[PATCH v4 11/17] arm64/build: Warn on orphan section placement

[Kees Cook <keescook@chromium.org>]

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.

Remove .eh_frame, since there are none left.

Add .plt, .data.rel.ro, .igot.*, and .iplt to discards as they are not
actually used. While .got.plt is also not used, it must be included
otherwise ld.bfd will fail to link with the error:

    aarch64-linux-gnu-ld: discarded output section: `.got.plt'

However, as it'd be better to validate that it stays effectively empty,
add an assert, as suggested by Ard Biesheuvel.

Explicitly include debug sections when they're present.

Finally, enable orphan section warnings.

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 arch/arm64/Makefile             |  4 ++++
 arch/arm64/kernel/vmlinux.lds.S | 11 ++++++++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 895486606f74..fb3aa2d7de4d 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -29,6 +29,10 @@ LDFLAGS_vmlinux	+= --fix-cortex-a53-843419
   endif
 endif
 
+# We never want expected sections to be placed heuristically by the
+# linker. All sections should be explicitly named in the linker script.
+LDFLAGS_vmlinux += --orphan-handling=warn
+
 ifeq ($(CONFIG_ARM64_USE_LSE_ATOMICS), y)
   ifneq ($(CONFIG_ARM64_LSE_ATOMICS), y)
 $(warning LSE atomics not supported by binutils)
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index b5a94ec1eada..320ba5ec2adc 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -94,7 +94,8 @@ SECTIONS
 	/DISCARD/ : {
 		*(.interp .dynamic)
 		*(.dynsym .dynstr .hash .gnu.hash)
-		*(.eh_frame)
+		*(.plt) *(.data.rel.ro)
+		*(.igot.*) *(.iplt)
 	}
 
 	. = KIMAGE_VADDR + TEXT_OFFSET;
@@ -244,9 +245,17 @@ SECTIONS
 	_end = .;
 
 	STABS_DEBUG
+	DWARF_DEBUG
 	ELF_DETAILS
 
 	HEAD_SYMBOLS
+
+	/*
+	 * Make sure that the .got.plt is either completely empty or it
+	 * contains only the lazy dispatch entries.
+	 */
+	.got.plt (INFO) : { *(.got.plt) }
+	ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, ".got.plt not empty")
 }
 
 #include "image-vars.h"
-- 
2.25.1