Fedora 33 VM running on Ubuntu 20.04 LTS host. All updates applied to both.
[roland@localhost VBox_GAs_6.1.16]$ uname -mrs
Linux 5.11.15-200.fc33.x86_64 x86_64
Tried to insert Guest Additions today.
./tools/objtool/objtool orc generate --module --no-fp --retpoline --uaccess /tmp/vbox.0/common/string/strformatrt.o
gcc -Wp,-MMD,/tmp/vbox.0/.combined-os-specific.o.d -nostdinc -isystem /usr/lib/gcc/x86_64-redhat-linux/10/include -I./arch/x86/include -I./arch/x86/include/generated -I./include -I./arch/x86/include/uapi -I./arch/x86/include/generated/uapi -I./include/uapi -I./include/generated/uapi -include ./include/linux/kconfig.h -include ./include/linux/compiler_types.h -D__KERNEL__ -fmacro-prefix-map=./= -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE -Werror=implicit-function-declaration -Werror=implicit-int -Wno-address-of-packed-member -Werror=return-type -Wno-format-security -std=gnu89 -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -fcf-protection=none -m64 -falign-jumps=1 -falign-loops=1 -mno-80387 -mno-fp-ret-in-387 -mpreferred-stack-boundary=3 -mskip-rax-setup -mtune=generic -mno-red-zone -mcmodel=kernel -Wno-sign-compare -fno-asynchronous-unwind-tables -mindirect-branch=thunk-extern -mindirect-branch-register -fno-jump-tables -fno-delete-null-pointer-checks -Wno-frame-address -Wno-format-truncation -Wno-format-overflow -Wno-address-of-packed-member -O2 -fno-allow-store-data-races -Wframe-larger-than=2048 -fstack-protector -Wno-unused-but-set-variable -Wimplicit-fallthrough -Wno-unused-const-variable -g -pg -mrecord-mcount -mfentry -DCC_USING_FENTRY -Wdeclaration-after-statement -Wvla -Wno-pointer-sign -Wno-stringop-truncation -Wno-zero-length-bounds -Wno-array-bounds -Wno-stringop-overflow -Wno-restrict -Wno-maybe-uninitialized -fno-strict-overflow -fno-stack-check -fconserve-stack -Werror=date-time -Werror=incompatible-pointer-types -Werror=designated-init -Wno-packed-not-aligned -Wno-declaration-after-statement -fno-pie -include /tmp/vbox.0/include/VBox/VBoxGuestMangling.h -I./include -I/tmp/vbox.0/ -I/tmp/vbox.0/include -I/tmp/vbox.0/r0drv/linux -D__KERNEL__ -DMODULE -DRT_WITHOUT_PRAGMA_ONCE -DVBOX -DRT_OS_LINUX -DIN_RING0 -DIN_RT_R0 -DIN_GUEST -DIN_GUEST_R0 -DIN_MODULE -DRT_WITH_VBOX -DVBGL_VBOXGUEST -DVBOX_WITH_HGCM -DVBOX_WITH_64_BITS_GUESTS -DRT_ARCH_AMD64 -DMODULE -DKBUILD_BASENAME='"combined_os_specific"' -DKBUILD_MODNAME='"vboxguest"' -c -o /tmp/vbox.0/combined-os-specific.o /tmp/vbox.0/combined-os-specific.c
In file included from /tmp/vbox.0/combined-os-specific.c:33:
/tmp/vbox.0/r0drv/linux/memobj-r0drv-linux.c: In function ‘rtR0MemObjLinuxVMap’:
/tmp/vbox.0/r0drv/linux/memobj-r0drv-linux.c:560:34: error: implicit declaration of function ‘alloc_vm_area’ [-Werror=implicit-function-declaration]
560 | pMemLnx->pArea = alloc_vm_area(pMemLnx->Core.cb, papPtes); /* Note! pArea->nr_pages is not set. */
| ^~~~~~~~~~~~~
/tmp/vbox.0/r0drv/linux/memobj-r0drv-linux.c:560:32: warning: assignment to ‘struct vm_struct *’ from ‘int’ makes pointer from integer without a cast [-Wint-conversion]
560 | pMemLnx->pArea = alloc_vm_area(pMemLnx->Core.cb, papPtes); /* Note! pArea->nr_pages is not set. */
| ^
cc1: some warnings being treated as errors
make[2]: *** [scripts/Makefile.build:279: /tmp/vbox.0/combined-os-specific.o] Error 1
make[2]: *** Waiting for unfinished jobs....
./tools/objtool/objtool orc generate --module --no-fp --retpoline --uaccess /tmp/vbox.0/combined-agnostic.o
make[1]: *** [Makefile:1821: /tmp/vbox.0] Error 2
make: *** [/tmp/vbox.0/Makefile-footer.gmk:117: vboxguest] Error 2
[roland@localhost VBox_GAs_6.1.16]$
Since Guest Additions installs in all of the other VMs I’m not certain I should finger Oracle for this. It looks like a tweak to default compiler options. Using the flag to treat all warnings as errors. Might not be default on other platforms.