Thank you. That is correct.
Alexander Bluhm <alexander.bl...@gmx.net> wrote:
> Hi,
>
> After enabling -fcf-protection=branch for the kernel, we have a new
> .plt section in the kernel. It was not there before.
>
> $ objdump -s .../snapshots/amd64/bsd
> ...
> ffffffff82048540 c7c13140 0682c9e9 c43646ff ..1@.....6F.
> Contents of section .plt:
> ffffffff82048550 00000000 00000000 00000000 00000000 ................
> Contents of section .rodata:
> ffffffff82049000 cccccccc cccccccc cccccccc cccccccc ................
>
> This is caused by compiling gapdummy.c with -fcf-protection=branch.
> Then gapdummy.o gets a new .note.gnu.property section.
>
> $ objdump -s arch/amd64/compile/GENERIC.MP/obj/gapdummy.o
>
> arch/amd64/compile/GENERIC.MP/obj/gapdummy.o: file format elf64-x86-64
>
> Contents of section .note.gnu.property:
> 0000 04000000 10000000 05000000 474e5500 ............GNU.
> 0010 020000c0 04000000 01000000 00000000 ................
>
> When creating gap.o from that, the linker adds a .plt section which
> finally shows up in the kernel.
>
> Diff below restores old behavior. I stumbled over this when linking
> kernel with sorted and aligned objects for my performance tests.
> There it resulted in unexpexted gaps within the kernel image.
>
> I am not sure if we want to fix this. If not, I can probly add
> another workaround for my use case. But this new .plt does not
> make much sense.
>
> ok?
>
> bluhm
>
> Index: sys/arch/amd64/conf/Makefile.amd64
> ===================================================================
> RCS file: /mount/openbsd/cvs/src/sys/arch/amd64/conf/Makefile.amd64,v
> retrieving revision 1.131
> diff -u -p -r1.131 Makefile.amd64
> --- sys/arch/amd64/conf/Makefile.amd64 17 Apr 2023 01:14:24 -0000
> 1.131
> +++ sys/arch/amd64/conf/Makefile.amd64 20 Apr 2023 21:25:05 -0000
> @@ -177,7 +177,7 @@ ld.script: ${_machdir}/conf/ld.script
>
> gapdummy.o:
> echo '__asm(".section .rodata,\"a\"");' > gapdummy.c
> - ${CC} -c ${CFLAGS} ${CPPFLAGS} gapdummy.c -o $@
> + ${CC} -c ${CFLAGS} ${CPPFLAGS} -fcf-protection=none gapdummy.c -o $@
>
> makegap.sh:
> cp $S/conf/makegap.sh $@
>
