: Add .text.startup to setup.ld |
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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@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.
*(.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@google.com>
>> >> ----- >> >> 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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