linux-kernel - [PATCH v2 2/2] arm64/build: Warn on orphan section placement

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Message-Id: <20200622205815.2988115-3-keescook@chromium.org>
Date:   Mon, 22 Jun 2020 13:58:15 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Will Deacon <will@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Collingbourne <pcc@...gle.com>,
        James Morse <james.morse@....com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        clang-built-linux@...glegroups.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2 2/2] arm64/build: Warn on orphan section placement

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.

Explicitly include debug sections when they're present. Add .eh_frame*
to discard as it seems that these are still generated even though
-fno-asynchronous-unwind-tables is being specified. Add .plt and
.data.rel.ro to discards as they are not actually used. Add .got.plt
to the image as it does appear to be mapped near .data. Finally enable
orphan section warnings.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 arch/arm64/Makefile             | 4 ++++
 arch/arm64/kernel/vmlinux.lds.S | 5 ++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index a0d94d063fa8..3e628983445a 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 5427f502c3a6..c9ecb3b2007d 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)
+		*(.eh_frame) *(.init.eh_frame)
 	}
 
 	. = KIMAGE_VADDR + TEXT_OFFSET;
@@ -209,6 +210,7 @@ SECTIONS
 	_data = .;
 	_sdata = .;
 	RW_DATA(L1_CACHE_BYTES, PAGE_SIZE, THREAD_ALIGN)
+	.got.plt : ALIGN(8) { *(.got.plt) }
 
 	/*
 	 * Data written with the MMU off but read with the MMU on requires
@@ -244,6 +246,7 @@ SECTIONS
 	_end = .;
 
 	STABS_DEBUG
+	DWARF_DEBUG
 
 	HEAD_SYMBOLS
 }
-- 
2.25.1