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https://github.com/ipxe/ipxe
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[build] Fix the REQUIRE_SYMBOL mechanism
At some point in the past few years, binutils became more aggressive at removing unused symbols. To function as a symbol requirement, a relocation record must now be in a section marked with @progbits and must not be in a section which gets discarded during the link (either via --gc-sections or via /DISCARD/). Update REQUIRE_SYMBOL() to generate relocation records meeting these criteria. To minimise the impact upon the final binary size, we use existing symbols (specified via the REQUIRING_SYMBOL() macro) as the relocation targets where possible. We use R_386_NONE or R_X86_64_NONE relocation types to prevent any actual unwanted relocation taking place. Where no suitable symbol exists for REQUIRING_SYMBOL() (such as in config.c), the macro PROVIDE_REQUIRING_SYMBOL() can be used to generate a one-byte-long symbol to act as the relocation target. If there are versions of binutils for which this approach fails, then the fallback will probably involve killing off REQUEST_SYMBOL(), redefining REQUIRE_SYMBOL() to use the current definition of REQUEST_SYMBOL(), and postprocessing the linked ELF file with something along the lines of "nm -u | wc -l" to check that there are no undefined symbols remaining. Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown
@@ -57,54 +57,101 @@
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* @{
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*/
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/** Provide a symbol within this object file */
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#ifdef ASSEMBLY
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#define PROVIDE_SYMBOL( _sym ) \
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.section ".provided", "a", @nobits ; \
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.hidden _sym ; \
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.globl _sym ; \
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_sym: ; \
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.previous
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#else /* ASSEMBLY */
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#define PROVIDE_SYMBOL( _sym ) \
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char _sym[0] \
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__attribute__ (( section ( ".provided" ) ))
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#endif /* ASSEMBLY */
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/** Require a symbol within this object file
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/**
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* Provide a symbol within this object file
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*
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* The symbol is referenced by a relocation in a discarded section, so
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* if it is not available at link time the link will fail.
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* @v symbol Symbol name
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*/
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#ifdef ASSEMBLY
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#define REQUIRE_SYMBOL( _sym ) \
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.section ".discard", "a", @progbits ; \
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.extern _sym ; \
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.long _sym ; \
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#define PROVIDE_SYMBOL( symbol ) \
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.section ".provided", "a", @nobits ; \
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.hidden symbol ; \
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.globl symbol ; \
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symbol: ; \
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.previous
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#else /* ASSEMBLY */
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#define REQUIRE_SYMBOL( _sym ) \
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extern char _sym; \
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static char * _C2 ( _C2 ( __require_, _sym ), _C2 ( _, __LINE__ ) ) \
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__attribute__ (( section ( ".discard" ), used )) \
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= &_sym
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#else
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#define PROVIDE_SYMBOL( symbol ) \
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char symbol[0] \
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__attribute__ (( section ( ".provided" ) ))
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#endif
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/** Request that a symbol be available at runtime
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/**
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* Request a symbol
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*
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* The requested symbol is entered as undefined into the symbol table
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* for this object, so the linker will pull in other object files as
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* necessary to satisfy the reference. However, the undefined symbol
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* is not referenced in any relocations, so the link can still succeed
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* if no file contains it.
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* @v symbol Symbol name
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*
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* Request a symbol to be included within the link. If the symbol
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* cannot be found, the link will succeed anyway.
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*/
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#ifdef ASSEMBLY
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#define REQUEST_SYMBOL( _sym ) \
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.equ __need_ ## _sym, _sym
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#else /* ASSEMBLY */
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#define REQUEST_SYMBOL( _sym ) \
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__asm__ ( ".equ\t__need_" #_sym ", " #_sym )
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#endif /* ASSEMBLY */
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#define REQUEST_SYMBOL( symbol ) \
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.equ __request_ ## symbol, symbol
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#else
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#define REQUEST_SYMBOL( symbol ) \
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__asm__ ( ".equ __request_" #symbol ", " #symbol )
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#endif
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/**
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* Require a symbol
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*
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* @v symbol Symbol name
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*
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* Require a symbol to be included within the link. If the symbol
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* cannot be found, the link will fail.
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*
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* To use this macro within a file, you must also specify the file's
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* "requiring symbol" using the REQUIRING_SYMBOL() or
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* PROVIDE_REQUIRING_SYMBOL() macros.
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*/
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#ifdef ASSEMBLY
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#define REQUIRE_SYMBOL( symbol ) \
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.reloc __requiring_symbol__, RELOC_TYPE_NONE, symbol
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#else
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#define REQUIRE_SYMBOL( symbol ) \
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__asm__ ( ".reloc __requiring_symbol__, " \
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_S2 ( RELOC_TYPE_NONE ) ", " #symbol )
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#endif
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/**
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* Specify the file's requiring symbol
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*
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* @v symbol Symbol name
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*
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* REQUIRE_SYMBOL() works by defining a dummy relocation record
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* against a nominated "requiring symbol". The presence of the
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* nominated requiring symbol will drag in all of the symbols
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* specified using REQUIRE_SYMBOL().
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*/
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#ifdef ASSEMBLY
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#define REQUIRING_SYMBOL( symbol ) \
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.equ __requiring_symbol__, symbol
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#else
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#define REQUIRING_SYMBOL( symbol ) \
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__asm__ ( ".equ __requiring_symbol__, " #symbol )
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#endif
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/**
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* Provide a file's requiring symbol
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*
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* If the file contains no symbols that can be used as the requiring
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* symbol, you can provide a dummy one-byte-long symbol using
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* PROVIDE_REQUIRING_SYMBOL().
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*/
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#ifdef ASSEMBLY
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#define PROVIDE_REQUIRING_SYMBOL() \
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.section ".tbl.requiring_symbols", "a", @progbits ; \
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__requiring_symbol__: .byte 0 ; \
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.size __requiring_symbol__, . - __requiring_symbol__ ; \
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.previous
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#else
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#define PROVIDE_REQUIRING_SYMBOL() \
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__asm__ ( ".section \".tbl.requiring_symbols\", " \
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" \"a\", @progbits\n" \
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"__requiring_symbol__:\t.byte 0\n" \
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".size __requiring_symbol__, " \
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" . - __requiring_symbol__\n" \
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".previous" )
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#endif
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/** @} */
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@@ -119,11 +166,29 @@
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/** Always provide the symbol for the current object (defined by -DOBJECT) */
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PROVIDE_SYMBOL ( OBJECT_SYMBOL );
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/** Explicitly require another object */
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#define REQUIRE_OBJECT( _obj ) REQUIRE_SYMBOL ( obj_ ## _obj )
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/**
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* Request an object
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*
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* @v object Object name
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*
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* Request an object to be included within the link. If the object
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* cannot be found, the link will succeed anyway.
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*/
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#define REQUEST_OBJECT( object ) REQUEST_SYMBOL ( obj_ ## object )
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/** Pull in another object if it exists */
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#define REQUEST_OBJECT( _obj ) REQUEST_SYMBOL ( obj_ ## _obj )
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/**
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* Require an object
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*
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* @v object Object name
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*
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* Require an object to be included within the link. If the object
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* cannot be found, the link will fail.
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*
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* To use this macro within a file, you must also specify the file's
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* "requiring symbol" using the REQUIRING_SYMBOL() or
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* PROVIDE_REQUIRING_SYMBOL() macros.
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*/
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#define REQUIRE_OBJECT( object ) REQUIRE_SYMBOL ( obj_ ## object )
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/** @} */
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@@ -685,8 +750,8 @@ int __debug_disable;
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/** @} */
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/* This file itself is under GPLv2-or-later */
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FILE_LICENCE ( GPL2_OR_LATER );
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/* This file itself is under GPLv2+/UBDL */
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FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
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#include <bits/compiler.h>
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@@ -276,6 +276,7 @@ static inline void * legacy_isa_get_drvdata ( void *hwdev ) {
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_name ## _isa_legacy_remove ( struct isa_device *isa ) { \
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return legacy_remove ( isa, legacy_isa_get_drvdata, \
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_name ## _disable ); \
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}
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} \
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PROVIDE_REQUIRING_SYMBOL()
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#endif /* NIC_H */
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