[PATCH v2 0/6] Trim unused exported kernel symbols

[PATCH v2 0/6] Trim unused exported kernel symbols

[Nicolas Pitre]

This patch series provides the option to omit exported symbols from
the kernel and modules that are never referenced by any of the selected
modules in the current kernel configuration.

The original cover letter with lots of test results can be found here:

	https://lkml.org/lkml/2016/2/8/813

A git branch with those patches is also available here:

	http://git.linaro.org/people/nicolas.pitre/linux.git autoksyms

Changes from v1:

- Replaced "exp" that doesn't convey the right meaning as noted by
  Sam Ravnborg. The "ksym" identifier is actually what the kernel
  already uses for this. Therefore:
  - CONFIG_TRIM_UNUSED_EXPSYMS --> CONFIG_TRIM_UNUSED_KSYMS
  - include/generated/expsyms.h --> include/generated/autoksyms.h
  - #define __EXPSYM_* --> #define __KSYM_*

- Some sed regexp improvements as suggested by Al Viro.

- Renamed vmlinux_recursive target to autoksyms_recursive.

- Accept EXPORT_SYMBOL variants with a prefix, e.g. ACPI_EXPORT_SYMBOL.

- Minor commit log clarifications.

- Added Rusty's ACK.
 

[PATCH v2 1/6] kbuild: record needed exported symbols for modules

[Nicolas Pitre]

Kernel modules are partially linked object files with some undefined
symbols that are expected to be matched with EXPORT_SYMBOL() entries
from elsewhere.

Each .tmp_versions/*.mod file currently contains two line of text
separated by a newline character. The first line has the actual module
file name while the second line has a list of object files constituting
that module. Those files are parsed by modpost (scripts/mod/sumversion.c),
scripts/Makefile.modpost, scripts/Makefile.modsign, etc.  Only the
modpost utility cares about the second line while the others retrieve
only the first line.

Therefore we can add a third line to record the list of undefined symbols
aka required EXPORT_SYMBOL() entries for each module into that file
without breaking anything. Like for the second line, symbols are separated
by a blank and the list is terminated with a newline character.

To avoid needless build overhead, the undefined symbols extraction is
performed only when CONFIG_TRIM_UNUSED_KSYMS is selected.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
---
 scripts/Makefile.build | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 2c47f9c305..f4b4320e0d 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -253,6 +253,13 @@ define rule_cc_o_c
 	mv -f $(dot-target).tmp $(dot-target).cmd
 endef
 
+# List module undefined symbols (or empty line if not enabled)
+ifdef CONFIG_TRIM_UNUSED_KSYMS
+cmd_undef_syms = $(NM) $@ | sed -n 's/^ \+U //p' | xargs echo
+else
+cmd_undef_syms = echo
+endif
+
 # Built-in and composite module parts
 $(obj)/%.o: $(src)/%.c $(recordmcount_source) FORCE
 	$(call cmd,force_checksrc)
@@ -263,7 +270,8 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) FORCE
 $(single-used-m): $(obj)/%.o: $(src)/%.c $(recordmcount_source) FORCE
 	$(call cmd,force_checksrc)
 	$(call if_changed_rule,cc_o_c)
-	@{ echo $(@:.o=.ko); echo $@; } > $(MODVERDIR)/$(@F:.o=.mod)
+	@{ echo $(@:.o=.ko); echo $@; \
+	   $(cmd_undef_syms); } > $(MODVERDIR)/$(@F:.o=.mod)
 
 quiet_cmd_cc_lst_c = MKLST   $@
       cmd_cc_lst_c = $(CC) $(c_flags) -g -c -o $*.o $< && \
@@ -393,7 +401,8 @@ $(call multi_depend, $(multi-used-y), .o, -objs -y)
 
 $(multi-used-m): FORCE
 	$(call if_changed,link_multi-m)
-	@{ echo $(@:.o=.ko); echo $(link_multi_deps); } > $(MODVERDIR)/$(@F:.o=.mod)
+	@{ echo $(@:.o=.ko); echo $(link_multi_deps); \
+	   $(cmd_undef_syms); } > $(MODVERDIR)/$(@F:.o=.mod)
 $(call multi_depend, $(multi-used-m), .o, -objs -y -m)
 
 targets += $(multi-used-y) $(multi-used-m)
-- 
2.5.0

[PATCH v2 2/6] allow for per-symbol configurable EXPORT_SYMBOL()

[Nicolas Pitre]

Similar to include/generated/autoconf.h, include/generated/autoksyms.h
will contain a list of defines for each EXPORT_SYMBOL() that we want
active. The format is:

  #define __KSYM_<symbol_name> 1

This list will be auto-generated with another patch.  For now we only
include the preprocessor magic to automatically create or omit the
corresponding struct kernel_symbol declaration.

Given the content of include/generated/autoksyms.h may not be known in
advance, an empty file is created early on to let the build proceed.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
---
 Makefile               |  2 ++
 include/linux/export.h | 22 ++++++++++++++++++++--
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 6c1a3c2479..e916428cf7 100644
--- a/Makefile
+++ b/Makefile
@@ -986,6 +986,8 @@ prepare2: prepare3 outputmakefile asm-generic
 prepare1: prepare2 $(version_h) include/generated/utsrelease.h \
                    include/config/auto.conf
 	$(cmd_crmodverdir)
+	$(Q)test -e include/generated/autoksyms.h || \
+	    touch   include/generated/autoksyms.h
 
 archprepare: archheaders archscripts prepare1 scripts_basic
 
diff --git a/include/linux/export.h b/include/linux/export.h
index 96e45ea463..77afdb2a25 100644
--- a/include/linux/export.h
+++ b/include/linux/export.h
@@ -38,7 +38,7 @@ extern struct module __this_module;
 
 #ifdef CONFIG_MODULES
 
-#ifndef __GENKSYMS__
+#if defined(__KERNEL__) && !defined(__GENKSYMS__)
 #ifdef CONFIG_MODVERSIONS
 /* Mark the CRC weak since genksyms apparently decides not to
  * generate a checksums for some symbols */
@@ -53,7 +53,7 @@ extern struct module __this_module;
 #endif
 
 /* For every exported symbol, place a struct in the __ksymtab section */
-#define __EXPORT_SYMBOL(sym, sec)				\
+#define ___EXPORT_SYMBOL(sym, sec)				\
 	extern typeof(sym) sym;					\
 	__CRC_SYMBOL(sym, sec)					\
 	static const char __kstrtab_##sym[]			\
@@ -65,6 +65,24 @@ extern struct module __this_module;
 	__attribute__((section("___ksymtab" sec "+" #sym), unused))	\
 	= { (unsigned long)&sym, __kstrtab_##sym }
 
+#ifdef CONFIG_TRIM_UNUSED_KSYMS
+
+#include <linux/kconfig.h>
+#include <generated/autoksyms.h>
+
+#define __EXPORT_SYMBOL(sym, sec)				\
+	__cond_export_sym(sym, sec, config_enabled(__KSYM_##sym))
+#define __cond_export_sym(sym, sec, conf)			\
+	___cond_export_sym(sym, sec, conf)
+#define ___cond_export_sym(sym, sec, enabled)			\
+	__cond_export_sym_##enabled(sym, sec)
+#define __cond_export_sym_1(sym, sec) ___EXPORT_SYMBOL(sym, sec)
+#define __cond_export_sym_0(sym, sec) /* nothing */
+
+#else
+#define __EXPORT_SYMBOL ___EXPORT_SYMBOL
+#endif
+
 #define EXPORT_SYMBOL(sym)					\
 	__EXPORT_SYMBOL(sym, "")
 
-- 
2.5.0

[PATCH v2 3/6] fixdep: minor cleanup

[Nicolas Pitre]

Simple code reorganization to make coming changes more obvious.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
---
 scripts/basic/fixdep.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/scripts/basic/fixdep.c b/scripts/basic/fixdep.c
index 5b327c67a8..63f129021d 100644
--- a/scripts/basic/fixdep.c
+++ b/scripts/basic/fixdep.c
@@ -236,21 +236,20 @@ static void parse_config_file(const char *map, size_t len)
 			continue;
 		p += 7;
 		for (q = p; q < map + len; q++) {
-			if (!(isalnum(*q) || *q == '_'))
-				goto found;
+			if (!(isalnum(*q) || *q == '_')) {
+				if (!memcmp(q - 7, "_MODULE", 7))
+					q -= 7;
+				if (q - p > 0)
+					use_config(p, q - p);
+				break;
+			}
 		}
 		continue;
 
-	found:
-		if (!memcmp(q - 7, "_MODULE", 7))
-			q -= 7;
-		if (q - p < 0)
-			continue;
-		use_config(p, q - p);
 	}
 }
 
-/* test is s ends in sub */
+/* test if s ends in sub */
 static int strrcmp(const char *s, const char *sub)
 {
 	int slen = strlen(s);
-- 
2.5.0

[PATCH v2 4/6] fixdep: add fine grained build dependencies for exported symbols

[Nicolas Pitre]

Like for kconfig options, we now have the ability to compile in and
out individual EXPORT_SYMBOL() declarations based on the content of
include/generated/autoksyms.h.  However we don't want the entire
world to be rebuilt whenever that file is touched.

Let's apply the same build dependency trick used with config symbols
to those EXPORT_SYMBOL() instances. The key in this case is the actual
symbol name being exported. All symbol names are collapsed to lowercase
path names whose time stamp provide fine grained dependencies.

Because of the lowercasing, there might be name collisions triggering
spurious rebuilds for similar symbols. But this shouldn't be a big issue
in practice. (This is the case for CONFIG symbols and I didn't want to
be different here, whatever the original reason for doing so.)

To avoid needless build overhead, the exported symbol name parsing is
performed only when CONFIG_TRIM_UNUSED_KSYMS is selected. A new cmdline
argument is added to that effect.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
---
 scripts/Kbuild.include |  3 ++-
 scripts/Makefile.build |  3 ++-
 scripts/basic/fixdep.c | 69 ++++++++++++++++++++++++++++++++++++++++++--------
 3 files changed, 62 insertions(+), 13 deletions(-)

diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 1db6d73c8d..e28afc039a 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -257,7 +257,8 @@ if_changed = $(if $(strip $(any-prereq) $(arg-check)),                       \
 if_changed_dep = $(if $(strip $(any-prereq) $(arg-check) ),                  \
 	@set -e;                                                             \
 	$(echo-cmd) $(cmd_$(1));                                             \
-	scripts/basic/fixdep $(depfile) $@ '$(make-cmd)' > $(dot-target).tmp;\
+	scripts/basic/fixdep $(if $(CONFIG_TRIM_UNUSED_KSYMS),-e)	     \
+		 $(depfile) $@ '$(make-cmd)' > $(dot-target).tmp;	     \
 	rm -f $(depfile);                                                    \
 	mv -f $(dot-target).tmp $(dot-target).cmd)
 
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index f4b4320e0d..01b3eae7ed 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -247,7 +247,8 @@ define rule_cc_o_c
 	$(cmd_modversions)						  \
 	$(call echo-cmd,record_mcount)					  \
 	$(cmd_record_mcount)						  \
-	scripts/basic/fixdep $(depfile) $@ '$(call make-cmd,cc_o_c)' >    \
+	scripts/basic/fixdep $(if $(CONFIG_TRIM_UNUSED_KSYMS),-e)	  \
+	                     $(depfile) $@ '$(call make-cmd,cc_o_c)' >    \
 	                                              $(dot-target).tmp;  \
 	rm -f $(depfile);						  \
 	mv -f $(dot-target).tmp $(dot-target).cmd
diff --git a/scripts/basic/fixdep.c b/scripts/basic/fixdep.c
index 63f129021d..3fa3e00aca 100644
--- a/scripts/basic/fixdep.c
+++ b/scripts/basic/fixdep.c
@@ -120,13 +120,20 @@
 #define INT_NFIG ntohl(0x4e464947)
 #define INT_FIG_ ntohl(0x4649475f)
 
+#define INT_EXPO ntohl(0x4558504f)
+#define INT_XPOR ntohl(0x58504f52)
+#define INT_PORT ntohl(0x504f5254)
+#define INT_ORT_ ntohl(0x4f52545f)
+
+int parse_export_symbol;
 char *target;
 char *depfile;
 char *cmdline;
 
 static void usage(void)
 {
-	fprintf(stderr, "Usage: fixdep <depfile> <target> <cmdline>\n");
+	fprintf(stderr, "Usage: fixdep [-e] <depfile> <target> <cmdline>\n");
+	fprintf(stderr, " -e  parse EXPORT_SYMBOL_* too.\n");
 	exit(1);
 }
 
@@ -140,6 +147,7 @@ static void print_cmdline(void)
 
 struct item {
 	struct item	*next;
+	const char	*type;
 	unsigned int	len;
 	unsigned int	hash;
 	char		name[0];
@@ -161,12 +169,12 @@ static unsigned int strhash(const char *str, unsigned int sz)
 /*
  * Lookup a value in the configuration string.
  */
-static int is_defined_config(const char *name, int len, unsigned int hash)
+static int is_defined_string(const char *type, const char *name, int len, unsigned int hash)
 {
 	struct item *aux;
 
 	for (aux = hashtab[hash % HASHSZ]; aux; aux = aux->next) {
-		if (aux->hash == hash && aux->len == len &&
+		if (aux->hash == hash && aux->len == len && aux->type == type &&
 		    memcmp(aux->name, name, len) == 0)
 			return 1;
 	}
@@ -176,7 +184,7 @@ static int is_defined_config(const char *name, int len, unsigned int hash)
 /*
  * Add a new value to the configuration string.
  */
-static void define_config(const char *name, int len, unsigned int hash)
+static void define_string(const char *type, const char *name, int len, unsigned int hash)
 {
 	struct item *aux = malloc(sizeof(*aux) + len);
 
@@ -186,25 +194,26 @@ static void define_config(const char *name, int len, unsigned int hash)
 	}
 	memcpy(aux->name, name, len);
 	aux->len = len;
+	aux->type = type;
 	aux->hash = hash;
 	aux->next = hashtab[hash % HASHSZ];
 	hashtab[hash % HASHSZ] = aux;
 }
 
 /*
- * Record the use of a CONFIG_* word.
+ * Record the use of a CONFIG_* word, or the argument to EXPORT_MODULE*()
  */
-static void use_config(const char *m, int slen)
+static void use_dep(const char *type, const char *m, int slen)
 {
 	unsigned int hash = strhash(m, slen);
 	int c, i;
 
-	if (is_defined_config(m, slen, hash))
+	if (is_defined_string(type, m, slen, hash))
 	    return;
 
-	define_config(m, slen, hash);
+	define_string(type, m, slen, hash);
 
-	printf("    $(wildcard include/config/");
+	printf("    $(wildcard include/%s/", type);
 	for (i = 0; i < slen; i++) {
 		c = m[i];
 		if (c == '_')
@@ -228,7 +237,17 @@ static void parse_config_file(const char *map, size_t len)
 		if (*m == INT_ONFI) { p = (char *) m-1; goto conf; }
 		if (*m == INT_NFIG) { p = (char *) m-2; goto conf; }
 		if (*m == INT_FIG_) { p = (char *) m-3; goto conf; }
+
+		if (!parse_export_symbol)
+			continue;
+
+		if (*m == INT_EXPO) { p = (char *) m  ; goto export; }
+		if (*m == INT_XPOR) { p = (char *) m-1; goto export; }
+		if (*m == INT_PORT) { p = (char *) m-2; goto export; }
+		if (*m == INT_ORT_) { p = (char *) m-3; goto export; }
+
 		continue;
+
 	conf:
 		if (p > map + len - 7)
 			continue;
@@ -240,12 +259,36 @@ static void parse_config_file(const char *map, size_t len)
 				if (!memcmp(q - 7, "_MODULE", 7))
 					q -= 7;
 				if (q - p > 0)
-					use_config(p, q - p);
+					use_dep("config", p, q - p);
 				break;
 			}
 		}
 		continue;
 
+	export:
+		if (p[-1] == '_' && (p >= map + 2 && p[-2] == '_'))
+			continue;
+		if (p > map + len - 7 || memcmp(p, "EXPORT_", 7) != 0)
+			continue;
+		p += 7;
+		if (p <= map + len - 6 && memcmp(p, "EARLY_", 6) == 0)
+			p += 6;
+		if (p <= map + len - 8 && memcmp(p, "PER_CPU_", 8) == 0)
+			p += 8;
+		if (p > map + len - 6 || memcmp(p, "SYMBOL", 6) != 0)
+			continue;
+		p += 6;
+		while (p < map + len && *p != '(' && *p != '\n')
+			p++;
+		q = p + 1;
+		while (q < map + len && *q != ')' && *q != '\n' && *q != '#')
+			q++;
+		if (q >= map + len || *p != '(' || *q != ')')
+			continue;
+		while (isblank(*++p));
+		while (isblank(q[-1])) q--;
+		if (q - p > 0)
+			use_dep("config/ksym", p, q - p);
 	}
 }
 
@@ -330,6 +373,7 @@ static void parse_dep_file(void *map, size_t len)
 			if (strrcmp(s, "include/generated/autoconf.h") &&
 			    strrcmp(s, "arch/um/include/uml-config.h") &&
 			    strrcmp(s, "include/linux/kconfig.h") &&
+			    strrcmp(s, "include/generated/autoksyms.h") &&
 			    strrcmp(s, ".ver")) {
 				/*
 				 * Do not list the source file as dependency,
@@ -429,7 +473,10 @@ int main(int argc, char *argv[])
 {
 	traps();
 
-	if (argc != 4)
+	if (argc == 5 && !strcmp(argv[1], "-e")) {
+		parse_export_symbol = 1;
+		argv++;
+	} else if (argc != 4)
 		usage();
 
 	depfile = argv[1];
-- 
2.5.0

[PATCH v2 5/6] create/adjust generated/autoksyms.h

[Nicolas Pitre]

Given the list of exported symbols needed by all modules, we can create
a header file containing preprocessor defines for each of those symbols.
Also, when some symbols are added and/or removed from the list, we can
update the time on the corresponding files used as build dependencies for
those symbols. And finally, if any symbol did change state, the
corresponding source files must be rebuilt.

The insertion or removal of an EXPORT_SYMBOL() entry within a module may
create or remove the need for another exported symbol.  This is why this
operation has to be repeated until the list of needed exported symbols
becomes stable. Only then the final kernel and modules link take place.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
---
 Makefile                    | 13 ++++++
 scripts/adjust_autoksyms.sh | 97 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 110 insertions(+)
 create mode 100755 scripts/adjust_autoksyms.sh

diff --git a/Makefile b/Makefile
index e916428cf7..bb865095ca 100644
--- a/Makefile
+++ b/Makefile
@@ -921,6 +921,10 @@ quiet_cmd_link-vmlinux = LINK    $@
 # Include targets which we want to
 # execute if the rest of the kernel build went well.
 vmlinux: scripts/link-vmlinux.sh $(vmlinux-deps) FORCE
+ifdef CONFIG_TRIM_UNUSED_KSYMS
+	$(Q)$(CONFIG_SHELL) scripts/adjust_autoksyms.sh \
+	  "$(MAKE) KBUILD_MODULES=1 -f $(srctree)/Makefile autoksyms_recursive"
+endif
 ifdef CONFIG_HEADERS_CHECK
 	$(Q)$(MAKE) -f $(srctree)/Makefile headers_check
 endif
@@ -935,6 +939,15 @@ ifdef CONFIG_GDB_SCRIPTS
 endif
 	+$(call if_changed,link-vmlinux)
 
+autoksyms_recursive: $(vmlinux-deps)
+	$(Q)$(CONFIG_SHELL) scripts/adjust_autoksyms.sh \
+	  "$(MAKE) KBUILD_MODULES=1 -f $(srctree)/Makefile autoksyms_recursive"
+PHONY += autoksyms_recursive
+
+# standalone target for easier testing
+include/generated/autoksyms.h: FORCE
+	$(Q)$(CONFIG_SHELL) scripts/adjust_autoksyms.sh true
+
 # The actual objects are generated when descending,
 # make sure no implicit rule kicks in
 $(sort $(vmlinux-deps)): $(vmlinux-dirs) ;
diff --git a/scripts/adjust_autoksyms.sh b/scripts/adjust_autoksyms.sh
new file mode 100755
index 0000000000..0cc8ac3349
--- /dev/null
+++ b/scripts/adjust_autoksyms.sh
@@ -0,0 +1,97 @@
+#!/bin/sh
+
+# Script to create/update include/generated/autoksyms.h and dependency files
+#
+# Copyright:	(C) 2016  Linaro Limited
+# Created by:	Nicolas Pitre, January 2016
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 as
+# published by the Free Software Foundation.
+
+# Create/update the include/generated/autoksyms.h file from the list
+# of all module's needed symbols as recorded on the third line of
+# .tmp_versions/*.mod files.
+#
+# For each symbol being added or removed, the corresponding dependency
+# file's timestamp is updated to force a rebuild of the affected source
+# file. All arguments passed to this script are assumed to be a command
+# to be exec'd to trigger a rebuild of those files.
+
+set -e
+
+cur_ksyms_file="include/generated/autoksyms.h"
+new_ksyms_file="include/generated/autoksyms.h.tmpnew"
+
+info() { [ "$quiet" != "silent_" ] && printf "  %-7s %s\n" "$1" "$2"; }
+
+info "CHK" "$cur_ksyms_file"
+
+# Use "make V=1" to debug this script.
+case "$KBUILD_VERBOSE" in
+*1*)
+	set -x
+	;;
+esac
+
+# We need access to CONFIG_ symbols
+case "${KCONFIG_CONFIG}" in
+*/*)
+	. "${KCONFIG_CONFIG}"
+	;;
+*)
+	# Force using a file from the current directory
+	. "./${KCONFIG_CONFIG}"
+esac
+
+# In case it doesn't exist yet...
+[ -e "$cur_ksyms_file" ] || touch "$cur_ksyms_file"
+
+# Generate a new ksym list file with symbols needed by the current
+# set of modules.
+cat > "$new_ksyms_file" << EOT
+/*
+ * Automatically generated file; DO NOT EDIT.
+ */
+
+EOT
+sed -ns -e '3s/ /\n/gp' "$MODVERDIR"/*.mod | sort -u |
+while read sym; do
+	if [ -n "$CONFIG_HAVE_UNDERSCORE_SYMBOL_PREFIX" ]; then
+		sym=$(sed 's/^_//' <<< "$sym")
+	fi
+	echo "#define __KSYM_${sym} 1"
+done >> "$new_ksyms_file"
+
+# Special case for modversions (see modpost.c)
+if [ -n "$CONFIG_MODVERSIONS" ]; then
+	echo "#define __KSYM_module_layout 1" >> "$new_ksyms_file"
+fi
+
+# Extract changes between old and new list and touch corresponding
+# dependency files.
+# Note: sort -m doesn't work well with underscore prefixed symbols so we
+# use 'cat ... | sort' instead.
+changed=0
+while read sympath; do
+	[ -z "$sympath" ] && continue
+	depfile="include/config/ksym/${sympath}.h"
+	mkdir -p "$(dirname "$depfile")"
+	touch "$depfile"
+	changed=$((changed + 1))
+done <<< "$(
+	cat "$cur_ksyms_file" "$new_ksyms_file" | sort | uniq -u |
+	sed -n 's/^#define __KSYM_\(.*\) 1/\1/p' | tr "A-Z_" "a-z/"  )"
+
+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: $old old, $new 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
-- 
2.5.0

[PATCH v2 6/6] kconfig option for TRIM_UNUSED_KSYMS

[Nicolas Pitre]

The config option to enable it all.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
---
 init/Kconfig | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/init/Kconfig b/init/Kconfig
index 22320804fb..e6f666331b 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1990,6 +1990,22 @@ config MODULE_COMPRESS_XZ
 
 endchoice
 
+config TRIM_UNUSED_KSYMS
+	bool "Trim unused exported kernel symbols"
+	depends on MODULES && !UNUSED_SYMBOLS
+	help
+	  The kernel and some modules make many symbols available for
+	  other modules to use via EXPORT_SYMBOL() and variants. Depending
+	  on the set of modules being selected in your kernel configuration,
+	  many of those exported symbols might never be used.
+
+	  This option allows for unused exported symbols to be dropped from
+	  the build. In turn, this provides the compiler more opportunities
+	  (especially when using LTO) for optimizing the code and reducing
+	  binary size.  This might have some security advantages as well.
+
+	  If unsure say N.
+
 endif # MODULES
 
 config MODULES_TREE_LOOKUP
-- 
2.5.0