kbuild: generate KSYMTAB entries by modpost

Commit 7b45371 ("kbuild: link symbol CRCs at final link, removing
CONFIG_MODULE_REL_CRCS") made modpost output CRCs in the same way
whether the EXPORT_SYMBOL() is placed in *.c or *.S.

For further cleanups, this commit applies a similar approach to the
entire data structure of EXPORT_SYMBOL().

The EXPORT_SYMBOL() compilation is split into two stages.

When a source file is compiled, EXPORT_SYMBOL() will be converted into
a dummy symbol in the .export_symbol section.

For example,

    EXPORT_SYMBOL(foo);
    EXPORT_SYMBOL_NS_GPL(bar, BAR_NAMESPACE);

will be encoded into the following assembly code:

    .section ".export_symbol","a"
    __export_symbol_foo:
            .asciz ""                      /* license */
            .asciz ""                      /* name space */
            .balign 8
            .quad foo                      /* symbol reference */
    .previous

    .section ".export_symbol","a"
    __export_symbol_bar:
            .asciz "GPL"                   /* license */
            .asciz "BAR_NAMESPACE"         /* name space */
            .balign 8
            .quad bar                      /* symbol reference */
    .previous

They are mere markers to tell modpost the name, license, and namespace
of the symbols. They will be dropped from the final vmlinux and modules
because the *(.export_symbol) will go into /DISCARD/ in the linker script.

Then, modpost extracts all the information about EXPORT_SYMBOL() from the
.export_symbol section, and generates the final C code:

    KSYMTAB_FUNC(foo, "", "");
    KSYMTAB_FUNC(bar, "_gpl", "BAR_NAMESPACE");

KSYMTAB_FUNC() (or KSYMTAB_DATA() if it is data) is expanded to struct
kernel_symbol that will be linked to the vmlinux or a module.

With this change, EXPORT_SYMBOL() works in the same way for *.c and *.S
files, providing the following benefits.

[1] Deprecate EXPORT_DATA_SYMBOL()

In the old days, EXPORT_SYMBOL() was only available in C files. To export
a symbol in *.S, EXPORT_SYMBOL() was placed in a separate *.c file.
arch/arm/kernel/armksyms.c is one example written in the classic manner.

Commit 22823ab ("EXPORT_SYMBOL() for asm") removed this limitation.
Since then, EXPORT_SYMBOL() can be placed close to the symbol definition
in *.S files. It was a nice improvement.

However, as that commit mentioned, you need to use EXPORT_DATA_SYMBOL()
for data objects on some architectures.

In the new approach, modpost checks symbol's type (STT_FUNC or not),
and outputs KSYMTAB_FUNC() or KSYMTAB_DATA() accordingly.

There are only two users of EXPORT_DATA_SYMBOL:

  EXPORT_DATA_SYMBOL_GPL(empty_zero_page)    (arch/ia64/kernel/head.S)
  EXPORT_DATA_SYMBOL(ia64_ivt)               (arch/ia64/kernel/ivt.S)

They are transformed as follows and output into .vmlinux.export.c

  KSYMTAB_DATA(empty_zero_page, "_gpl", "");
  KSYMTAB_DATA(ia64_ivt, "", "");

The other EXPORT_SYMBOL users in ia64 assembly are output as
KSYMTAB_FUNC().

EXPORT_DATA_SYMBOL() is now deprecated.

[2] merge <linux/export.h> and <asm-generic/export.h>

There are two similar header implementations:

  include/linux/export.h        for .c files
  include/asm-generic/export.h  for .S files

Ideally, the functionality should be consistent between them, but they
tend to diverge.

Commit 8651ec0 ("module: add support for symbol namespaces.") did
not support the namespace for *.S files.

This commit shifts the essential implementation part to C, which supports
EXPORT_SYMBOL_NS() for *.S files.

<asm/export.h> and <asm-generic/export.h> will remain as a wrapper of
<linux/export.h> for a while.

They will be removed after #include <asm/export.h> directives are all
replaced with #include <linux/export.h>.

[3] Implement CONFIG_TRIM_UNUSED_KSYMS in one-pass algorithm (by a later commit)

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.

We can do this better now; modpost can selectively emit KSYMTAB entries
that are really used by modules.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

arch/ia64/include/asm/Kbuild

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 generated-y += syscall_table.h
 generic-y += agp.h
+generic-y += export.h
 generic-y += kvm_para.h
 generic-y += mcs_spinlock.h
 generic-y += vtime.h

include/asm-generic/export.h

@@ -3,86 +3,12 @@
 #define __ASM_GENERIC_EXPORT_H
 
 /*
- * This comment block is used by fixdep. Please do not remove.
- *
- * When CONFIG_MODVERSIONS is changed from n to y, all source files having
- * EXPORT_SYMBOL variants must be re-compiled because genksyms is run as a
- * side effect of the *.o build rule.
+ * <asm/export.h> and <asm-generic/export.h> are deprecated.
+ * Please include <linux/export.h> directly.
  */
+#include <linux/export.h>
 
-#ifndef KSYM_FUNC
-#define KSYM_FUNC(x) x
-#endif
-#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
-#define KSYM_ALIGN 4
-#elif defined(CONFIG_64BIT)
-#define KSYM_ALIGN 8
-#else
-#define KSYM_ALIGN 4
-#endif
-
-.macro __put, val, name
-#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
-	.long	\val - ., \name - ., 0
-#elif defined(CONFIG_64BIT)
-	.quad	\val, \name, 0
-#else
-	.long	\val, \name, 0
-#endif
-.endm
-
-/*
- * note on .section use: we specify progbits since usage of the "M" (SHF_MERGE)
- * section flag requires it. Use '%progbits' instead of '@progbits' since the
- * former apparently works on all arches according to the binutils source.
- */
-
-.macro ___EXPORT_SYMBOL name,val,sec
-#if defined(CONFIG_MODULES) && !defined(__DISABLE_EXPORTS)
-	.section ___ksymtab\sec+\name,"a"
-	.balign KSYM_ALIGN
-__ksymtab_\name:
-	__put \val, __kstrtab_\name
-	.previous
-	.section __ksymtab_strings,"aMS",%progbits,1
-__kstrtab_\name:
-	.asciz "\name"
-	.previous
-#endif
-.endm
-
-#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)
-#define ___cond_export_sym(sym, val, sec, enabled)		\
-	__cond_export_sym_##enabled(sym, val, sec)
-#define __cond_export_sym_1(sym, val, sec) ___EXPORT_SYMBOL sym, val, sec
-#define __cond_export_sym_0(sym, val, sec) /* nothing */
-
-#else
-#define __EXPORT_SYMBOL(sym, val, sec) ___EXPORT_SYMBOL sym, val, sec
-#endif
-
-#define EXPORT_SYMBOL(name)					\
-	__EXPORT_SYMBOL(name, KSYM_FUNC(name),)
-#define EXPORT_SYMBOL_GPL(name) 				\
-	__EXPORT_SYMBOL(name, KSYM_FUNC(name), _gpl)
-#define EXPORT_DATA_SYMBOL(name)				\
-	__EXPORT_SYMBOL(name, name,)
-#define EXPORT_DATA_SYMBOL_GPL(name)				\
-	__EXPORT_SYMBOL(name, name,_gpl)
+#define EXPORT_DATA_SYMBOL(name)	EXPORT_SYMBOL(name)
+#define EXPORT_DATA_SYMBOL_GPL(name)	EXPORT_SYMBOL_GPL(name)
 
 #endif

include/asm-generic/vmlinux.lds.h

@@ -1006,6 +1006,7 @@
 	PATCHABLE_DISCARDS						\
 	*(.discard)							\
 	*(.discard.*)							\
+	*(.export_symbol)						\
 	*(.modinfo)							\
 	/* ld.bfd warns about .gnu.version* even when not emitted */	\
 	*(.gnu.version*)						\

include/linux/export-internal.h

@@ -10,6 +10,55 @@
 #include <linux/compiler.h>
 #include <linux/types.h>
 
+#if defined(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS)
+/*
+ * relative reference: this reduces the size by half on 64-bit architectures,
+ * and eliminates the need for absolute relocations that require runtime
+ * processing on relocatable kernels.
+ */
+#define __KSYM_REF(sym)		".long " #sym "- ."
+#elif defined(CONFIG_64BIT)
+#define __KSYM_REF(sym)		".quad " #sym
+#else
+#define __KSYM_REF(sym)		".long " #sym
+#endif
+
+/*
+ * For every exported symbol, do the following:
+ *
+ * - Put the name of the symbol and namespace (empty string "" for none) in
+ *   __ksymtab_strings.
+ * - Place a struct kernel_symbol entry in the __ksymtab section.
+ *
+ * Note on .section use: we specify progbits since usage of the "M" (SHF_MERGE)
+ * section flag requires it. Use '%progbits' instead of '@progbits' since the
+ * former apparently works on all arches according to the binutils source.
+ */
+#define __KSYMTAB(name, sym, sec, ns)						\
+	asm("	.section \"__ksymtab_strings\",\"aMS\",%progbits,1"	"\n"	\
+	    "__kstrtab_" #name ":"					"\n"	\
+	    "	.asciz \"" #name "\""					"\n"	\
+	    "__kstrtabns_" #name ":"					"\n"	\
+	    "	.asciz \"" ns "\""					"\n"	\
+	    "	.previous"						"\n"	\
+	    "	.section \"___ksymtab" sec "+" #name "\", \"a\""	"\n"	\
+	    "	.balign	4"						"\n"	\
+	    "__ksymtab_" #name ":"					"\n"	\
+		__KSYM_REF(sym)						"\n"	\
+		__KSYM_REF(__kstrtab_ ##name)				"\n"	\
+		__KSYM_REF(__kstrtabns_ ##name)				"\n"	\
+	    "	.previous"						"\n"	\
+	)
+
+#ifdef CONFIG_IA64
+#define KSYM_FUNC(name)		@fptr(name)
+#else
+#define KSYM_FUNC(name)		name
+#endif
+
+#define KSYMTAB_FUNC(name, sec, ns)	__KSYMTAB(name, KSYM_FUNC(name), sec, ns)
+#define KSYMTAB_DATA(name, sec, ns)	__KSYMTAB(name, name, sec, ns)
+
 #define SYMBOL_CRC(sym, crc, sec)   \
 	asm(".section \"___kcrctab" sec "+" #sym "\",\"a\""	"\n" \
 	    "__crc_" #sym ":"					"\n" \

include/linux/export.h

@@ -2,6 +2,8 @@
 #ifndef _LINUX_EXPORT_H
 #define _LINUX_EXPORT_H
 
+#include <linux/compiler.h>
+#include <linux/linkage.h>
 #include <linux/stringify.h>
 
 /*
@@ -28,72 +30,41 @@ extern struct module __this_module;
 #else
 #define THIS_MODULE ((struct module *)0)
 #endif
+#endif /* __ASSEMBLY__ */
 
-#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
-#include <linux/compiler.h>
-/*
- * Emit the ksymtab entry as a pair of relative references: this reduces
- * the size by half on 64-bit architectures, and eliminates the need for
- * absolute relocations that require runtime processing on relocatable
- * kernels.
- */
-#define __KSYMTAB_ENTRY(sym, sec)					\
-	__ADDRESSABLE(sym)						\
-	asm("	.section \"___ksymtab" sec "+" #sym "\", \"a\"	\n"	\
-	    "	.balign	4					\n"	\
-	    "__ksymtab_" #sym ":				\n"	\
-	    "	.long	" #sym "- .				\n"	\
-	    "	.long	__kstrtab_" #sym "- .			\n"	\
-	    "	.long	__kstrtabns_" #sym "- .			\n"	\
-	    "	.previous					\n")
-
-struct kernel_symbol {
-	int value_offset;
-	int name_offset;
-	int namespace_offset;
-};
+#ifdef CONFIG_64BIT
+#define __EXPORT_SYMBOL_REF(sym)			\
+	.balign 8				ASM_NL	\
+	.quad sym
 #else
-#define __KSYMTAB_ENTRY(sym, sec)					\
-	static const struct kernel_symbol __ksymtab_##sym		\
-	__attribute__((section("___ksymtab" sec "+" #sym), used))	\
-	__aligned(sizeof(void *))					\
-	= { (unsigned long)&sym, __kstrtab_##sym, __kstrtabns_##sym }
-
-struct kernel_symbol {
-	unsigned long value;
-	const char *name;
-	const char *namespace;
-};
+#define __EXPORT_SYMBOL_REF(sym)			\
+	.balign 4				ASM_NL	\
+	.long sym
 #endif
 
+#define ____EXPORT_SYMBOL(sym, license, ns)		\
+	.section ".export_symbol","a"		ASM_NL	\
+	__export_symbol_##sym:			ASM_NL	\
+		.asciz license			ASM_NL	\
+		.asciz ns			ASM_NL	\
+		__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
 
-/*
- * For every exported symbol, do the following:
- *
- * - Put the name of the symbol and namespace (empty string "" for none) in
- *   __ksymtab_strings.
- * - Place a struct kernel_symbol entry in the __ksymtab section.
- *
- * note on .section use: we specify progbits since usage of the "M" (SHF_MERGE)
- * section flag requires it. Use '%progbits' instead of '@progbits' since the
- * former apparently works on all arches according to the binutils source.
- */
-#define ___EXPORT_SYMBOL(sym, sec, ns)						\
-	extern typeof(sym) sym;							\
-	extern const char __kstrtab_##sym[];					\
-	extern const char __kstrtabns_##sym[];					\
-	asm("	.section \"__ksymtab_strings\",\"aMS\",%progbits,1	\n"	\
-	    "__kstrtab_" #sym ":					\n"	\
-	    "	.asciz 	\"" #sym "\"					\n"	\
-	    "__kstrtabns_" #sym ":					\n"	\
-	    "	.asciz 	\"" ns "\"					\n"	\
-	    "	.previous						\n");	\
-	__KSYMTAB_ENTRY(sym, sec)
+#define ___EXPORT_SYMBOL(sym, license, ns)			\
+	extern typeof(sym) sym;					\
+	__ADDRESSABLE(sym)					\
+	asm(__stringify(____EXPORT_SYMBOL(sym, license, ns)))
 
 #endif
 
@@ -117,9 +88,21 @@ struct kernel_symbol {
  * from the $(NM) output (see scripts/gen_ksymdeps.sh). These symbols are
  * discarded in the final link stage.
  */
+
+#ifdef __ASSEMBLY__
+
+#define __ksym_marker(sym)					\
+	.section ".discard.ksym","a" ;				\
+__ksym_marker_##sym: ;						\
+	.previous
+
+#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))
@@ -148,10 +131,8 @@ struct kernel_symbol {
 #endif
 
 #define EXPORT_SYMBOL(sym)		_EXPORT_SYMBOL(sym, "")
-#define EXPORT_SYMBOL_GPL(sym)		_EXPORT_SYMBOL(sym, "_gpl")
+#define EXPORT_SYMBOL_GPL(sym)		_EXPORT_SYMBOL(sym, "GPL")
 #define EXPORT_SYMBOL_NS(sym, ns)	__EXPORT_SYMBOL(sym, "", __stringify(ns))
-#define EXPORT_SYMBOL_NS_GPL(sym, ns)	__EXPORT_SYMBOL(sym, "_gpl", __stringify(ns))
-
-#endif /* !__ASSEMBLY__ */
+#define EXPORT_SYMBOL_NS_GPL(sym, ns)	__EXPORT_SYMBOL(sym, "GPL", __stringify(ns))
 
 #endif /* _LINUX_EXPORT_H */

include/linux/pm.h

@@ -389,9 +389,9 @@ const struct dev_pm_ops name = { \
 #endif
 
 #define EXPORT_DEV_PM_OPS(name) _EXPORT_DEV_PM_OPS(name, "", "")
-#define EXPORT_GPL_DEV_PM_OPS(name) _EXPORT_DEV_PM_OPS(name, "_gpl", "")
+#define EXPORT_GPL_DEV_PM_OPS(name) _EXPORT_DEV_PM_OPS(name, "GPL", "")
 #define EXPORT_NS_DEV_PM_OPS(name, ns) _EXPORT_DEV_PM_OPS(name, "", #ns)
-#define EXPORT_NS_GPL_DEV_PM_OPS(name, ns) _EXPORT_DEV_PM_OPS(name, "_gpl", #ns)
+#define EXPORT_NS_GPL_DEV_PM_OPS(name, ns) _EXPORT_DEV_PM_OPS(name, "GPL", #ns)
 
 /*
  * Use this if you want to use the same suspend and resume callbacks for suspend

kernel/module/internal.h

@@ -32,6 +32,18 @@
 /* Maximum number of characters written by module_flags() */
 #define MODULE_FLAGS_BUF_SIZE (TAINT_FLAGS_COUNT + 4)
 
+struct kernel_symbol {
+#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
+	int value_offset;
+	int name_offset;
+	int namespace_offset;
+#else
+	unsigned long value;
+	const char *name;
+	const char *namespace;
+#endif
+};
+
 extern struct mutex module_mutex;
 extern struct list_head modules;
 

scripts/Makefile.build

@@ -163,15 +163,15 @@ quiet_cmd_cc_o_c = CC $(quiet_modtag)  $@
 ifdef CONFIG_MODVERSIONS
 # When module versioning is enabled the following steps are executed:
 # o compile a <file>.o from <file>.c
-# o if <file>.o doesn't contain a __ksymtab version, i.e. does
+# o if <file>.o doesn't contain a __export_symbol_*, i.e. does
 #   not export symbols, it's done.
 # o otherwise, we calculate symbol versions using the good old
 #   genksyms on the preprocessed source and dump them into the .cmd file.
 # o modpost will extract versions from that file and create *.c files that will
 #   be compiled and linked to the kernel and/or modules.
 
 gen_symversions =								\
-	if $(NM) $@ 2>/dev/null | grep -q __ksymtab; then			\
+	if $(NM) $@ 2>/dev/null | grep -q ' __export_symbol_'; then		\
 		$(call cmd_gensymtypes_$(1),$(KBUILD_SYMTYPES),$(@:.o=.symtypes)) \
 			>> $(dot-target).cmd;					\
 	fi
@@ -342,9 +342,7 @@ $(obj)/%.ll: $(src)/%.rs FORCE
 cmd_gensymtypes_S =                                                         \
    { echo "\#include <linux/kernel.h>" ;                                    \
      echo "\#include <asm/asm-prototypes.h>" ;                              \
-    $(CPP) $(a_flags) $< |                                                  \
-     grep "\<___EXPORT_SYMBOL\>" |                                          \
-     sed 's/.*___EXPORT_SYMBOL[[:space:]]*\([a-zA-Z0-9_]*\)[[:space:]]*,.*/EXPORT_SYMBOL(\1);/' ; } | \
+     $(NM) $@ | sed -n 's/.* __export_symbol_\(.*\)/EXPORT_SYMBOL(\1);/p' ; } | \
     $(CPP) -D__GENKSYMS__ $(c_flags) -xc - | $(genksyms)
 
 quiet_cmd_cc_symtypes_S = SYM $(quiet_modtag) $@

scripts/check-local-export

@@ -46,9 +46,9 @@ BEGIN {
 { symbol_types[$3]=$2 }
 
 # append the exported symbol to the array
-($3 ~ /^__ksymtab_/) {
+($3 ~ /^__export_symbol_.*/) {
 	export_symbols[i] = $3
-	sub(/^__ksymtab_/, "", export_symbols[i])
+	sub(/^__export_symbol_/, "", export_symbols[i])
 	i++
 }