Commit ddb5cdba authored by Masahiro Yamada's avatar Masahiro Yamada
Browse files

kbuild: generate KSYMTAB entries by modpost 



Commit 7b453719 ("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 22823ab4 ("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 8651ec01 ("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: default avatarMasahiro Yamada <masahiroy@kernel.org>
Reviewed-by: default avatarNick Desaulniers <ndesaulniers@google.com>
parent 94d6cb68

arch/ia64/include/asm/Kbuild

+1 −0
Original line number Diff line number Diff line 
# 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

arch/ia64/include/asm/export.h

deleted100644 → 0
+0 −3
Original line number Diff line number Diff line 
/* EXPORT_DATA_SYMBOL != EXPORT_SYMBOL here */

#define KSYM_FUNC(name) @fptr(name)

#include <asm-generic/export.h>

include/asm-generic/export.h

+5 −79
Original line number Diff line number Diff line
@@ -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

+1 −0
Original line number Diff line number Diff line
@@ -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

+49 −0
Original line number Diff line number Diff line
@@ -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" \