Messages in this thread | | | Date | Fri, 27 Oct 2023 22:32:32 +1000 | Subject | Re: [PATCH v5 16/21] kbuild: generate KSYMTAB entries by modpost | From | Greg Ungerer <> |
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On 27/10/23 19:56, Masahiro Yamada wrote: > On Fri, Oct 27, 2023 at 1:30 PM Greg Ungerer <gregungerer@westnet.com.au> wrote: >> >> Hi Masahiro, >> >> On 15/5/23 01:27, Masahiro Yamada wrote: >>> Commit 7b4537199a4a ("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. >>> >>> This commit applies a similar approach to the entire data structure of >>> EXPORT_SYMBOL() for further cleanups. The EXPORT_SYMBOL() compilation >>> is split into two stages. >>> >>> When a source file is compiled, EXPORT_SYMBOL() is 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 "" >>> .balign 8 >>> .quad foo >>> .previous >>> >>> .section ".export_symbol","a" >>> __export_symbol_gpl_bar: >>> .asciz "BAR_NAMESPACE" >>> .balign 8 >>> .quad bar >>> .previous >>> >>> They are just 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 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 22823ab419d8 ("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 8651ec01daed ("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> >> >> This breaks building kernels with an m68k-uclinux-gcc toolchain that have >> modules configured. Before this change they built and ran fine. >> They build and run fine if CONFIG_MODULES is not set. >> >> A few hundred errors like this spew out: >> >> scripts/mod/modpost -o Module.symvers -T modules.order vmlinux.o >> ERROR: modpost: vmlinux: .export_symbol section references '', but it does not seem to be an export symbol >> ERROR: modpost: vmlinux: .export_symbol section references '', but it does not seem to be an export symbol >> ERROR: modpost: vmlinux: .export_symbol section references '', but it does not seem to be an export symbol >> ... > > > > Where can I download a prebuilt m68k-uclinux-gcc?
https://sourceforge.net/projects/uclinux/files/Tools/m68k-uclinux-tools-20231026.tar.xz/download
Regards Greg
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