[PATCH 04/22] kbuild: lto: fix recordmcount

Sami Tolvanen
2020-06-24T20:33:04Z

With LTO, LLVM bitcode won't be compiled into native code until
modpost_link. This change postpones calls to recordmcount until after
this step.

In order to exclude specific functions from inspection, we add a new
code section .text..nomcount, which we tell recordmcount to ignore, and
a __nomcount attribute for moving functions to this section.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
Makefile | 2 +-
arch/Kconfig | 2 +-
include/asm-generic/vmlinux.lds.h | 1 +
include/linux/compiler-clang.h | 4 ++++
include/linux/compiler_types.h | 4 ++++
kernel/trace/ftrace.c | 1 +
scripts/Makefile.build | 9 +++++++++
scripts/Makefile.modfinal | 18 ++++++++++++++++--
scripts/link-vmlinux.sh | 29 +++++++++++++++++++++++++++++
scripts/recordmcount.c | 3 ++-
10 files changed, 68 insertions(+), 5 deletions(-)

diff --git a/Makefile b/Makefile
index 161ad0d1f77f..3a7e5e5c17b9 100644
--- a/Makefile
+++ b/Makefile
@@ -861,7 +861,7 @@ KBUILD_AFLAGS += $(CC_FLAGS_USING)
ifdef CONFIG_DYNAMIC_FTRACE
ifdef CONFIG_HAVE_C_RECORDMCOUNT
BUILD_C_RECORDMCOUNT := y
- export BUILD_C_RECORDMCOUNT
+ export BUILD_C_RECORDMCOUNT RECORDMCOUNT_WARN
endif
endif
endif
diff --git a/arch/Kconfig b/arch/Kconfig
index 87488fe1e6b8..85b2044b927d 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -598,7 +598,7 @@ config LTO_CLANG
depends on $(success,$(NM) --help | head -n 1 | grep -qi llvm)
depends on $(success,$(AR) --help | head -n 1 | grep -qi llvm)
depends on ARCH_SUPPORTS_LTO_CLANG
- depends on !FTRACE_MCOUNT_RECORD
+ depends on !FTRACE_MCOUNT_RECORD || HAVE_C_RECORDMCOUNT
depends on !KASAN
select LTO
help
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 78079000c05a..a1c902b808d0 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -565,6 +565,7 @@
*(.text.hot TEXT_MAIN .text.fixup .text.unlikely) \
NOINSTR_TEXT \
*(.text..refcount) \
+ *(.text..nomcount) \
*(.ref.text) \
MEM_KEEP(init.text*) \
MEM_KEEP(exit.text*) \
diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
index ee37256ec8bd..fd78475c0642 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -55,3 +55,7 @@
#if __has_feature(shadow_call_stack)
# define __noscs __attribute__((__no_sanitize__("shadow-call-stack")))
#endif
+
+#if defined(CONFIG_LTO_CLANG) && defined(CONFIG_FTRACE_MCOUNT_RECORD)
+#define __nomcount __attribute__((__section__(".text..nomcount")))
+#endif
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index e368384445b6..1470c9703a25 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -233,6 +233,10 @@ struct ftrace_likely_data {
# define __noscs
#endif

+#ifndef __nomcount
+# define __nomcount
+#endif
+
#ifndef asm_volatile_goto
#define asm_volatile_goto(x...) asm goto(x)
#endif
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 1903b80db6eb..8e3ddb8123d9 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -6062,6 +6062,7 @@ static int ftrace_cmp_ips(const void *a, const void *b)
return 0;
}

+__nomcount
static int ftrace_process_locs(struct module *mod,
unsigned long *start,
unsigned long *end)
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 5c0bbb6ddfcf..64e99f4baa5b 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -187,6 +187,9 @@ endif

ifdef CONFIG_FTRACE_MCOUNT_RECORD
ifndef CC_USING_RECORD_MCOUNT
+ifndef CC_USING_PATCHABLE_FUNCTION_ENTRY
+# With LTO, we postpone recordmcount until we compile a native binary
+ifndef CONFIG_LTO_CLANG
# compiler will not generate __mcount_loc use recordmcount or recordmcount.pl
ifdef BUILD_C_RECORDMCOUNT
ifeq ("$(origin RECORDMCOUNT_WARN)", "command line")
@@ -200,6 +203,8 @@ sub_cmd_record_mcount = \
if [ $(@) != "scripts/mod/empty.o" ]; then \
$(objtree)/scripts/recordmcount $(RECORDMCOUNT_FLAGS) "$(@)"; \
fi;
+endif # CONFIG_LTO_CLANG
+
recordmcount_source := $(srctree)/scripts/recordmcount.c \
$(srctree)/scripts/recordmcount.h
else
@@ -209,11 +214,15 @@ sub_cmd_record_mcount = perl $(srctree)/scripts/recordmcount.pl "$(ARCH)" \
"$(OBJDUMP)" "$(OBJCOPY)" "$(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS)" \
"$(LD) $(KBUILD_LDFLAGS)" "$(NM)" "$(RM)" "$(MV)" \
"$(if $(part-of-module),1,0)" "$(@)";
+
recordmcount_source := $(srctree)/scripts/recordmcount.pl
endif # BUILD_C_RECORDMCOUNT
+ifndef CONFIG_LTO_CLANG
cmd_record_mcount = $(if $(findstring $(strip $(CC_FLAGS_FTRACE)),$(_c_flags)), \
$(sub_cmd_record_mcount))
+endif # CONFIG_LTO_CLANG
endif # CC_USING_RECORD_MCOUNT
+endif # CC_USING_PATCHABLE_FUNCTION_ENTRY
endif # CONFIG_FTRACE_MCOUNT_RECORD

ifdef CONFIG_STACK_VALIDATION
diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 1005b147abd0..d168f0cfe67c 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -34,10 +34,24 @@ ifdef CONFIG_LTO_CLANG
# With CONFIG_LTO_CLANG, reuse the object file we compiled for modpost to
# avoid a second slow LTO link
prelink-ext := .lto
-endif
+
+# ELF processing was skipped earlier because we didn't have native code,
+# so let's now process the prelinked binary before we link the module.
+
+ifdef CONFIG_FTRACE_MCOUNT_RECORD
+ifndef CC_USING_RECORD_MCOUNT
+ifndef CC_USING_PATCHABLE_FUNCTION_ENTRY
+cmd_ld_ko_o += $(objtree)/scripts/recordmcount $(RECORDMCOUNT_FLAGS) \
+ $(@:.ko=$(prelink-ext).o);
+
+endif # CC_USING_PATCHABLE_FUNCTION_ENTRY
+endif # CC_USING_RECORD_MCOUNT
+endif # CONFIG_FTRACE_MCOUNT_RECORD
+
+endif # CONFIG_LTO_CLANG

quiet_cmd_ld_ko_o = LD [M] $@
- cmd_ld_ko_o = \
+ cmd_ld_ko_o += \
$(LD) -r $(KBUILD_LDFLAGS) \
$(KBUILD_LDFLAGS_MODULE) $(LDFLAGS_MODULE) \
$(addprefix -T , $(KBUILD_LDS_MODULE)) \
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 69a6d7254e28..c72f5d0238f1 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -108,6 +108,29 @@ objtool_link()
fi
}

+# If CONFIG_LTO_CLANG is selected, we postpone running recordmcount until
+# we have compiled LLVM IR to an object file.
+recordmcount()
+{
+ if [ "${CONFIG_LTO_CLANG} ${CONFIG_FTRACE_MCOUNT_RECORD}" != "y y" ]; then
+ return
+ fi
+
+ if [ -n "${CC_USING_RECORD_MCOUNT}" ]; then
+ return
+ fi
+ if [ -n "${CC_USING_PATCHABLE_FUNCTION_ENTRY}" ]; then
+ return
+ fi
+
+ local flags=""
+
+ [ -n "${RECORDMCOUNT_WARN}" ] && flags="-w"
+
+ info MCOUNT $*
+ ${objtree}/scripts/recordmcount ${flags} $*
+}
+
# Link of vmlinux
# ${1} - output file
# ${2}, ${3}, ... - optional extra .o files
@@ -316,6 +339,12 @@ objtool_link vmlinux.o
# modpost vmlinux.o to check for section mismatches
${MAKE} -f "${srctree}/scripts/Makefile.modpost" MODPOST_VMLINUX=1

+if [ -n "${CONFIG_LTO_CLANG}" ]; then
+ # If we postponed ELF processing steps due to LTO, process
+ # vmlinux.o instead.
+ recordmcount vmlinux.o
+fi
+
info MODINFO modules.builtin.modinfo
${OBJCOPY} -j .modinfo -O binary vmlinux.o modules.builtin.modinfo
info GEN modules.builtin
diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
index 7225107a9aaf..9e9f10b4d649 100644
--- a/scripts/recordmcount.c
+++ b/scripts/recordmcount.c
@@ -404,7 +404,8 @@ static uint32_t (*w2)(uint16_t);
/* Names of the sections that could contain calls to mcount. */
static int is_mcounted_section_name(char const *const txtname)
{
- return strncmp(".text", txtname, 5) == 0 ||
+ return (strncmp(".text", txtname, 5) == 0 &&
+ strcmp(".text..nomcount", txtname) != 0) ||
strcmp(".init.text", txtname) == 0 ||
strcmp(".ref.text", txtname) == 0 ||
strcmp(".sched.text", txtname) == 0 ||
--
2.27.0.212.ge8ba1cc988-goog