s390: always build relocatable kernel

Nathan Chancellor reported several link errors on s390 with
CONFIG_RELOCATABLE disabled, after binutils commit 906f69cf65da ("IBM
zSystems: Issue error for *DBL relocs on misaligned symbols"). The binutils
commit reveals potential miscompiles that might have happened already
before with linker script defined symbols at odd addresses.

A similar bug was recently fixed in the kernel with commit c9305b6
("s390: fix nospec table alignments").

See ClangBuiltLinux#1747 for an analysis
from Ulich Weigand.

Therefore always build a relocatable kernel to avoid this problem. There is
hardly any use-case for non-relocatable kernels, so this shouldn't be
controversial.

Link: ClangBuiltLinux#1747
Signed-off-by: Heiko Carstens <[email protected]>
Reported-by: Nathan Chancellor <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexander Gordeev <[email protected]>

Author: hcahca

arch/s390/Kconfig

@@ -568,8 +568,7 @@ config EXPOLINE_FULL
 endchoice
 
 config RELOCATABLE
-	bool "Build a relocatable kernel"
-	default y
+	def_bool y
 	help
 	  This builds a kernel image that retains relocation information
 	  so it can be loaded at an arbitrary address.
@@ -578,10 +577,11 @@ config RELOCATABLE
 	  bootup process.
 	  The relocations make the kernel image about 15% larger (compressed
 	  10%), but are discarded at runtime.
+	  Note: this option exists only for documentation purposes, please do
+	  not remove it.
 
 config RANDOMIZE_BASE
 	bool "Randomize the address of the kernel image (KASLR)"
-	depends on RELOCATABLE
 	default y
 	help
 	  In support of Kernel Address Space Layout Randomization (KASLR),

arch/s390/Makefile

@@ -14,10 +14,8 @@ KBUILD_AFLAGS_MODULE += -fPIC
 KBUILD_CFLAGS_MODULE += -fPIC
 KBUILD_AFLAGS	+= -m64
 KBUILD_CFLAGS	+= -m64
-ifeq ($(CONFIG_RELOCATABLE),y)
 KBUILD_CFLAGS	+= -fPIE
 LDFLAGS_vmlinux	:= -pie
-endif
 aflags_dwarf	:= -Wa,-gdwarf-2
 KBUILD_AFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -D__ASSEMBLY__
 ifndef CONFIG_AS_IS_LLVM

arch/s390/boot/Makefile

@@ -37,9 +37,8 @@ CFLAGS_sclp_early_core.o += -I$(srctree)/drivers/s390/char
 
 obj-y	:= head.o als.o startup.o mem_detect.o ipl_parm.o ipl_report.o
 obj-y	+= string.o ebcdic.o sclp_early_core.o mem.o ipl_vmparm.o cmdline.o
-obj-y	+= version.o pgm_check_info.o ctype.o ipl_data.o
+obj-y	+= version.o pgm_check_info.o ctype.o ipl_data.o machine_kexec_reloc.o
 obj-$(findstring y, $(CONFIG_PROTECTED_VIRTUALIZATION_GUEST) $(CONFIG_PGSTE))	+= uv.o
-obj-$(CONFIG_RELOCATABLE)	+= machine_kexec_reloc.o
 obj-$(CONFIG_RANDOMIZE_BASE)	+= kaslr.o
 obj-y	+= $(if $(CONFIG_KERNEL_UNCOMPRESSED),,decompressor.o) info.o
 obj-$(CONFIG_KERNEL_ZSTD) += clz_ctz.o

arch/s390/boot/startup.c

@@ -291,8 +291,7 @@ void startup_kernel(void)
 
 	clear_bss_section();
 	copy_bootdata();
-	if (IS_ENABLED(CONFIG_RELOCATABLE))
-		handle_relocs(__kaslr_offset);
+	handle_relocs(__kaslr_offset);
 
 	if (__kaslr_offset) {
 		/*