[PATCH v3 08/15] kbuild: link symbol CRCs at final link, removing CONFIG_MODULE_REL_CRCS

From: Masahiro Yamada
Date: Thu May 05 2022 - 03:27:06 EST

include/{linux,asm-generic}/export.h defines a weak symbol, __crc_*
as a placeholder.

Genksyms writes the version CRCs into the linker script, which will be
used for filling the __crc_* symbols. The linker script format depends
on CONFIG_MODULE_REL_CRCS. If it is enabled, __crc_* holds the offset
to the reference of CRC.

We are ready to get rid of this complexity.

Now that modpost parses text files (.*.cmd) to collect all the CRCs,
it can generate C code that will be linked to the vmlinux or modules.

Generate a new C file, .vmlinux-symver.c, which contains the CRCs of
symbols exported by vmlinux. It is compiled and linked to vmlinux in
scripts/link-vmlinux.sh.

Put the CRCs of symbols exported by modules into the existing *.mod.c
files. No additional build step is needed for modules. As usual,
*.mod.c are compiled and linked to *.ko in scripts/Makefile.modfinal.

Please note we no longer use the linker magic. The new C implementation
works in the same way, whether CONFIG_RELOCATABLE is enabled or not.
CONFIG_MODULE_REL_CRCS is no longer needed.

Previously, Kbuild invoked additional $(LD) to update the CRCs in
objects, but this step is unneeded too.

Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
---

Changes in v3:
- New patch

arch/powerpc/Kconfig | 1 -
arch/s390/Kconfig | 1 -
arch/um/Kconfig | 1 -
include/asm-generic/export.h | 22 ++++++++--------------
include/linux/export.h | 30 ++++++++----------------------
include/linux/symversion.h | 13 +++++++++++++
init/Kconfig | 4 ----
kernel/module.c | 10 +---------
scripts/Makefile.build | 27 ++++-----------------------
scripts/genksyms/genksyms.c | 17 ++++-------------
scripts/link-vmlinux.sh | 20 +++++++++++++-------
scripts/mod/modpost.c | 31 +++++++++++++++++++++++++++----
12 files changed, 78 insertions(+), 99 deletions(-)
create mode 100644 include/linux/symversion.h

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 174edabb74fa..a4e8dd889e29 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -566,7 +566,6 @@ config RELOCATABLE
bool "Build a relocatable kernel"
depends on PPC64 || (FLATMEM && (44x || FSL_BOOKE))
select NONSTATIC_KERNEL
- select MODULE_REL_CRCS if MODVERSIONS
help
This builds a kernel image that is capable of running at the
location the kernel is loaded at. For ppc32, there is no any
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 77b5a03de13a..aa5848004c76 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -567,7 +567,6 @@ endchoice

config RELOCATABLE
bool "Build a relocatable kernel"
- select MODULE_REL_CRCS if MODVERSIONS
default y
help
This builds a kernel image that retains relocation information
diff --git a/arch/um/Kconfig b/arch/um/Kconfig
index 4d398b80aea8..e8983d098e73 100644
--- a/arch/um/Kconfig
+++ b/arch/um/Kconfig
@@ -106,7 +106,6 @@ config LD_SCRIPT_DYN
bool
default y
depends on !LD_SCRIPT_STATIC
- select MODULE_REL_CRCS if MODVERSIONS

config LD_SCRIPT_DYN_RPATH
bool "set rpath in the binary" if EXPERT
diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h
index 07a36a874dca..51ce72ce80fa 100644
--- a/include/asm-generic/export.h
+++ b/include/asm-generic/export.h
@@ -2,6 +2,14 @@
#ifndef __ASM_GENERIC_EXPORT_H
#define __ASM_GENERIC_EXPORT_H

+/*
+ * This comment block is used by fixdep. Please do not remove.
+ *
+ * When CONFIG_MODVERSIONS is changed from n to y, all source files having
+ * EXPORT_SYMBOL variants must be re-compiled because genksyms is run as a
+ * side effect of the .o build rule.
+ */
+
#ifndef KSYM_FUNC
#define KSYM_FUNC(x) x
#endif
@@ -12,9 +20,6 @@
#else
#define KSYM_ALIGN 4
#endif
-#ifndef KCRC_ALIGN
-#define KCRC_ALIGN 4
-#endif

.macro __put, val, name
#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
@@ -43,17 +48,6 @@ __ksymtab_\name:
__kstrtab_\name:
.asciz "\name"
.previous
-#ifdef CONFIG_MODVERSIONS
- .section ___kcrctab\sec+\name,"a"
- .balign KCRC_ALIGN
-#if defined(CONFIG_MODULE_REL_CRCS)
- .long __crc_\name - .
-#else
- .long __crc_\name
-#endif
- .weak __crc_\name
- .previous
-#endif
#endif
.endm

diff --git a/include/linux/export.h b/include/linux/export.h
index 27d848712b90..6c8e24e723bd 100644
--- a/include/linux/export.h
+++ b/include/linux/export.h
@@ -11,6 +11,14 @@
* hackers place grumpy comments in header files.
*/

+/*
+ * This comment block is used by fixdep. Please do not remove.
+ *
+ * When CONFIG_MODVERSIONS is changed from n to y, all source files having
+ * EXPORT_SYMBOL variants must be re-compiled because genksyms is run as a
+ * side effect of the .o build rule.
+ */
+
#ifndef __ASSEMBLY__
#ifdef MODULE
extern struct module __this_module;
@@ -19,26 +27,6 @@ extern struct module __this_module;
#define THIS_MODULE ((struct module *)0)
#endif

-#ifdef CONFIG_MODVERSIONS
-/* Mark the CRC weak since genksyms apparently decides not to
- * generate a checksums for some symbols */
-#if defined(CONFIG_MODULE_REL_CRCS)
-#define __CRC_SYMBOL(sym, sec) \
- asm(" .section \"___kcrctab" sec "+" #sym "\", \"a\" \n" \
- " .weak __crc_" #sym " \n" \
- " .long __crc_" #sym " - . \n" \
- " .previous \n")
-#else
-#define __CRC_SYMBOL(sym, sec) \
- asm(" .section \"___kcrctab" sec "+" #sym "\", \"a\" \n" \
- " .weak __crc_" #sym " \n" \
- " .long __crc_" #sym " \n" \
- " .previous \n")
-#endif
-#else
-#define __CRC_SYMBOL(sym, sec)
-#endif
-
#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
#include <linux/compiler.h>
/*
@@ -85,7 +73,6 @@ struct kernel_symbol {
/*
* For every exported symbol, do the following:
*
- * - If applicable, place a CRC entry in the __kcrctab section.
* - Put the name of the symbol and namespace (empty string "" for none) in
* __ksymtab_strings.
* - Place a struct kernel_symbol entry in the __ksymtab section.
@@ -98,7 +85,6 @@ struct kernel_symbol {
extern typeof(sym) sym; \
extern const char __kstrtab_##sym[]; \
extern const char __kstrtabns_##sym[]; \
- __CRC_SYMBOL(sym, sec); \
asm(" .section \"__ksymtab_strings\",\"aMS\",%progbits,1 \n" \
"__kstrtab_" #sym ": \n" \
" .asciz \"" #sym "\" \n" \
diff --git a/include/linux/symversion.h b/include/linux/symversion.h
new file mode 100644
index 000000000000..09971145710a
--- /dev/null
+++ b/include/linux/symversion.h
@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/* This header is used by C files generated by modpost. */
+
+#ifndef __LINUX_SYMVERSION_H__
+#define __LINUX_SYMVERSION_H__
+
+#include <linux/compiler.h>
+#include <linux/types.h>
+
+#define SYMBOL_CRC(sym, crc, sec) \
+ u32 __section("___kcrctab" sec "+" #sym) __crc_##sym = crc
+
+#endif /* __LINUX_SYMVERSION_H__ */
diff --git a/init/Kconfig b/init/Kconfig
index ddcbefe535e9..f5b14318dfcb 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -2136,10 +2136,6 @@ config ASM_MODVERSIONS
assembly. This can be enabled only when the target architecture
supports it.

-config MODULE_REL_CRCS
- bool
- depends on MODVERSIONS
-
config MODULE_SRCVERSION_ALL
bool "Source checksum for all modules"
help
diff --git a/kernel/module.c b/kernel/module.c
index 6cea788fd965..c9e2342da28e 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -1231,11 +1231,6 @@ static int try_to_force_load(struct module *mod, const char *reason)

#ifdef CONFIG_MODVERSIONS

-static u32 resolve_rel_crc(const s32 *crc)
-{
- return *(u32 *)((void *)crc + *crc);
-}
-
static int check_version(const struct load_info *info,
const char *symname,
struct module *mod,
@@ -1264,10 +1259,7 @@ static int check_version(const struct load_info *info,
if (strcmp(versions[i].name, symname) != 0)
continue;

- if (IS_ENABLED(CONFIG_MODULE_REL_CRCS))
- crcval = resolve_rel_crc(crc);
- else
- crcval = *crc;
+ crcval = *crc;
if (versions[i].crc == crcval)
return 1;
pr_debug("Found checksum %X vs module %lX\n",
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index a1023868775f..ddd9080fc028 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -128,7 +128,6 @@ $(obj)/%.i: $(src)/%.c FORCE

genksyms = scripts/genksyms/genksyms \
$(if $(1), -T $(2)) \
- $(if $(CONFIG_MODULE_REL_CRCS), -R) \
$(if $(KBUILD_PRESERVE), -p) \
-r $(or $(wildcard $(2:.symtypes=.symref)), /dev/null)

@@ -162,19 +161,11 @@ ifdef CONFIG_MODVERSIONS
# o if <file>.o doesn't contain a __ksymtab version, i.e. does
# not export symbols, it's done.
# o otherwise, we calculate symbol versions using the good old
-# genksyms on the preprocessed source and postprocess them in a way
-# that they are usable as a linker script
-# o generate .tmp_<file>.o from <file>.o using the linker to
-# replace the unresolved symbols __crc_exported_symbol with
-# the actual value of the checksum generated by genksyms
-# o remove .tmp_<file>.o to <file>.o
+# genksyms on the preprocessed source and dump them into the .cmd file.
+# o modpost will extract versions from that file and create *.c files that will
+# be compiled and linked to the kernel and/or modules.

-# Generate .o.symversions files for each .o with exported symbols, and link these
-# to the kernel and/or modules at the end.
-
-genksyms_format_rel_crc := [^_]*__crc_\([^ ]*\) = \.; LONG(\([^)]*\)).*
-genksyms_format_normal := __crc_\(.*\) = \(.*\);
-genksyms_format := $(if $(CONFIG_MODULE_REL_CRCS),$(genksyms_format_rel_crc),$(genksyms_format_normal))
+genksyms_format := __crc_\(.*\) = \(.*\);

gen_symversions = \
if $(NM) $@ 2>/dev/null | grep -q __ksymtab; then \
@@ -188,12 +179,6 @@ gen_symversions = \

cmd_gen_symversions_c = $(call gen_symversions,c)

-cmd_modversions = \
- if [ -r $@.symversions ]; then \
- $(LD) $(KBUILD_LDFLAGS) -r -o $(@D)/.tmp_$(@F) $@ \
- -T $@.symversions; \
- mv -f $(@D)/.tmp_$(@F) $@; \
- fi
endif

ifdef CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT
@@ -273,7 +258,6 @@ define rule_cc_o_c
$(call cmd,checkdoc)
$(call cmd,gen_objtooldep)
$(call cmd,gen_symversions_c)
- $(if $(CONFIG_LTO_CLANG),,$(call cmd,modversions))
$(call cmd,record_mcount)
endef

@@ -282,7 +266,6 @@ define rule_as_o_S
$(call cmd,gen_ksymdeps)
$(call cmd,gen_objtooldep)
$(call cmd,gen_symversions_S)
- $(call cmd,modversions)
endef

# Built-in and composite module parts
@@ -296,8 +279,6 @@ ifneq ($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT),)
quiet_cmd_cc_prelink_modules = LD [M] $@
cmd_cc_prelink_modules = \
$(LD) $(ld_flags) -r -o $@ \
- $(shell [ -s $(@:.prelink.o=.o.symversions) ] && \
- echo -T $(@:.prelink.o=.o.symversions)) \
--whole-archive $(filter-out FORCE,$^) \
$(cmd_objtool)

diff --git a/scripts/genksyms/genksyms.c b/scripts/genksyms/genksyms.c
index 4827c5abe5b7..6e6933ae7911 100644
--- a/scripts/genksyms/genksyms.c
+++ b/scripts/genksyms/genksyms.c
@@ -33,7 +33,7 @@ char *cur_filename;
int in_source_file;

static int flag_debug, flag_dump_defs, flag_reference, flag_dump_types,
- flag_preserve, flag_warnings, flag_rel_crcs;
+ flag_preserve, flag_warnings;

static int errors;
static int nsyms;
@@ -681,10 +681,7 @@ void export_symbol(const char *name)
fputs(">\n", debugfile);

/* Used as a linker script. */
- printf(!flag_rel_crcs ? "__crc_%s = 0x%08lx;\n" :
- "SECTIONS { .rodata : ALIGN(4) { "
- "__crc_%s = .; LONG(0x%08lx); } }\n",
- name, crc);
+ printf("__crc_%s = 0x%08lx;\n", name, crc);
}
}

@@ -733,7 +730,6 @@ static void genksyms_usage(void)
" -q, --quiet Disable warnings (default)\n"
" -h, --help Print this message\n"
" -V, --version Print the release version\n"
- " -R, --relative-crc Emit section relative symbol CRCs\n"
#else /* __GNU_LIBRARY__ */
" -s Select symbol prefix\n"
" -d Increment the debug level (repeatable)\n"
@@ -745,7 +741,6 @@ static void genksyms_usage(void)
" -q Disable warnings (default)\n"
" -h Print this message\n"
" -V Print the release version\n"
- " -R Emit section relative symbol CRCs\n"
#endif /* __GNU_LIBRARY__ */
, stderr);
}
@@ -766,14 +761,13 @@ int main(int argc, char **argv)
{"preserve", 0, 0, 'p'},
{"version", 0, 0, 'V'},
{"help", 0, 0, 'h'},
- {"relative-crc", 0, 0, 'R'},
{0, 0, 0, 0}
};

- while ((o = getopt_long(argc, argv, "s:dwqVDr:T:phR",
+ while ((o = getopt_long(argc, argv, "s:dwqVDr:T:ph",
&long_opts[0], NULL)) != EOF)
#else /* __GNU_LIBRARY__ */
- while ((o = getopt(argc, argv, "s:dwqVDr:T:phR")) != EOF)
+ while ((o = getopt(argc, argv, "s:dwqVDr:T:ph")) != EOF)
#endif /* __GNU_LIBRARY__ */
switch (o) {
case 'd':
@@ -813,9 +807,6 @@ int main(int argc, char **argv)
case 'h':
genksyms_usage();
return 0;
- case 'R':
- flag_rel_crcs = 1;
- break;
default:
genksyms_usage();
return 1;
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index eceb3ee7ec06..2742b7dd089a 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -88,11 +88,6 @@ modpost_link()
gen_initcalls
lds="-T .tmp_initcalls.lds"

- if is_enabled CONFIG_MODVERSIONS; then
- gen_symversions
- lds="${lds} -T .tmp_symversions.lds"
- fi
-
# This might take a while, so indicate that we're doing
# an LTO link
info LTO ${1}
@@ -277,7 +272,7 @@ kallsyms_step()
kallsymso=${kallsyms_vmlinux}.o
kallsyms_S=${kallsyms_vmlinux}.S

- vmlinux_link ${kallsyms_vmlinux} "${kallsymso_prev}" ${btf_vmlinux_bin_o}
+ vmlinux_link ${kallsyms_vmlinux} "${kallsymso_prev}" ${vmlinux_symver_o} ${btf_vmlinux_bin_o}
kallsyms ${kallsyms_vmlinux} ${kallsyms_S}

info AS ${kallsyms_S}
@@ -312,6 +307,7 @@ cleanup()
rm -f vmlinux.o
rm -f .vmlinux.d
rm -f .vmlinux.objs
+ rm -f .vmlinux-symver.c
}

# Use "make V=1" to debug this script
@@ -373,6 +369,16 @@ if is_enabled CONFIG_DEBUG_INFO_BTF; then
fi
fi

+vmlinux_symver_o=
+if is_enabled CONFIG_MODVERSIONS; then
+ vmlinux_symver_o=.vmlinux-symver.o
+ info CC ${vmlinux_symver_o}
+ ${CC} ${NOSTDINC_FLAGS} ${LINUXINCLUDE} \
+ ${KBUILD_CPPFLAGS} ${KBUILD_CFLAGS} \
+ ${KBUILD_CFLAGS_KERNEL} ${CFLAGS_KERNEL} \
+ -c -o ${vmlinux_symver_o} ${vmlinux_symver_o%.o}.c
+fi
+
kallsymso=""
kallsymso_prev=""
kallsyms_vmlinux=""
@@ -413,7 +419,7 @@ if is_enabled CONFIG_KALLSYMS; then
fi
fi

-vmlinux_link vmlinux "${kallsymso}" ${btf_vmlinux_bin_o}
+vmlinux_link vmlinux "${kallsymso}" ${vmlinux_symver_o} ${btf_vmlinux_bin_o}

# fill in BTF IDs
if is_enabled CONFIG_DEBUG_INFO_BTF && is_enabled CONFIG_BPF; then
diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 92ee1f454e29..be7d8adc5d31 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -2276,20 +2276,30 @@ static void add_header(struct buffer *b, struct module *mod)
buf_printf(b, "\nMODULE_INFO(staging, \"Y\");\n");
}

-static void check_symversions(struct module *mod)
+/* Record CRCs for exported symbols */
+static void add_exported_symversions(struct buffer *buf, struct module *mod)
{
struct symbol *sym;

if (!modversions)
return;

+ buf_printf(buf,
+ "\n"
+ "#include <linux/symversion.h>\n"
+ "\n");
+
list_for_each_entry(sym, &mod->exported_symbols, list) {
if (!sym->crc_valid) {
warn("EXPORT symbol \"%s\" [%s%s] version generation failed, symbol will not be versioned.\n"
"Is \"%s\" prototyped in <asm/asm-prototypes.h>?\n",
sym->name, mod->name, mod->is_vmlinux ? "" : ".ko",
sym->name);
+ continue;
}
+
+ buf_printf(buf, "SYMBOL_CRC(%s, 0x%08x, \"%s\");\n",
+ sym->name, sym->crc, sym->is_gpl_only ? "_gpl" : "");
}
}

@@ -2426,6 +2436,18 @@ static void write_if_changed(struct buffer *b, const char *fname)
write_buf(b, fname);
}

+static void write_vmlinux_symver_c_file(struct module *mod)
+{
+ struct buffer buf = { };
+
+ if (!modversions)
+ return;
+
+ add_exported_symversions(&buf, mod);
+ write_if_changed(&buf, ".vmlinux-symver.c");
+ free(buf.p);
+}
+
/* do sanity checks, and generate *.mod.c file */
static void write_mod_c_file(struct module *mod)
{
@@ -2437,6 +2459,7 @@ static void write_mod_c_file(struct module *mod)
check_exports(mod);

add_header(&buf, mod);
+ add_exported_symversions(&buf, mod);
add_versions(&buf, mod);
add_depends(&buf, mod);
add_moddevtable(&buf, mod);
@@ -2633,9 +2656,9 @@ int main(int argc, char **argv)
if (mod->from_dump)
continue;

- check_symversions(mod);
-
- if (!mod->is_vmlinux)
+ if (mod->is_vmlinux)
+ write_vmlinux_symver_c_file(mod);
+ else
write_mod_c_file(mod);
}

--
2.32.0