[PATCH v3 4/9] x86/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.

Discards the unused rela, plt, and got sections that are not needed
in the final vmlinux, stop emitting kprobe sections without kprobes,
and enable orphan section warnings.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
 arch/x86/Makefile             | 4 ++++
 arch/x86/include/asm/asm.h    | 6 +++++-
 arch/x86/kernel/vmlinux.lds.S | 6 ++++++
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 00e378de8bc0..f8a5b2333729 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -51,6 +51,10 @@ ifdef CONFIG_X86_NEED_RELOCS
         LDFLAGS_vmlinux := --emit-relocs --discard-none
 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
+
 #
 # Prevent GCC from generating any FP code by mistake.
 #
diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
index 0f63585edf5f..92feec0f0a12 100644
--- a/arch/x86/include/asm/asm.h
+++ b/arch/x86/include/asm/asm.h
@@ -138,11 +138,15 @@
 # define _ASM_EXTABLE_FAULT(from, to)				\
 	_ASM_EXTABLE_HANDLE(from, to, ex_handler_fault)
 
-# define _ASM_NOKPROBE(entry)					\
+# ifdef CONFIG_KPROBES
+#  define _ASM_NOKPROBE(entry)					\
 	.pushsection "_kprobe_blacklist","aw" ;			\
 	_ASM_ALIGN ;						\
 	_ASM_PTR (entry);					\
 	.popsection
+# else
+#  define _ASM_NOKPROBE(entry)
+# endif
 
 #else
 # define _EXPAND_EXTABLE_HANDLE(x) #x
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 3bfc8dd8a43d..bb085ceeaaad 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -412,6 +412,12 @@ SECTIONS
 	DWARF_DEBUG
 
 	DISCARDS
+	/DISCARD/ : {
+		*(.rela.*) *(.rela_*)
+		*(.rel.*) *(.rel_*)
+		*(.got) *(.got.*)
+		*(.igot.*) *(.iplt)
+	}
 }
 
 
-- 
2.25.1