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linux-next/include/linux/export.h
Jessica Yu ce2b617ce8 export.h: reduce __ksymtab_strings string duplication by using "MS" section flags 
Commit c3a6cf19e6 ("export: avoid code duplication in
include/linux/export.h") refactors export.h quite nicely, but introduces
a slight increase in memory usage due to using the empty string ""
instead of NULL to indicate that an exported symbol has no namespace. As
mentioned in that commit, this meant an increase of 1 byte per exported
symbol without a namespace. For example, if a kernel configuration has
about 10k exported symbols, this would mean that the size of
__ksymtab_strings would increase by roughly 10kB.

We can alleviate this situation by utilizing the SHF_MERGE and
SHF_STRING section flags. SHF_MERGE|SHF_STRING indicate to the linker
that the data in the section are null-terminated strings that can be
merged to eliminate duplication. More specifically, from the binutils
documentation - "for sections with both M and S, a string which is a
suffix of a larger string is considered a duplicate. Thus "def" will be
merged with "abcdef"; A reference to the first "def" will be changed to
a reference to "abcdef"+3". Thus, all the empty strings would be merged
as well as any strings that can be merged according to the cited method
above. For example, "memset" and "__memset" would be merged to just
"__memset" in __ksymtab_strings.

As of v5.4-rc5, the following statistics were gathered with x86
defconfig with approximately 10.7k exported symbols.

Size of __ksymtab_strings in vmlinux:
-------------------------------------
v5.4-rc5: 213834 bytes
v5.4-rc5 with commit c3a6cf19e6: 224455 bytes
v5.4-rc5 with this patch: 205759 bytes

So, we already see memory savings of ~8kB compared to vanilla -rc5 and
savings of nearly 18.7kB compared to -rc5 with commit c3a6cf19e6 on top.

Unfortunately, as of this writing, strings will not get deduplicated for
kernel modules, as ld does not do the deduplication for
SHF_MERGE|SHF_STRINGS sections for relocatable files (ld -r), which
kernel modules are. A patch for ld is currently being worked on to
hopefully allow for string deduplication in relocatable files in the
future.

Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Jessica Yu <jeyu@kernel.org>
2019-12-16 10:35:33 +01:00

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/* SPDX-License-Identifier: GPL-2.0-only */
#ifndef _LINUX_EXPORT_H
#define _LINUX_EXPORT_H
/*
* Export symbols from the kernel to modules. Forked from module.h
* to reduce the amount of pointless cruft we feed to gcc when only
* exporting a simple symbol or two.
*
* Try not to add #includes here. It slows compilation and makes kernel
* hackers place grumpy comments in header files.
*/
#ifndef __ASSEMBLY__
#ifdef MODULE
extern struct module __this_module;
#define THIS_MODULE (&__this_module)
#else
#define THIS_MODULE ((struct module *)0)
#endif
#ifdef CONFIG_MODVERSIONS
/* Mark the CRC weak since genksyms apparently decides not to
* generate a checksums for some symbols */
#if defined(CONFIG_MODULE_REL_CRCS)
#define __CRC_SYMBOL(sym, sec) \
asm(" .section \"___kcrctab" sec "+" #sym "\", \"a\" \n" \
" .weak __crc_" #sym " \n" \
" .long __crc_" #sym " - . \n" \
" .previous \n")
#else
#define __CRC_SYMBOL(sym, sec) \
asm(" .section \"___kcrctab" sec "+" #sym "\", \"a\" \n" \
" .weak __crc_" #sym " \n" \
" .long __crc_" #sym " \n" \
" .previous \n")
#endif
#else
#define __CRC_SYMBOL(sym, sec)
#endif
#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;
};
#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;
};
#endif
#ifdef __GENKSYMS__
#define ___EXPORT_SYMBOL(sym, sec, ns) __GENKSYMS_EXPORT_SYMBOL(sym)
#else
/*
* For every exported symbol, do the following:
*
* - If applicable, place a CRC entry in the __kcrctab section.
* - 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[]; \
__CRC_SYMBOL(sym, sec); \
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)
#endif
#if !defined(CONFIG_MODULES) || defined(__DISABLE_EXPORTS)
/*
* Allow symbol exports to be disabled completely so that C code may
* be reused in other execution contexts such as the UEFI stub or the
* decompressor.
*/
#define __EXPORT_SYMBOL(sym, sec, ns)
#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, ns) \
__ksym_marker(sym); \
__cond_export_sym(sym, sec, ns, __is_defined(__KSYM_##sym))
#define __cond_export_sym(sym, sec, ns, conf) \
___cond_export_sym(sym, sec, ns, conf)
#define ___cond_export_sym(sym, sec, ns, enabled) \
__cond_export_sym_##enabled(sym, sec, ns)
#define __cond_export_sym_1(sym, sec, ns) ___EXPORT_SYMBOL(sym, sec, ns)
#define __cond_export_sym_0(sym, sec, ns) /* nothing */
#else
#define __EXPORT_SYMBOL(sym, sec, ns) ___EXPORT_SYMBOL(sym, sec, ns)
#endif /* CONFIG_MODULES */
#ifdef DEFAULT_SYMBOL_NAMESPACE
#include <linux/stringify.h>
#define _EXPORT_SYMBOL(sym, sec) __EXPORT_SYMBOL(sym, sec, __stringify(DEFAULT_SYMBOL_NAMESPACE))
#else
#define _EXPORT_SYMBOL(sym, sec) __EXPORT_SYMBOL(sym, sec, "")
#endif
#define EXPORT_SYMBOL(sym) _EXPORT_SYMBOL(sym, "")
#define EXPORT_SYMBOL_GPL(sym) _EXPORT_SYMBOL(sym, "_gpl")
#define EXPORT_SYMBOL_GPL_FUTURE(sym) _EXPORT_SYMBOL(sym, "_gpl_future")
#define EXPORT_SYMBOL_NS(sym, ns) __EXPORT_SYMBOL(sym, "", #ns)
#define EXPORT_SYMBOL_NS_GPL(sym, ns) __EXPORT_SYMBOL(sym, "_gpl", #ns)
#ifdef CONFIG_UNUSED_SYMBOLS
#define EXPORT_UNUSED_SYMBOL(sym) _EXPORT_SYMBOL(sym, "_unused")
#define EXPORT_UNUSED_SYMBOL_GPL(sym) _EXPORT_SYMBOL(sym, "_unused_gpl")
#else
#define EXPORT_UNUSED_SYMBOL(sym)
#define EXPORT_UNUSED_SYMBOL_GPL(sym)
#endif
#endif /* !__ASSEMBLY__ */
#endif /* _LINUX_EXPORT_H */
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