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 <[e-mail]>
Reviewed-by: Nicolas Pitre <[e-mail]>

include/asm-generic/export.h

···
 .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)

include/linux/export.h

···
  */
 #define __EXPORT_SYMBOL(sym, sec)
 
-#elif defined(__KSYM_DEPS__)
-
-/*
- * 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).
- */
-#define __EXPORT_SYMBOL(sym, sec)	=== __KSYM_##sym ===
-
 #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 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 __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)

scripts/Kbuild.include

···
 	@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,

scripts/Makefile.build

···
 	      $(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)						  \
···
 
 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

scripts/basic/fixdep.c

···
 
 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);
 }
 
···
 		prev_c = c;
 	}
 	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 {
···
  * 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;
···
 		exit(1);
 	}
 
-	if (insert_extra_deps)
-		do_extra_deps();
-
 	printf("\n%s: $(deps_%s)\n\n", target, target);
 	printf("$(deps_%s):\n", target);
 }
···
 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];
···
 	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;

scripts/gen_ksymdeps.sh

···
+#!/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):"