x86: Add prefix symbols for function padding

JIRA: https://issues.redhat.com/browse/RHEL-255

Conflicts:
	scripts/Makefile.lib
	- cs-9 doesn't have b42d230 ("kbuild: factor out the
	  common objtool arguments") or 9ec6ab6 ("kbuild: use
	  objtool-args-y to clean up objtool arguments"), which moved
	  moved objtool_args from scripts/Makefile.build and objtooltopt
	  from scripts/link-vmlinux.sh to scripts/Makefile.lib.  Add the
	  --prefix option to both of former files instead.

	  At the same time, provide scripts/link-vmlinux.sh workarounds
	  to read the CONFIG_FUNCTION_PADDING_BYTES value and only
	  enable objtool for vmlinux.o given the same configuration
	  conditions in b42d230.

commit b341b20
Author: Peter Zijlstra <peterz@infradead.org>
Date:   Fri Oct 28 21:08:19 2022 +0200

    x86: Add prefix symbols for function padding

    When code is compiled with:

      -fpatchable-function-entry=${PADDING_BYTES},${PADDING_BYTES}

    functions will have PADDING_BYTES of NOP in front of them. Unwinders
    and other things that symbolize code locations will typically
    attribute these bytes to the preceding function.

    Given that these bytes nominally belong to the following symbol this
    mis-attribution is confusing.

    Inspired by the fact that CFI_CLANG emits __cfi_##name symbols to
    claim these bytes, use objtool to emit __pfx_##name symbols to do
    the same when CFI_CLANG is not used.

    This then shows the callthunk for symbol 'name' as:

      __pfx_##name+0x6/0x10

    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Tested-by: Yujie Liu <yujie.liu@intel.com>
    Link: https://lkml.kernel.org/r/20221028194453.592512209@infradead.org

Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>

Author: joe-lawrence

arch/x86/Kconfig

@@ -2484,6 +2484,10 @@ config CALL_THUNKS
 	def_bool n
 	select FUNCTION_ALIGNMENT_16B
 
+config PREFIX_SYMBOLS
+	def_bool y
+	depends on CALL_THUNKS && !CFI_CLANG
+
 menuconfig SPECULATION_MITIGATIONS
 	bool "Mitigations for speculative execution vulnerabilities"
 	default y

scripts/Makefile.build

@@ -247,7 +247,8 @@ objtool_args =								\
 	$(if $(CONFIG_X86_SMAP), --uaccess)				\
 	$(if $(linked-object), --link)					\
 	$(if $(part-of-module), --module)				\
-	$(if $(CONFIG_GCOV_KERNEL), --no-unreachable)
+	$(if $(CONFIG_GCOV_KERNEL), --no-unreachable)			\
+	$(if $(CONFIG_PREFIX_SYMBOLS), --prefix=$(CONFIG_FUNCTION_PADDING_BYTES))
 
 cmd_objtool = $(if $(objtool-enabled), ; $(objtool) $(objtool_args) $@)
 cmd_gen_objtooldep = $(if $(objtool-enabled), { echo ; echo '$@: $$(wildcard $(objtool))' ; } >> $(dot-target).cmd)

scripts/link-vmlinux.sh

@@ -38,6 +38,13 @@ is_enabled() {
 	grep -q "^$1=y" include/config/auto.conf
 }
 
+# RHEL-only workaround for missing upstream b42d23065024
+# ("kbuild: factor out the common objtool arguments"), provide a means
+# to read configuration values for a key like CONFIG_FUNCTION_PADDING_BYTES
+config_value() {
+	gawk -vkey="$1" -F= '$1==key{ print $NF }' include/config/auto.conf 2>/dev/null
+}
+
 # Nice output in kbuild format
 # Will be supressed by "make -s"
 info()
@@ -112,6 +119,19 @@ objtool_link()
 		return;
 	fi
 
+	# RHEL-only workaround for missing upstream b42d23065024
+	# ("kbuild: factor out the common objtool arguments"), objtool
+	# is only enabled for vmlinux.o under the following conditions:
+	#
+	# scripts/Makefile.lib
+	#   delay-objtool := $(or $(CONFIG_LTO_CLANG),$(CONFIG_X86_KERNEL_IBT))
+	#
+	# scripts/Makefile.vmlinux_o
+	#   objtool-enabled := $(or $(delay-objtool),$(CONFIG_NOINSTR_VALIDATION))
+	if ! (is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT || is_enabled CONFIG_NOINSTR_VALIDATION); then
+		return
+	fi
+
 	if is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT; then
 
 		# For LTO and IBT, objtool doesn't run on individual
@@ -165,6 +185,10 @@ objtool_link()
 		fi
 	fi
 
+	if is_enabled CONFIG_PREFIX_SYMBOLS; then
+		objtoolopt="${objtoolopt} --prefix=$(config_value "CONFIG_FUNCTION_PADDING_BYTES")"
+	fi
+
 	if [ -n "${objtoolopt}" ]; then
 
 		if is_enabled CONFIG_GCOV_KERNEL; then