Re: [PATCH v6 15/20] kbuild: implement CONFIG_TRIM_UNUSED_KSYMS without recursion
From: Nick Desaulniers
Date: Thu May 25 2023 - 14:15:18 EST
On Sun, May 21, 2023 at 9:05 AM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
>
> 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:
>
> - 5cf0fd591f2e ("Kbuild: disable TRIM_UNUSED_KSYMS option")
> - a555bdd0c58c ("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 f73edc8951b2 ("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@xxxxxxxxxxx/
>
> 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.
I think this patch removes the only 2 references to
scripts/gen_autoksyms.sh in the tree. Can or should that be removed as
well?
The rest of the patch LGTM.
>
> Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> ---
>
> Changes in v5:
> - Clean up more
>
> .gitignore | 1 -
> Makefile | 19 +---------
> include/linux/export.h | 65 +++++----------------------------
> scripts/Makefile.build | 7 ----
> scripts/Makefile.modpost | 7 ++++
> scripts/adjust_autoksyms.sh | 73 -------------------------------------
> scripts/basic/fixdep.c | 3 +-
> scripts/gen_ksymdeps.sh | 30 ---------------
> scripts/mod/modpost.c | 54 ++++++++++++++++++++++++---
> scripts/remove-stale-files | 2 +
> 10 files changed, 70 insertions(+), 191 deletions(-)
> delete mode 100755 scripts/adjust_autoksyms.sh
> delete mode 100755 scripts/gen_ksymdeps.sh
>
> diff --git a/.gitignore b/.gitignore
> index 7f86e0837909..172e3874adfd 100644
> --- a/.gitignore
> +++ b/.gitignore
> @@ -112,7 +112,6 @@ modules.order
> #
> /include/config/
> /include/generated/
> -/include/ksym/
> /arch/*/include/generated/
>
> # stgit generated dirs
> diff --git a/Makefile b/Makefile
> index f836936fb4d8..ffc2c9b632fd 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1193,28 +1193,13 @@ 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 +1208,7 @@ 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 +1264,7 @@ 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
> diff --git a/include/linux/export.h b/include/linux/export.h
> index 32461a01608c..9bf081ff9903 100644
> --- a/include/linux/export.h
> +++ b/include/linux/export.h
> @@ -37,30 +37,13 @@ extern struct module __this_module;
> #define __EXPORT_SYMBOL_REF(sym) .balign 4; .long sym
> #endif
>
> -#define ____EXPORT_SYMBOL(sym, license, ns) \
> +#define ___EXPORT_SYMBOL(sym, license, ns) \
> .section ".export_symbol","a" ; \
> __export_symbol_##license##_##sym: ; \
> .asciz ns ; \
> __EXPORT_SYMBOL_REF(sym) ; \
> .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) \
> - __ADDRESSABLE(sym) \
> - asm(__stringify(____EXPORT_SYMBOL(sym, license, ns)))
> -
> -#endif
> -
> #if !defined(CONFIG_MODULES) || defined(__DISABLE_EXPORTS)
>
> /*
> @@ -70,50 +53,20 @@ 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) \
> + __ADDRESSABLE(sym) \
> + asm(__stringify(___EXPORT_SYMBOL(sym, license, ns)))
>
> #endif /* CONFIG_MODULES */
>
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index bd4123795299..8154bd962eea 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -215,18 +215,12 @@ 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)
> @@ -237,7 +231,6 @@ 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)
> diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
> index 0980c58d8afc..1e0b47cbabd9 100644
> --- a/scripts/Makefile.modpost
> +++ b/scripts/Makefile.modpost
> @@ -90,6 +90,13 @@ 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 -W, $(ksym-wl))
> +modpost-deps += $(ksym-wl)
> +endif
> +
> ifeq ($(wildcard vmlinux.o),)
> missing-input := vmlinux.o
> output-symdump := modules-only.symvers
> diff --git a/scripts/adjust_autoksyms.sh b/scripts/adjust_autoksyms.sh
> deleted file mode 100755
> index f1b5ac818411..000000000000
> --- a/scripts/adjust_autoksyms.sh
> +++ /dev/null
> @@ -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
> diff --git a/scripts/basic/fixdep.c b/scripts/basic/fixdep.c
> index fa562806c2be..84b6efa849f4 100644
> --- a/scripts/basic/fixdep.c
> +++ b/scripts/basic/fixdep.c
> @@ -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 */
> diff --git a/scripts/gen_ksymdeps.sh b/scripts/gen_ksymdeps.sh
> deleted file mode 100755
> index 8ee533f33659..000000000000
> --- a/scripts/gen_ksymdeps.sh
> +++ /dev/null
> @@ -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):"
> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> index f14fe9301ae6..516323c3910a 100644
> --- a/scripts/mod/modpost.c
> +++ b/scripts/mod/modpost.c
> @@ -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 */
> @@ -217,6 +220,7 @@ 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[];
> };
>
> @@ -1772,6 +1776,7 @@ 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;
> @@ -1795,6 +1800,23 @@ 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;
> @@ -1865,10 +1887,14 @@ 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;
> @@ -1876,6 +1902,9 @@ 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",
> @@ -2039,9 +2068,6 @@ 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);
> @@ -2175,12 +2201,13 @@ 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:awENd:")) != -1) {
> + while ((opt = getopt(argc, argv, "ei:mntT:tW:o:awENd:")) != -1) {
> switch (opt) {
> case 'e':
> external_module = true;
> @@ -2205,6 +2232,12 @@ int main(int argc, char **argv)
> case 'T':
> files_source = optarg;
> break;
> + case 't':
> + trim_unused_exports = true;
> + break;
> + case 'W':
> + unused_exports_white_list = optarg;
> + break;
> case 'w':
> warn_unresolved = true;
> break;
> @@ -2234,6 +2267,17 @@ 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;
> diff --git a/scripts/remove-stale-files b/scripts/remove-stale-files
> index 7f432900671a..8502a17d47df 100755
> --- a/scripts/remove-stale-files
> +++ b/scripts/remove-stale-files
> @@ -33,3 +33,5 @@ rm -f rust/target.json
> rm -f scripts/bin2c
>
> rm -f .scmversion
> +
> +rm -rf include/ksym
> --
> 2.39.2
>
--
Thanks,
~Nick Desaulniers