Re: [PATCH v2 3/3] x86/boot: Warn on orphan section placement

[Fangrui Song]

On 2020-06-22, Kees Cook wrote:
> We don't want to depend on the linker's orphan section placement
> heuristics as these can vary between linkers, and may change between
> versions. All sections need to be explicitly named in the linker
> script.
>
> Add the common debugging sections. Discard the unused note, rel, plt,
> dyn, and hash sections that are not needed in the compressed vmlinux.
> Disable .eh_frame generation in the linker and enable orphan section
> warnings.
>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> arch/x86/boot/compressed/Makefile      |  3 ++-
> arch/x86/boot/compressed/vmlinux.lds.S | 11 +++++++++++
> 2 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 7619742f91c9..646720a05f89 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -48,6 +48,7 @@ GCOV_PROFILE := n
> UBSAN_SANITIZE :=n
>
> KBUILD_LDFLAGS := -m elf_$(UTS_MACHINE)
> +KBUILD_LDFLAGS += $(call ld-option,--no-ld-generated-unwind-info)
> # Compressed kernel should be built as PIE since it may be loaded at any
> # address by the bootloader.
> ifeq ($(CONFIG_X86_32),y)
> @@ -59,7 +60,7 @@ else
> KBUILD_LDFLAGS += $(shell $(LD) --help 2>&1 | grep -q "\-z noreloc-overflow" \
> 	&& echo "-z noreloc-overflow -pie --no-dynamic-linker")
> endif
> -LDFLAGS_vmlinux := -T
> +LDFLAGS_vmlinux := --orphan-handling=warn -T
>
> hostprogs	:= mkpiggy
> HOST_EXTRACFLAGS += -I$(srctree)/tools/include
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index 8f1025d1f681..6fe3ecdfd685 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -75,5 +75,16 @@ SECTIONS
> 	. = ALIGN(PAGE_SIZE);	/* keep ZO size page aligned */
> 	_end = .;
>
> +	STABS_DEBUG
> +	DWARF_DEBUG
> +
> 	DISCARDS
> +	/DISCARD/ : {
> +		*(.note.*)
> +		*(.rela.*) *(.rela_*)
> +		*(.rel.*) *(.rel_*)
> +		*(.plt) *(.plt.*)
> +		*(.dyn*)
> +		*(.hash) *(.gnu.hash)
> +	}
> }
> --
> 2.25.1

LLD may report warnings for 3 synthetic sections if they are orphans:

ld.lld: warning: <internal>:(.symtab) is being placed in '.symtab'
ld.lld: warning: <internal>:(.shstrtab) is being placed in '.shstrtab'
ld.lld: warning: <internal>:(.strtab) is being placed in '.strtab'

Are they described?