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kbuild: implement CONFIG_TRIM_UNUSED_KSYMS without recursion

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When CONFIG_TRIM_UNUSED_KSYMS is enabled, Kbuild recursively traverses
the directory tree to determine which EXPORT_SYMBOL to trim. If an
EXPORT_SYMBOL turns out to be unused by anyone, Kbuild begins the
second traverse, where some source files are recompiled with their
EXPORT_SYMBOL() tuned into a no-op.

Linus stated negative opinions about this slowness in commits:

 - 5cf0fd591f ("Kbuild: disable TRIM_UNUSED_KSYMS option")
 - a555bdd0c5 ("Kbuild: enable TRIM_UNUSED_KSYMS again, with some guarding")

We can do this better now. The final data structures of EXPORT_SYMBOL
are generated by the modpost stage, so modpost can selectively emit
KSYMTAB entries that are really used by modules.

Commit f73edc8951 ("kbuild: unify two modpost invocations") is another
ground-work to do this in a one-pass algorithm. With the list of modules,
modpost sets sym->used if it is used by a module. modpost emits KSYMTAB
only for symbols with sym->used==true.

BTW, Nicolas explained why the trimming was implemented with recursion:

  https://lore.kernel.org/all/2o2rpn97-79nq-p7s2-nq5-8p83391473r@syhkavp.arg/

Actually, we never achieved that level of optimization where the chain
reaction of trimming comes into play because:

 - CONFIG_LTO_CLANG cannot remove any unused symbols
 - CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is enabled only for vmlinux,
   but not modules

If deeper trimming is required, we need to revisit this, but I guess
that is unlikely to happen.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
This commit is contained in:
Masahiro Yamada 2023-06-12 00:50:57 +09:00
parent 700c48b439
commit 5e9e95cc91
11 changed files with 78 additions and 264 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

2
.gitignore vendored
View file

@ -51,7 +51,6 @@
*.symversions
*.tab.[ch]
*.tar
*.usyms
*.xz
*.zst
Module.symvers
@ -112,7 +111,6 @@ modules.order
#
/include/config/
/include/generated/
/include/ksym/
/arch/*/include/generated/
# stgit generated dirs

22
Makefile
View file

@ -1193,28 +1193,12 @@ endif
export KBUILD_VMLINUX_LIBS
export KBUILD_LDS := arch/$(SRCARCH)/kernel/vmlinux.lds
# Recurse until adjust_autoksyms.sh is satisfied
PHONY += autoksyms_recursive
ifdef CONFIG_TRIM_UNUSED_KSYMS
# For the kernel to actually contain only the needed exported symbols,
# we have to build modules as well to determine what those symbols are.
# (this can be evaluated only once include/config/auto.conf has been included)
KBUILD_MODULES := 1
autoksyms_recursive: $(build-dir) modules.order
$(Q)$(CONFIG_SHELL) $(srctree)/scripts/adjust_autoksyms.sh \
"$(MAKE) -f $(srctree)/Makefile autoksyms_recursive"
endif
autoksyms_h := $(if $(CONFIG_TRIM_UNUSED_KSYMS), include/generated/autoksyms.h)
quiet_cmd_autoksyms_h = GEN $@
cmd_autoksyms_h = mkdir -p $(dir $@); \
$(CONFIG_SHELL) $(srctree)/scripts/gen_autoksyms.sh $@
$(autoksyms_h):
$(call cmd,autoksyms_h)
# '$(AR) mPi' needs 'T' to workaround the bug of llvm-ar <= 14
quiet_cmd_ar_vmlinux.a = AR $@
cmd_ar_vmlinux.a = \
@ -1223,7 +1207,7 @@ quiet_cmd_ar_vmlinux.a = AR $@
$(AR) mPiT $$($(AR) t $@ | sed -n 1p) $@ $$($(AR) t $@ | grep -F -f $(srctree)/scripts/head-object-list.txt)
targets += vmlinux.a
vmlinux.a: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt autoksyms_recursive FORCE
vmlinux.a: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt FORCE
$(call if_changed,ar_vmlinux.a)
PHONY += vmlinux_o
@ -1279,7 +1263,7 @@ scripts: scripts_basic scripts_dtc
PHONY += prepare archprepare
archprepare: outputmakefile archheaders archscripts scripts include/config/kernel.release \
asm-generic $(version_h) $(autoksyms_h) include/generated/utsrelease.h \
asm-generic $(version_h) include/generated/utsrelease.h \
include/generated/compile.h include/generated/autoconf.h remove-stale-files
prepare0: archprepare
@ -2039,7 +2023,7 @@ clean: $(clean-dirs)
-o -name '*.dtb.S' -o -name '*.dtbo.S' \
-o -name '*.dt.yaml' \
-o -name '*.dwo' -o -name '*.lst' \
-o -name '*.su' -o -name '*.mod' -o -name '*.usyms' \
-o -name '*.su' -o -name '*.mod' \
-o -name '.*.d' -o -name '.*.tmp' -o -name '*.mod.c' \
-o -name '*.lex.c' -o -name '*.tab.[ch]' \
-o -name '*.asn1.[ch]' \

67
include/linux/export.h
View file

@ -42,7 +42,7 @@ extern struct module __this_module;
.long sym
#endif
#define ____EXPORT_SYMBOL(sym, license, ns) \
#define ___EXPORT_SYMBOL(sym, license, ns) \
.section ".export_symbol","a" ASM_NL \
__export_symbol_##sym: ASM_NL \
.asciz license ASM_NL \
@ -50,24 +50,6 @@ extern struct module __this_module;
__EXPORT_SYMBOL_REF(sym) ASM_NL \
.previous
#ifdef __GENKSYMS__
#define ___EXPORT_SYMBOL(sym, sec, ns) __GENKSYMS_EXPORT_SYMBOL(sym)
#elif defined(__ASSEMBLY__)
#define ___EXPORT_SYMBOL(sym, license, ns) \
____EXPORT_SYMBOL(sym, license, ns)
#else
#define ___EXPORT_SYMBOL(sym, license, ns) \
extern typeof(sym) sym; \
__ADDRESSABLE(sym) \
asm(__stringify(____EXPORT_SYMBOL(sym, license, ns)))
#endif
#if !defined(CONFIG_MODULES) || defined(__DISABLE_EXPORTS)
/*
@ -77,50 +59,21 @@ extern struct module __this_module;
*/
#define __EXPORT_SYMBOL(sym, sec, ns)
#elif defined(CONFIG_TRIM_UNUSED_KSYMS)
#elif defined(__GENKSYMS__)
#include <generated/autoksyms.h>
#define __EXPORT_SYMBOL(sym, sec, ns) __GENKSYMS_EXPORT_SYMBOL(sym)
/*
* For fine grained build dependencies, we want to tell the build system
* about each possible exported symbol even if they're not actually exported.
* We use a symbol pattern __ksym_marker_<symbol> that the build system filters
* from the $(NM) output (see scripts/gen_ksymdeps.sh). These symbols are
* discarded in the final link stage.
*/
#elif defined(__ASSEMBLY__)
#ifdef __ASSEMBLY__
#define __ksym_marker(sym) \
.section ".discard.ksym","a" ; \
__ksym_marker_##sym: ; \
.previous
#define __EXPORT_SYMBOL(sym, license, ns) \
___EXPORT_SYMBOL(sym, license, ns)
#else
#define __ksym_marker(sym) \
static int __ksym_marker_##sym[0] __section(".discard.ksym") __used
#endif
#define __EXPORT_SYMBOL(sym, sec, ns) \
__ksym_marker(sym); \
__cond_export_sym(sym, sec, ns, __is_defined(__KSYM_##sym))
#define __cond_export_sym(sym, sec, ns, conf) \
___cond_export_sym(sym, sec, ns, conf)
#define ___cond_export_sym(sym, sec, ns, enabled) \
__cond_export_sym_##enabled(sym, sec, ns)
#define __cond_export_sym_1(sym, sec, ns) ___EXPORT_SYMBOL(sym, sec, ns)
#ifdef __GENKSYMS__
#define __cond_export_sym_0(sym, sec, ns) __GENKSYMS_EXPORT_SYMBOL(sym)
#else
#define __cond_export_sym_0(sym, sec, ns) /* nothing */
#endif
#else
#define __EXPORT_SYMBOL(sym, sec, ns) ___EXPORT_SYMBOL(sym, sec, ns)
#define __EXPORT_SYMBOL(sym, license, ns) \
extern typeof(sym) sym; \
__ADDRESSABLE(sym) \
asm(__stringify(___EXPORT_SYMBOL(sym, license, ns)))
#endif /* CONFIG_MODULES */

15
scripts/Makefile.build
View file

@ -82,7 +82,7 @@ ifdef need-builtin
targets-for-builtin += $(obj)/built-in.a
endif
targets-for-modules := $(foreach x, o mod $(if $(CONFIG_TRIM_UNUSED_KSYMS), usyms), \
targets-for-modules := $(foreach x, o mod, \
$(patsubst %.o, %.$x, $(filter %.o, $(obj-m))))
ifdef need-modorder
@ -217,18 +217,12 @@ is-standard-object = $(if $(filter-out y%, $(OBJECT_FILES_NON_STANDARD_$(basetar
$(obj)/%.o: objtool-enabled = $(if $(is-standard-object),$(if $(delay-objtool),$(is-single-obj-m),y))
ifdef CONFIG_TRIM_UNUSED_KSYMS
cmd_gen_ksymdeps = \
$(CONFIG_SHELL) $(srctree)/scripts/gen_ksymdeps.sh $@ >> $(dot-target).cmd
endif
ifneq ($(findstring 1, $(KBUILD_EXTRA_WARN)),)
cmd_warn_shared_object = $(if $(word 2, $(modname-multi)),$(warning $(kbuild-file): $*.o is added to multiple modules: $(modname-multi)))
endif
define rule_cc_o_c
$(call cmd_and_fixdep,cc_o_c)
$(call cmd,gen_ksymdeps)
$(call cmd,checksrc)
$(call cmd,checkdoc)
$(call cmd,gen_objtooldep)
@ -239,7 +233,6 @@ endef
define rule_as_o_S
$(call cmd_and_fixdep,as_o_S)
$(call cmd,gen_ksymdeps)
$(call cmd,gen_objtooldep)
$(call cmd,gen_symversions_S)
$(call cmd,warn_shared_object)
@ -258,12 +251,6 @@ cmd_mod = printf '%s\n' $(call real-search, $*.o, .o, -objs -y -m) | \
$(obj)/%.mod: FORCE
$(call if_changed,mod)
# List module undefined symbols
cmd_undefined_syms = $(NM) $< | sed -n 's/^ *U //p' > $@
$(obj)/%.usyms: $(obj)/%.o FORCE
$(call if_changed,undefined_syms)
quiet_cmd_cc_lst_c = MKLST $@
cmd_cc_lst_c = $(CC) $(c_flags) -g -c -o $*.o $< && \
$(CONFIG_SHELL) $(srctree)/scripts/makelst $*.o \

7
scripts/Makefile.modpost
View file

@ -91,6 +91,13 @@ targets += .vmlinux.objs
.vmlinux.objs: vmlinux.a $(KBUILD_VMLINUX_LIBS) FORCE
$(call if_changed,vmlinux_objs)
ifdef CONFIG_TRIM_UNUSED_KSYMS
ksym-wl := $(CONFIG_UNUSED_KSYMS_WHITELIST)
ksym-wl := $(if $(filter-out /%, $(ksym-wl)),$(srctree)/)$(ksym-wl)
modpost-args += -t $(addprefix -u , $(ksym-wl))
modpost-deps += $(ksym-wl)
endif
ifeq ($(wildcard vmlinux.o),)
missing-input := vmlinux.o
output-symdump := modules-only.symvers

73
scripts/adjust_autoksyms.sh
View file

@ -1,73 +0,0 @@
#!/bin/sh
# SPDX-License-Identifier: GPL-2.0-only
# Script to update include/generated/autoksyms.h and dependency files
#
# Copyright: (C) 2016 Linaro Limited
# Created by: Nicolas Pitre, January 2016
#
# Update the include/generated/autoksyms.h file.
#
# For each symbol being added or removed, the corresponding dependency
# file's timestamp is updated to force a rebuild of the affected source
# file. All arguments passed to this script are assumed to be a command
# to be exec'd to trigger a rebuild of those files.
set -e
cur_ksyms_file="include/generated/autoksyms.h"
new_ksyms_file="include/generated/autoksyms.h.tmpnew"
info() {
if [ "$quiet" != "silent_" ]; then
printf " %-7s %s\n" "$1" "$2"
fi
}
info "CHK" "$cur_ksyms_file"
# Use "make V=1" to debug this script.
case "$KBUILD_VERBOSE" in
*1*)
set -x
;;
esac
# Generate a new symbol list file
$CONFIG_SHELL $srctree/scripts/gen_autoksyms.sh --modorder "$new_ksyms_file"
# Extract changes between old and new list and touch corresponding
# dependency files.
changed=$(
count=0
sort "$cur_ksyms_file" "$new_ksyms_file" | uniq -u |
sed -n 's/^#define __KSYM_\(.*\) 1/\1/p' |
while read sympath; do
if [ -z "$sympath" ]; then continue; fi
depfile="include/ksym/${sympath}"
mkdir -p "$(dirname "$depfile")"
touch "$depfile"
# Filesystems with coarse time precision may create timestamps
# equal to the one from a file that was very recently built and that
# needs to be rebuild. Let's guard against that by making sure our
# dep files are always newer than the first file we created here.
while [ ! "$depfile" -nt "$new_ksyms_file" ]; do
touch "$depfile"
done
echo $((count += 1))
done | tail -1 )
changed=${changed:-0}
if [ $changed -gt 0 ]; then
# Replace the old list with tne new one
old=$(grep -c "^#define __KSYM_" "$cur_ksyms_file" || true)
new=$(grep -c "^#define __KSYM_" "$new_ksyms_file" || true)
info "KSYMS" "symbols: before=$old, after=$new, changed=$changed"
info "UPD" "$cur_ksyms_file"
mv -f "$new_ksyms_file" "$cur_ksyms_file"
# Then trigger a rebuild of affected source files
exec $@
else
rm -f "$new_ksyms_file"
fi

3
scripts/basic/fixdep.c
View file

@ -246,8 +246,7 @@ static void *read_file(const char *filename)
/* Ignore certain dependencies */
static int is_ignored_file(const char *s, int len)
{
return str_ends_with(s, len, "include/generated/autoconf.h") ||
str_ends_with(s, len, "include/generated/autoksyms.h");
return str_ends_with(s, len, "include/generated/autoconf.h");
}
/* Do not parse these files */

62
scripts/gen_autoksyms.sh
View file

@ -1,62 +0,0 @@
#!/bin/sh
# SPDX-License-Identifier: GPL-2.0-only
# Create an autoksyms.h header file from the list of all module's needed symbols
# as recorded in *.usyms files and the user-provided symbol whitelist.
set -e
# Use "make V=1" to debug this script.
case "$KBUILD_VERBOSE" in
*1*)
set -x
;;
esac
read_modorder=
if [ "$1" = --modorder ]; then
shift
read_modorder=1
fi
output_file="$1"
needed_symbols=
# Special case for modversions (see modpost.c)
if grep -q "^CONFIG_MODVERSIONS=y$" include/config/auto.conf; then
needed_symbols="$needed_symbols module_layout"
fi
ksym_wl=$(sed -n 's/^CONFIG_UNUSED_KSYMS_WHITELIST=\(.*\)$/\1/p' include/config/auto.conf)
if [ -n "$ksym_wl" ]; then
[ "${ksym_wl}" != "${ksym_wl#/}" ] || ksym_wl="$abs_srctree/$ksym_wl"
if [ ! -f "$ksym_wl" ] || [ ! -r "$ksym_wl" ]; then
echo "ERROR: '$ksym_wl' whitelist file not found" >&2
exit 1
fi
fi
# Generate a new ksym list file with symbols needed by the current
# set of modules.
cat > "$output_file" << EOT
/*
* Automatically generated file; DO NOT EDIT.
*/
EOT
{
[ -n "${read_modorder}" ] && sed 's/o$/usyms/' modules.order | xargs cat
echo "$needed_symbols"
[ -n "$ksym_wl" ] && cat "$ksym_wl"
} | sed -e 's/ /\n/g' | sed -n -e '/^$/!p' |
# Remove the dot prefix for ppc64; symbol names with a dot (.) hold entry
# point addresses.
sed -e 's/^\.//' |
sort -u |
# Ignore __this_module. It's not an exported symbol, and will be resolved
# when the final .ko's are linked.
grep -v '^__this_module$' |
sed -e 's/\(.*\)/#define __KSYM_\1 1/' >> "$output_file"

30
scripts/gen_ksymdeps.sh
View file

@ -1,30 +0,0 @@
#!/bin/sh
# SPDX-License-Identifier: GPL-2.0
set -e
# List of exported symbols
#
# If the object has no symbol, $NM warns 'no symbols'.
# Suppress the stderr.
# TODO:
# Use -q instead of 2>/dev/null when we upgrade the minimum version of
# binutils to 2.37, llvm to 13.0.0.
ksyms=$($NM $1 2>/dev/null | sed -n 's/.*__ksym_marker_\(.*\)/\1/p')
if [ -z "$ksyms" ]; then
exit 0
fi
echo
echo "ksymdeps_$1 := \\"
for s in $ksyms
do
printf ' $(wildcard include/ksym/%s) \\\n' "$s"
done
echo
echo "$1: \$(ksymdeps_$1)"
echo
echo "\$(ksymdeps_$1):"

57
scripts/mod/modpost.c
View file

@ -35,6 +35,9 @@ static bool warn_unresolved;
static int sec_mismatch_count;
static bool sec_mismatch_warn_only = true;
/* Trim EXPORT_SYMBOLs that are unused by in-tree modules */
static bool trim_unused_exports;
/* ignore missing files */
static bool ignore_missing_files;
/* If set to 1, only warn (instead of error) about missing ns imports */
@ -219,6 +222,7 @@ struct symbol {
bool weak;
bool is_func;
bool is_gpl_only; /* exported by EXPORT_SYMBOL_GPL */
bool used; /* there exists a user of this symbol */
char name[];
};
@ -1826,6 +1830,7 @@ static void check_exports(struct module *mod)
continue;
}
exp->used = true;
s->module = exp->module;
s->crc_valid = exp->crc_valid;
s->crc = exp->crc;
@ -1849,6 +1854,23 @@ static void check_exports(struct module *mod)
}
}
static void handle_white_list_exports(const char *white_list)
{
char *buf, *p, *name;
buf = read_text_file(white_list);
p = buf;
while ((name = strsep(&p, "\n"))) {
struct symbol *sym = find_symbol(name);
if (sym)
sym->used = true;
}
free(buf);
}
static void check_modname_len(struct module *mod)
{
const char *mod_name;
@ -1919,10 +1941,14 @@ static void add_exported_symbols(struct buffer *buf, struct module *mod)
/* generate struct for exported symbols */
buf_printf(buf, "\n");
list_for_each_entry(sym, &mod->exported_symbols, list)
list_for_each_entry(sym, &mod->exported_symbols, list) {
if (trim_unused_exports && !sym->used)
continue;
buf_printf(buf, "KSYMTAB_%s(%s, \"%s\", \"%s\");\n",
sym->is_func ? "FUNC" : "DATA", sym->name,
sym->is_gpl_only ? "_gpl" : "", sym->namespace);
}
if (!modversions)
return;
@ -1930,6 +1956,9 @@ static void add_exported_symbols(struct buffer *buf, struct module *mod)
/* record CRCs for exported symbols */
buf_printf(buf, "\n");
list_for_each_entry(sym, &mod->exported_symbols, list) {
if (trim_unused_exports && !sym->used)
continue;
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",
@ -2093,9 +2122,6 @@ static void write_mod_c_file(struct module *mod)
char fname[PATH_MAX];
int ret;
check_modname_len(mod);
check_exports(mod);
add_header(&buf, mod);
add_exported_symbols(&buf, mod);
add_versions(&buf, mod);
@ -2187,6 +2213,9 @@ static void write_dump(const char *fname)
if (mod->from_dump)
continue;
list_for_each_entry(sym, &mod->exported_symbols, list) {
if (trim_unused_exports && !sym->used)
continue;
buf_printf(&buf, "0x%08x\t%s\t%s\tEXPORT_SYMBOL%s\t%s\n",
sym->crc, sym->name, mod->name,
sym->is_gpl_only ? "_GPL" : "",
@ -2229,12 +2258,13 @@ int main(int argc, char **argv)
{
struct module *mod;
char *missing_namespace_deps = NULL;
char *unused_exports_white_list = NULL;
char *dump_write = NULL, *files_source = NULL;
int opt;
LIST_HEAD(dump_lists);
struct dump_list *dl, *dl2;
while ((opt = getopt(argc, argv, "ei:mnT:o:aWwENd:")) != -1) {
while ((opt = getopt(argc, argv, "ei:mnT:to:au:WwENd:")) != -1) {
switch (opt) {
case 'e':
external_module = true;
@ -2259,6 +2289,12 @@ int main(int argc, char **argv)
case 'T':
files_source = optarg;
break;
case 't':
trim_unused_exports = true;
break;
case 'u':
unused_exports_white_list = optarg;
break;
case 'W':
extra_warn = true;
break;
@ -2291,6 +2327,17 @@ int main(int argc, char **argv)
if (files_source)
read_symbols_from_files(files_source);
list_for_each_entry(mod, &modules, list) {
if (mod->from_dump || mod->is_vmlinux)
continue;
check_modname_len(mod);
check_exports(mod);
}
if (unused_exports_white_list)
handle_white_list_exports(unused_exports_white_list);
list_for_each_entry(mod, &modules, list) {
if (mod->from_dump)
continue;

4
scripts/remove-stale-files
View file

@ -33,3 +33,7 @@ rm -f rust/target.json
rm -f scripts/bin2c
rm -f .scmversion
rm -rf include/ksym
find . -name '*.usyms' | xargs rm -f