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

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Date:   Mon, 22 Jun 2020 15:06:28 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Borislav Petkov <bp@...e.de>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Arnd Bergmann <arnd@...db.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        clang-built-linux@...glegroups.com, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] x86/boot: Warn on orphan section placement

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@...omium.org>
>---
> 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?