Messages in this thread | | | Date | Fri, 27 Oct 2023 14:29:02 +1000 | From | Greg Ungerer <> | Subject | Re: [PATCH v5 16/21] kbuild: generate KSYMTAB entries by modpost |
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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 ...
This is still broken all the way through to the current 6.6-rc7, though the error messages are slightly better:
ERROR: modpost: vmlinux: local symbol 'system_state' was exported ERROR: modpost: vmlinux: local symbol 'static_key_initialized' was exported ERROR: modpost: vmlinux: local symbol 'reset_devices' was exported ...
I tried a bunch of different binutils and gcc versions (binutils-2.251 through 2.40 and gcc versions 8.3.0 through 12.3.0). If I compile with an m68k-linux targeted toolchain then it works - no modpost processing problems.
nm reports the same information for symbols in both cases, eg:
$ m68k-uclinux-nm vmlinux.o | grep system_state 00000000 r __export_symbol_system_state 00000008 B system_state 0000000c d __UNIQUE_ID___addressable_system_state320
$ m68k-linux-nm vmlinux.o | grep system_state 00000000 r __export_symbol_system_state 00000008 B system_state 0000000c d __UNIQUE_ID___addressable_system_state320
Tracing in scripts/mod/modpost.c I see that for this symbol example ("system_state") that ELF_ST_BIND(sym->st_info) is 0x0 for the m68k-uclinux toolchain case, so STB_LOCAL, whereas for the m68k-linux case it is 0x1, so STB_GLOBAL.
Any idea what is going on here?
Regards Greg
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