powerpc/ftrace: Add a postlink script to validate function tracer

Function tracer on powerpc can only work with vmlinux having a .text
size of up to ~64MB due to powerpc branch instruction having a limited
relative branch range of 32MB. Today, this is only detected on kernel
boot when ftrace is init'ed. Add a post-link script to check the size of
.text so that we can detect this at build time, and break the build if
necessary.

We add a dependency on !COMPILE_TEST for CONFIG_HAVE_FUNCTION_TRACER so
that allyesconfig and other test builds can continue to work without
enabling ftrace.

Signed-off-by: Naveen N Rao <[e-mail]>
Signed-off-by: Hari Bathini <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

arch/powerpc/Kconfig

···
 	select HAVE_FUNCTION_DESCRIPTORS	if PPC64_ELF_ABI_V1
 	select HAVE_FUNCTION_ERROR_INJECTION
 	select HAVE_FUNCTION_GRAPH_TRACER
-	select HAVE_FUNCTION_TRACER		if PPC64 || (PPC32 && CC_IS_GCC)
+	select HAVE_FUNCTION_TRACER		if !COMPILE_TEST && (PPC64 || (PPC32 && CC_IS_GCC))
 	select HAVE_GCC_PLUGINS			if GCC_VERSION >= 50200   # plugin support on gcc <= 5.1 is buggy on PPC
 	select HAVE_GENERIC_VDSO
 	select HAVE_HARDLOCKUP_DETECTOR_ARCH	if PPC_BOOK3S_64 && SMP

arch/powerpc/Makefile.postlink

···
 	$(CONFIG_SHELL) $(srctree)/arch/powerpc/tools/relocs_check.sh "$(OBJDUMP)" "$(NM)" "$@"
 endif
 
+quiet_cmd_ftrace_check = CHKFTRC $@
+      cmd_ftrace_check = $(CONFIG_SHELL) $(srctree)/arch/powerpc/tools/ftrace_check.sh "$(NM)" "$@"
+
 # `@true` prevents complaint when there is nothing to be done
 
 vmlinux: FORCE
···
 endif
 ifdef CONFIG_RELOCATABLE
 	$(call if_changed,relocs_check)
+endif
+ifdef CONFIG_FUNCTION_TRACER
+ifndef CONFIG_PPC64_ELF_ABI_V1
+	$(call cmd,ftrace_check)
+endif
 endif
 
 clean:

arch/powerpc/tools/ftrace_check.sh

···
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# This script checks vmlinux to ensure that all functions can call ftrace_caller() either directly,
+# or through the stub, ftrace_tramp_text, at the end of kernel text.
+
+# Error out if any command fails
+set -e
+
+# Allow for verbose output
+if [ "$V" = "1" ]; then
+	set -x
+fi
+
+if [ $# -lt 2 ]; then
+	echo "$0 [path to nm] [path to vmlinux]" 1>&2
+	exit 1
+fi
+
+# Have Kbuild supply the path to nm so we handle cross compilation.
+nm="$1"
+vmlinux="$2"
+
+stext_addr=$($nm "$vmlinux" | grep -e " [TA] _stext$" | \
+	cut -d' ' -f1 | tr '[:lower:]' '[:upper:]')
+ftrace_caller_addr=$($nm "$vmlinux" | grep -e " T ftrace_caller$" | \
+	cut -d' ' -f1 | tr '[:lower:]' '[:upper:]')
+ftrace_tramp_addr=$($nm "$vmlinux" | grep -e " T ftrace_tramp_text$" | \
+	cut -d' ' -f1 | tr '[:lower:]' '[:upper:]')
+
+ftrace_caller_offset=$(echo "ibase=16;$ftrace_caller_addr - $stext_addr" | bc)
+ftrace_tramp_offset=$(echo "ibase=16;$ftrace_tramp_addr - $ftrace_caller_addr" | bc)
+sz_32m=$(printf "%d" 0x2000000)
+sz_64m=$(printf "%d" 0x4000000)
+
+# ftrace_caller - _stext < 32M
+if [ "$ftrace_caller_offset" -ge "$sz_32m" ]; then
+	echo "ERROR: ftrace_caller (0x$ftrace_caller_addr) is beyond 32MiB of _stext" 1>&2
+	echo "ERROR: consider disabling CONFIG_FUNCTION_TRACER, or reducing the size \
+		of kernel text" 1>&2
+	exit 1
+fi
+
+# ftrace_tramp_text - ftrace_caller < 64M
+if [ "$ftrace_tramp_offset" -ge "$sz_64m" ]; then
+	echo "ERROR: kernel text extends beyond 64MiB from ftrace_caller" 1>&2
+	echo "ERROR: consider disabling CONFIG_FUNCTION_TRACER, or reducing the size \
+		of kernel text" 1>&2
+	exit 1
+fi