Re: [PATCH v3 4/9] x86/build: Warn on orphan section placement

[Kees Cook]

On Thu, Jun 25, 2020 at 02:36:27AM +0800, kernel test robot wrote:
> I love your patch! Perhaps something to improve:
> [...]
> config: x86_64-randconfig-a012-20200624 (attached as .config)

CONFIG_KCSAN=y

> compiler: clang version 11.0.0 (https://github.com/llvm/llvm-project 1d4c87335d5236ea1f35937e1014980ba961ae34)
> [...]
> All warnings (new ones prefixed by >>):
> 
>    ld.lld: warning: drivers/built-in.a(mfd/mt6397-irq.o):(.init_array.0) is being placed in '.init_array.0'

As far as I can tell, this is a Clang bug. But I don't know the
internals here, so I've opened:
https://bugs.llvm.org/show_bug.cgi?id=46478

and created a work-around patch for the kernel:


commit 915f2c343e59a14f00c68f4d7afcfdc621de0674
Author: Kees Cook <keescook@xxxxxxxxxxxx>
Date:   Sat Jun 27 08:07:54 2020 -0700

    vmlinux.lds.h: Avoid KCSAN's unwanted sections
    
    KCSAN (-fsanitize=thread) produces unwanted[1] .eh_frame and .init_array.*
    sections. Add them to DISCARDS, except with CONFIG_CONSTRUCTORS, which
    wants to keep .init_array.* sections.
    
    [1] https://bugs.llvm.org/show_bug.cgi?id=46478
    
    Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index f8a5b2333729..41c8c73de6c4 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -195,7 +195,9 @@ endif
 # Workaround for a gcc prelease that unfortunately was shipped in a suse release
 KBUILD_CFLAGS += -Wno-sign-compare
 #
-KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
+KBUILD_AFLAGS += -fno-asynchronous-unwind-tables -fno-unwind-tables
+KBUILD_CFLAGS += -fno-asynchronous-unwind-tables -fno-unwind-tables
+KBUILD_LDFLAGS += $(call ld-option,--no-ld-generated-unwind-info)
 
 # Avoid indirect branches in kernel to deal with Spectre
 ifdef CONFIG_RETPOLINE
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index b1dca0762fc5..a44ee16abc78 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -934,10 +934,28 @@
 	EXIT_DATA
 #endif
 
+/*
+ * Clang's -fsanitize=thread produces unwanted sections (.eh_frame
+ * and .init_array.*), but CONFIG_CONSTRUCTORS wants to keep any
+ * .init_array.* sections.
+ * https://bugs.llvm.org/show_bug.cgi?id=46478
+ */
+#if defined(CONFIG_KCSAN) && !defined(CONFIG_CONSTRUCTORS)
+#define KCSAN_DISCARDS	 						\
+	*(.init_array) *(.init_array.*)					\
+	*(.eh_frame)
+#elif defined(CONFIG_KCSAN) && defined(CONFIG_CONSTRUCTORS)
+#define KCSAN_DISCARDS	 						\
+	*(.eh_frame)
+#else
+#define KCSAN_DISCARDS
+#endif
+
 #define DISCARDS							\
 	/DISCARD/ : {							\
 	EXIT_DISCARDS							\
 	EXIT_CALL							\
+	KCSAN_DISCARDS							\
 	*(.discard)							\
 	*(.discard.*)							\
 	*(.modinfo)							\

-- 
Kees Cook