[PATCH] D109090: [FreeBSD] avoid marking __stack_chk_guard symbol as dso_local on FreeBSD

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added a comment.

In D109090#3113552 <https://reviews.llvm.org/D109090#3113552>, @dim wrote:

> Hmm this seems to cause some trouble when building kernels and kernel modules on FreeBSD, which are built in -ffreestanding mode, and then you get weird relocations in kernel modules:
>
>   clang13  -O2 -pipe -fno-common  -fno-strict-aliasing -Werror -D_KERNEL -DKLD_MODULE -DKLD_TIED -nostdinc   -DHAVE_KERNEL_OPTION_HEADERS -include /usr/home/kostik/work/DEV/src/sys/amd64/compile/X/opt_global.h -I. -I/usr/home/kostik/work/DEV/src/sys -I/usr/home/kostik/work/DEV/src/sys/contrib/ck/include -fno-common -gdwarf-2 -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -fdebug-prefix-map=./machine=/usr/home/kostik/work/DEV/src/sys/amd64/include -fdebug-prefix-map=./x86=/usr/home/kostik/work/DEV/src/sys/x86/include -I/usr/home/kostik/work/DEV/src/sys/amd64/compile/X     -MD  -MF.depend.ufs_vnops.o -MTufs_vnops.o -mcmodel=kernel -mno-red-zone -mno-mmx -mno-sse -msoft-float  -fno-asynchronous-unwind-tables -ffreestanding -fwrapv -fstack-protector -Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes -Wmissing-prototypes -Wpointer-arith -Wcast-qual -Wundef -Wno-pointer-sign -D__printf__=__freebsd_kprintf__ -Wmissing-include-dirs -fdiagnostics-show-option -Wno-unknown-pragmas -Wno-error=tautological-compare -Wno-error=empty-body -Wno-error=parentheses-equality -Wno-error=unused-function -Wno-error=pointer-sign -Wno-error=shift-negative-value -Wno-address-of-packed-member -Wno-error=unused-but-set-variable -Wno-format-zero-length   -mno-aes -mno-avx  -std=iso9899:1999 -c /usr/home/kostik/work/DEV/src/sys/ufs/ufs/ufs_vnops.c -o ufs_vnops.o
>   
>   % readelf -a ../src/sys/amd64/compile/X/modules/usr/home/kostik/work/DEV/src/sys/modules/ufs/ufs_vnops.o | grep GOT | head -3
>   0000000001f7  004a0000002a R_X86_64_REX_GOTP 0000000000000000 __stack_chk_guard - 4
>   00000000060d  004a0000002a R_X86_64_REX_GOTP 0000000000000000 __stack_chk_guard - 4
>   0000000007c8  004a0000002a R_X86_64_REX_GOTP 0000000000000000 __stack_chk_guard - 4
>
> Maybe this should have an additional check for freestanding mode?

`readelf -Wr`/`llvm-readobj -r` prints the full name.

The relocation type name is `R_X86_64_REX_GOTPCRELX`, which is correct (if the definition resides in libc.so) to avoid copy relocations.
Ideally the relevant FreeBSD code should handle the relocation type. It can be handled the same way as R_X86_64_GOTPCREL if no optimization is planned.
-fno-pic programs may use GOT-generating relocations as well (https://maskray.me/blog/2021-08-29-all-about-global-offset-table#undefined%20symbols).


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