s390/nospec: add an option to use thunk-extern

Currently with -mindirect-branch=thunk and -mfunction-return=thunk compiler
options expoline thunks are put into individual COMDAT group sections. s390
is the only architecture which has group sections and it has implications
for kpatch and objtool tools support.

Using -mindirect-branch=thunk-extern and -mfunction-return=thunk-extern
is an alternative, which comes with a need to generate all required
expoline thunks manually. Unfortunately modules area is too far away from
the kernel image, and expolines from the kernel image cannon be used.
But since all new distributions (except Debian) build kernels for machine
generations newer than z10, where "exrl" instruction is available, that
leaves only 16 expolines thunks possible.

Provide an option to build the kernel with
-mindirect-branch=thunk-extern and -mfunction-return=thunk-extern for
z10 or newer. This also requires to postlink expoline thunks into all
modules explicitly. Currently modules already contain most expolines
anyhow.

Unfortunately -mindirect-branch=thunk-extern and
-mfunction-return=thunk-extern options support is broken in gcc <= 11.2.
Additional compile test is required to verify proper gcc support.
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Co-developed-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/Kconfig

@@ -585,6 +585,7 @@ config KERNEL_NOBP
 
 config EXPOLINE
 	def_bool n
+	depends on $(cc-option,-mindirect-branch=thunk)
 	prompt "Avoid speculative indirect branches in the kernel"
 	help
 	  Compile the kernel with the expoline compiler options to guard
@@ -595,6 +596,20 @@ config EXPOLINE
 
 	  If unsure, say N.
 
+config EXPOLINE_EXTERN
+	def_bool n
+	depends on EXPOLINE
+	depends on HAVE_MARCH_Z10_FEATURES
+	depends on CC_IS_GCC && GCC_VERSION >= 110200
+	depends on $(success,$(srctree)/arch/s390/tools/gcc-thunk-extern.sh $(CC))
+	prompt "Generate expolines as extern functions."
+	help
+	  This option is required for some tooling like kpatch. The kernel is
+	  compiled with -mindirect-branch=thunk-extern and requires a newer
+	  compiler.
+
+	  If unsure, say N.
+
 choice
 	prompt "Expoline default"
 	depends on EXPOLINE

arch/s390/Makefile

@@ -81,14 +81,18 @@ ifneq ($(call cc-option,-mstack-size=8192 -mstack-guard=128),)
 endif
 
 ifdef CONFIG_EXPOLINE
-  ifneq ($(call cc-option,$(CC_FLAGS_MARCH) -mindirect-branch=thunk),)
+  ifdef CONFIG_EXPOLINE_EXTERN
+    KBUILD_LDFLAGS_MODULE += arch/s390/lib/expoline.o
+    CC_FLAGS_EXPOLINE := -mindirect-branch=thunk-extern
+    CC_FLAGS_EXPOLINE += -mfunction-return=thunk-extern
+  else
     CC_FLAGS_EXPOLINE := -mindirect-branch=thunk
     CC_FLAGS_EXPOLINE += -mfunction-return=thunk
-    CC_FLAGS_EXPOLINE += -mindirect-branch-table
-    export CC_FLAGS_EXPOLINE
-    cflags-y += $(CC_FLAGS_EXPOLINE) -DCC_USING_EXPOLINE
-    aflags-y += -DCC_USING_EXPOLINE
   endif
+  CC_FLAGS_EXPOLINE += -mindirect-branch-table
+  export CC_FLAGS_EXPOLINE
+  cflags-y += $(CC_FLAGS_EXPOLINE) -DCC_USING_EXPOLINE
+  aflags-y += -DCC_USING_EXPOLINE
 endif
 
 ifdef CONFIG_FUNCTION_TRACER

arch/s390/include/asm/nospec-insn.h

@@ -18,7 +18,11 @@ _LC_BR_R1 = __LC_BR_R1
  * the various thunks are merged into a single copy.
  */
 	.macro __THUNK_PROLOG_NAME name
+#ifdef CONFIG_EXPOLINE_EXTERN
+	.pushsection .text,"ax",@progbits
+#else
 	.pushsection .text.\name,"axG",@progbits,\name,comdat
+#endif
 	.globl \name
 	.hidden \name
 	.type \name,@function
@@ -115,7 +119,13 @@ _LC_BR_R1 = __LC_BR_R1
 555:	br	\reg
 	.endm
 
+#ifdef CONFIG_EXPOLINE_EXTERN
+	.macro GEN_BR_THUNK reg,ruse=%r1
+	.endm
+	.macro GEN_BR_THUNK_EXTERN reg,ruse=%r1
+#else
 	.macro GEN_BR_THUNK reg,ruse=%r1
+#endif
 	__DECODE_RR __THUNK_PROLOG_BR,\reg,\ruse
 	__THUNK_EX_BR \reg,\ruse
 	__THUNK_EPILOG

arch/s390/lib/Makefile

@@ -7,6 +7,7 @@ lib-y += delay.o string.o uaccess.o find.o spinlock.o
 obj-y += mem.o xor.o
 lib-$(CONFIG_KPROBES) += probes.o
 lib-$(CONFIG_UPROBES) += probes.o
+obj-$(CONFIG_EXPOLINE_EXTERN) += expoline.o
 obj-$(CONFIG_S390_KPROBES_SANITY_TEST) += test_kprobes_s390.o
 test_kprobes_s390-objs += test_kprobes_asm.o test_kprobes.o
 

arch/s390/lib/expoline.S

+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <asm/nospec-insn.h>
+#include <linux/linkage.h>
+
+.macro GEN_ALL_BR_THUNK_EXTERN
+	.irp r1,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15
+	GEN_BR_THUNK_EXTERN %r\r1
+	.endr
+.endm
+
+GEN_ALL_BR_THUNK_EXTERN

arch/s390/tools/gcc-thunk-extern.sh

+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+# Borrowed from gcc: gcc/testsuite/gcc.target/s390/nobp-section-type-conflict.c
+# Checks that we don't get error: section type conflict with ‘put_page’.
+
+cat << "END" | $@ -x c - -fno-PIE -march=z10 -mindirect-branch=thunk-extern -mfunction-return=thunk-extern -mindirect-branch-table -O2 -c -o /dev/null
+int a;
+int b (void);
+void c (int);
+
+static void
+put_page (void)
+{
+  if (b ())
+    c (a);
+}
+
+__attribute__ ((__section__ (".init.text"), __cold__)) void
+d (void)
+{
+  put_page ();
+  put_page ();
+}
+END

scripts/mod/modpost.c

@@ -658,6 +658,11 @@ static int ignore_undef_symbol(struct elf_info *info, const char *symname)
 		    strstarts(symname, "_savevr_") ||
 		    strcmp(symname, ".TOC.") == 0)
 			return 1;
+
+	if (info->hdr->e_machine == EM_S390)
+		/* Expoline thunks are linked on all kernel modules during final link of .ko */
+		if (strstarts(symname, "__s390_indirect_jump_r"))
+			return 1;
 	/* Do not ignore this symbol */
 	return 0;
 }