• Masahiro Yamada's avatar
    kbuild: generate KSYMTAB entries by modpost · ddb5cdba
    Masahiro Yamada authored
    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>
    ddb5cdba
export.h 369 Bytes