[RFC PATCH v4 10/17] powerpc/ftrace: Add a postlink script to validate function tracer

Naveen N Rao
2024-07-14T08:30:03Z

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 <naveen@kernel.org>
---
arch/powerpc/Kconfig | 2 +-
arch/powerpc/Makefile.postlink | 8 ++++++
arch/powerpc/tools/ftrace_check.sh | 45 ++++++++++++++++++++++++++++++
3 files changed, 54 insertions(+), 1 deletion(-)
create mode 100755 arch/powerpc/tools/ftrace_check.sh

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index f8891fbe7c16..68f0e7a5576f 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -244,7 +244,7 @@ config PPC
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
diff --git a/arch/powerpc/Makefile.postlink b/arch/powerpc/Makefile.postlink
index ae5a4256b03d..bb601be36173 100644
--- a/arch/powerpc/Makefile.postlink
+++ b/arch/powerpc/Makefile.postlink
@@ -24,6 +24,9 @@ else
$(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
@@ -34,6 +37,11 @@ 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:
rm -f .tmp_symbols.txt
diff --git a/arch/powerpc/tools/ftrace_check.sh b/arch/powerpc/tools/ftrace_check.sh
new file mode 100755
index 000000000000..33f2fd45e54d
--- /dev/null
+++ b/arch/powerpc/tools/ftrace_check.sh
@@ -0,0 +1,45 @@
+#!/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
--
2.45.2