kbuild: upgrade the orphan section warning to an error if CONFIG_WERROR is set

Andrew Cooper suggested upgrading the orphan section warning to a hard link
error. However Nathan Chancellor said outright turning the warning into an
error with no escape hatch might be too aggressive, as we have had these
warnings triggered by new compiler generated sections, and suggested turning
orphan sections into an error only if CONFIG_WERROR is set. Kees Cook echoed
and emphasized that the mandate from Linus is that we should avoid breaking
builds. It wrecks bisection, it causes problems across compiler versions, etc.

Thus upgrade the orphan section warning to a hard link error only if
CONFIG_WERROR is set.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Xin Li <xin3.li@intel.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221025073023.16137-2-xin3.li@intel.com

Makefile

@@ -1118,7 +1118,7 @@ endif
 # We never want expected sections to be placed heuristically by the
 # linker. All sections should be explicitly named in the linker script.
 ifdef CONFIG_LD_ORPHAN_WARN
-LDFLAGS_vmlinux += --orphan-handling=warn
+LDFLAGS_vmlinux += --orphan-handling=$(CONFIG_LD_ORPHAN_WARN_LEVEL)
 endif
 
 # Align the bit size of userspace programs with the kernel

arch/arm/boot/compressed/Makefile

@@ -123,7 +123,7 @@ LDFLAGS_vmlinux += --no-undefined
 LDFLAGS_vmlinux += -X
 # Report orphan sections
 ifdef CONFIG_LD_ORPHAN_WARN
-LDFLAGS_vmlinux += --orphan-handling=warn
+LDFLAGS_vmlinux += --orphan-handling=$(CONFIG_LD_ORPHAN_WARN_LEVEL)
 endif
 # Next argument is a linker script
 LDFLAGS_vmlinux += -T

arch/arm64/kernel/vdso/Makefile

@@ -27,7 +27,7 @@ ldflags-y := -shared -soname=linux-vdso.so.1 --hash-style=sysv	\
 	     -Bsymbolic --build-id=sha1 -n $(btildflags-y)
 
 ifdef CONFIG_LD_ORPHAN_WARN
-  ldflags-y += --orphan-handling=warn
+  ldflags-y += --orphan-handling=$(CONFIG_LD_ORPHAN_WARN_LEVEL)
 endif
 
 ldflags-y += -T

arch/arm64/kernel/vdso32/Makefile

@@ -104,7 +104,7 @@ VDSO_AFLAGS += -D__ASSEMBLY__
 VDSO_LDFLAGS += -Bsymbolic --no-undefined -soname=linux-vdso.so.1
 VDSO_LDFLAGS += -z max-page-size=4096 -z common-page-size=4096
 VDSO_LDFLAGS += -shared --hash-style=sysv --build-id=sha1
-VDSO_LDFLAGS += --orphan-handling=warn
+VDSO_LDFLAGS += --orphan-handling=$(CONFIG_LD_ORPHAN_WARN_LEVEL)
 
 
 # Borrow vdsomunge.c from the arm vDSO

arch/x86/boot/compressed/Makefile

@@ -68,7 +68,7 @@ KBUILD_LDFLAGS += $(call ld-option,--no-ld-generated-unwind-info)
 # address by the bootloader.
 LDFLAGS_vmlinux := -pie $(call ld-option, --no-dynamic-linker)
 ifdef CONFIG_LD_ORPHAN_WARN
-LDFLAGS_vmlinux += --orphan-handling=warn
+LDFLAGS_vmlinux += --orphan-handling=$(CONFIG_LD_ORPHAN_WARN_LEVEL)
 endif
 LDFLAGS_vmlinux += -z noexecstack
 ifeq ($(CONFIG_LD_IS_BFD),y)

init/Kconfig

@@ -159,10 +159,12 @@ config WERROR
 	help
 	  A kernel build should not cause any compiler warnings, and this
 	  enables the '-Werror' (for C) and '-Dwarnings' (for Rust) flags
-	  to enforce that rule by default.
+	  to enforce that rule by default. Certain warnings from other tools
+	  such as the linker may be upgraded to errors with this option as
+	  well.
 
-	  However, if you have a new (or very old) compiler with odd and
-	  unusual warnings, or you have some architecture with problems,
+	  However, if you have a new (or very old) compiler or linker with odd
+	  and unusual warnings, or you have some architecture with problems,
 	  you may need to disable this config option in order to
 	  successfully build the kernel.
 
@@ -1454,6 +1456,13 @@ config LD_ORPHAN_WARN
 	def_bool y
 	depends on ARCH_WANT_LD_ORPHAN_WARN
 	depends on $(ld-option,--orphan-handling=warn)
+	depends on $(ld-option,--orphan-handling=error)
+
+config LD_ORPHAN_WARN_LEVEL
+        string
+        depends on LD_ORPHAN_WARN
+        default "error" if WERROR
+        default "warn"
 
 config SYSCTL
 	bool