kbuild: simplify dependency generation for CONFIG_TRIM_UNUSED_KSYMS

My main motivation of this commit is to clean up scripts/Kbuild.include
and scripts/Makefile.build.

Currently, CONFIG_TRIM_UNUSED_KSYMS works with a tricky gimmick;
possibly exported symbols are detected by letting $(CPP) replace
EXPORT_SYMBOL* with a special string '=== __KSYM_*===', which is
post-processed by sed, and passed to fixdep. The extra preprocessing
is costly, and hacking cmd_and_fixdep is ugly.

I came up with a new way to find exported symbols; insert a dummy
symbol __ksym_marker_* to each potentially exported symbol. Those
dummy symbols are picked up by $(NM), post-processed by sed, then
appended to .*.cmd files. I collected the post-process part to a
new shell script scripts/gen_ksymdeps.sh for readability. The dummy
symbols are put into the .discard.* section so that the linker
script rips them off the final vmlinux or modules.

A nice side-effect is building with CONFIG_TRIM_UNUSED_KSYMS will
be much faster.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Nicolas Pitre <nico@linaro.org>

6 files changed, 53 insertions, 69 deletions

diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h
index 4d73e6e3c66c..294d6ae785d4 100644
--- a/include/asm-generic/export.h
+++ b/include/asm-generic/export.h
@@ -59,16 +59,19 @@ __kcrctab_\name:
 .endm
 #undef __put
 
-#if defined(__KSYM_DEPS__)
-
-#define __EXPORT_SYMBOL(sym, val, sec)  === __KSYM_##sym ===
-
-#elif defined(CONFIG_TRIM_UNUSED_KSYMS)
+#if defined(CONFIG_TRIM_UNUSED_KSYMS)
 
 #include <linux/kconfig.h>
 #include <generated/autoksyms.h>
 
+.macro __ksym_marker sym
+        .section ".discard.ksym","a"
+__ksym_marker_\sym:
+         .previous
+.endm
+
 #define __EXPORT_SYMBOL(sym, val, sec)                          \
+        __ksym_marker sym;                                      \
         __cond_export_sym(sym, val, sec, __is_defined(__KSYM_##sym))
 #define __cond_export_sym(sym, val, sec, conf)                  \
         ___cond_export_sym(sym, val, sec, conf)
diff --git a/include/linux/export.h b/include/linux/export.h
index ce764a5d2ee4..fd8711ed9ac4 100644
--- a/include/linux/export.h
+++ b/include/linux/export.h
@@ -92,22 +92,22 @@ struct kernel_symbol {
  */
 #define __EXPORT_SYMBOL(sym, sec)
 
-#elif defined(__KSYM_DEPS__)
+#elif defined(CONFIG_TRIM_UNUSED_KSYMS)
+
+#include <generated/autoksyms.h>
 
 /*
  * 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 string pattern that is unlikely to be valid code that the build
- * system filters out from the preprocessor output (see ksym_dep_filter
- * in scripts/Kbuild.include).
+ * 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.
  */
-#define __EXPORT_SYMBOL(sym, sec)       === __KSYM_##sym ===
-
-#elif defined(CONFIG_TRIM_UNUSED_KSYMS)
-
-#include <generated/autoksyms.h>
+#define __ksym_marker(sym)      \
+        static int __ksym_marker_##sym[0] __section(".discard.ksym") __used
 
 #define __EXPORT_SYMBOL(sym, sec)                               \
+        __ksym_marker(sym);                                     \
         __cond_export_sym(sym, sec, __is_defined(__KSYM_##sym))
 #define __cond_export_sym(sym, sec, conf)                       \
         ___cond_export_sym(sym, sec, conf)
diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 6cf6a8b83b97..4b943f4d2226 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -260,39 +260,11 @@ if_changed_dep = $(if $(strip $(any-prereq) $(arg-check) ),                  \
         @set -e;                                                             \
         $(cmd_and_fixdep), @:)
 
-ifndef CONFIG_TRIM_UNUSED_KSYMS
-
 cmd_and_fixdep =                                                             \
         $(echo-cmd) $(cmd_$(1));                                             \
         scripts/basic/fixdep $(depfile) $@ '$(make-cmd)' > $(dot-target).cmd;\
         rm -f $(depfile);
 
-else
-
-# Filter out exported kernel symbol names from the preprocessor output.
-# See also __KSYM_DEPS__ in include/linux/export.h.
-# We disable the depfile generation here, so as not to overwrite the existing
-# depfile while fixdep is parsing it.
-flags_nodeps = $(filter-out -Wp$(comma)-M%, $($(1)))
-ksym_dep_filter =                                                            \
-        case "$(1)" in                                                       \
-          cc_*_c|cpp_i_c)                                                    \
-            $(CPP) $(call flags_nodeps,c_flags) -D__KSYM_DEPS__ $< ;;        \
-          as_*_S|cpp_s_S)                                                    \
-            $(CPP) $(call flags_nodeps,a_flags) -D__KSYM_DEPS__ $< ;;        \
-          boot*|build*|cpp_its_S|*cpp_lds_S|dtc|host*|vdso*) : ;;            \
-          *) echo "Don't know how to preprocess $(1)" >&2; false ;;          \
-        esac | tr ";" "\n" | sed -n 's/^.*=== __KSYM_\(.*\) ===.*$$/_\1/p'
-
-cmd_and_fixdep =                                                             \
-        $(echo-cmd) $(cmd_$(1));                                             \
-        $(ksym_dep_filter) |                                                 \
-                scripts/basic/fixdep -e $(depfile) $@ '$(make-cmd)'          \
-                        > $(dot-target).cmd;                                 \
-        rm -f $(depfile);
-
-endif
-
 # Usage: $(call if_changed_rule,foo)
 # Will check if $(cmd_foo) or any of the prerequisites changed,
 # and if so will execute $(rule_foo).
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index cdb25d163b42..23ebf2508234 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -254,9 +254,15 @@ objtool_dep = $(objtool_obj)					\
               $(wildcard include/config/orc/unwinder.h          \
                          include/config/stack/validation.h)
 
+ifdef CONFIG_TRIM_UNUSED_KSYMS
+cmd_gen_ksymdeps = \
+        $(CONFIG_SHELL) $(srctree)/scripts/gen_ksymdeps.sh $@ >> $(dot-target).cmd;
+endif
+
 define rule_cc_o_c
         $(call echo-cmd,checksrc) $(cmd_checksrc)                         \
         $(call cmd_and_fixdep,cc_o_c)                                     \
+        $(cmd_gen_ksymdeps)                                               \
         $(cmd_checkdoc)                                                   \
         $(call echo-cmd,objtool) $(cmd_objtool)                           \
         $(cmd_modversions_c)                                              \
@@ -265,6 +271,7 @@ endef
 
 define rule_as_o_S
         $(call cmd_and_fixdep,as_o_S)                                     \
+        $(cmd_gen_ksymdeps)                                               \
         $(call echo-cmd,objtool) $(cmd_objtool)                           \
         $(cmd_modversions_S)
 endef
diff --git a/scripts/basic/fixdep.c b/scripts/basic/fixdep.c
index 850966f3d602..facbd603adf6 100644
--- a/scripts/basic/fixdep.c
+++ b/scripts/basic/fixdep.c
@@ -105,8 +105,7 @@
 
 static void usage(void)
 {
-        fprintf(stderr, "Usage: fixdep [-e] <depfile> <target> <cmdline>\n");
-        fprintf(stderr, " -e  insert extra dependencies given on stdin\n");
+        fprintf(stderr, "Usage: fixdep <depfile> <target> <cmdline>\n");
         exit(1);
 }
 
@@ -131,21 +130,6 @@ static void print_dep(const char *m, int slen, const char *dir)
         printf(".h) \\\n");
 }
 
-static void do_extra_deps(void)
-{
-        char buf[80];
-
-        while (fgets(buf, sizeof(buf), stdin)) {
-                int len = strlen(buf);
-
-                if (len < 2 || buf[len - 1] != '\n') {
-                        fprintf(stderr, "fixdep: bad data on stdin\n");
-                        exit(1);
-                }
-                print_dep(buf, len - 1, "include/ksym");
-        }
-}
-
 struct item {
         struct item     *next;
         unsigned int    len;
@@ -293,7 +277,7 @@ static int is_ignored_file(const char *s, int len)
  * assignments are parsed not only by make, but also by the rather simple
  * parser in scripts/mod/sumversion.c.
  */
-static void parse_dep_file(char *m, const char *target, int insert_extra_deps)
+static void parse_dep_file(char *m, const char *target)
 {
         char *p;
         int is_last, is_target;
@@ -369,9 +353,6 @@ static void parse_dep_file(char *m, const char *target, int insert_extra_deps)
                 exit(1);
         }
 
-        if (insert_extra_deps)
-                do_extra_deps();
-
         printf("\n%s: $(deps_%s)\n\n", target, target);
         printf("$(deps_%s):\n", target);
 }
@@ -379,13 +360,9 @@ static void parse_dep_file(char *m, const char *target, int insert_extra_deps)
 int main(int argc, char *argv[])
 {
         const char *depfile, *target, *cmdline;
-        int insert_extra_deps = 0;
         void *buf;
 
-        if (argc == 5 && !strcmp(argv[1], "-e")) {
-                insert_extra_deps = 1;
-                argv++;
-        } else if (argc != 4)
+        if (argc != 4)
                 usage();
 
         depfile = argv[1];
@@ -395,7 +372,7 @@ int main(int argc, char *argv[])
         printf("cmd_%s := %s\n\n", target, cmdline);
 
         buf = read_file(depfile);
-        parse_dep_file(buf, target, insert_extra_deps);
+        parse_dep_file(buf, target);
         free(buf);
 
         return 0;
diff --git a/scripts/gen_ksymdeps.sh b/scripts/gen_ksymdeps.sh
new file mode 100755
index 000000000000..1324986e1362
--- /dev/null
+++ b/scripts/gen_ksymdeps.sh
@@ -0,0 +1,25 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+
+set -e
+
+# List of exported symbols
+ksyms=$($NM $1 | sed -n 's/.*__ksym_marker_\(.*\)/\1/p' | tr A-Z a-z)
+
+if [ -z "$ksyms" ]; then
+        exit 0
+fi
+
+echo
+echo "ksymdeps_$1 := \\"
+
+for s in $ksyms
+do
+        echo $s | sed -e 's:^_*:    $(wildcard include/ksym/:' \
+                        -e 's:__*:/:g' -e 's/$/.h) \\/'
+done
+
+echo
+echo "$1: \$(ksymdeps_$1)"
+echo
+echo "\$(ksymdeps_$1):"