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[linker] Expand and correct symbol requirement macros
REQUIRE_SYMBOL() formerly used a formulation of symbol requirement
that would allow a link to succeed despite lacking a required symbol,
because it did not introduce any relocations. Fix by renaming it to
REQUEST_SYMBOL() (since the soft-requirement behavior can be useful)
and add a REQUIRE_SYMBOL() that truly requires.

Add EXPORT_SYMBOL() and IMPORT_SYMBOL() for REQUEST_SYMBOL()-like
behavior that allows one to make use of the symbol, by combining a
weak external on the symbol itself with a REQUEST_SYMBOL() of a second
symbol.

Signed-off-by: Marty Connor <mdc@etherboot.org>
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rwcr authored and Marty Connor committed Nov 21, 2009
1 parent 7a5aaec commit 572e617
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Showing 4 changed files with 98 additions and 10 deletions.
1 change: 1 addition & 0 deletions src/arch/i386/scripts/i386-kir.lds
Expand Up @@ -130,6 +130,7 @@ SECTIONS {
/DISCARD/ : {
*(.comment)
*(.note)
*(.discard)
}

/*
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1 change: 1 addition & 0 deletions src/arch/i386/scripts/i386.lds
Expand Up @@ -152,6 +152,7 @@ SECTIONS {
*(.eh_frame.*)
*(.rel)
*(.rel.*)
*(.discard)
}

/*
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1 change: 1 addition & 0 deletions src/arch/x86/scripts/efi.lds
Expand Up @@ -101,5 +101,6 @@ SECTIONS {
*(.eh_frame.*)
*(.rel)
*(.rel.*)
*(.discard)
}
}
105 changes: 95 additions & 10 deletions src/include/compiler.h
Expand Up @@ -37,6 +37,21 @@
#endif
#endif /* ASSEMBLY */

#undef _S1
#undef _S2
#undef _C1
#undef _C2

/** Concatenate non-expanded arguments */
#define _C1( x, y ) x ## y
/** Concatenate expanded arguments */
#define _C2( x, y ) _C1 ( x, y )

/** Stringify non-expanded argument */
#define _S1( x ) #x
/** Stringify expanded argument */
#define _S2( x ) _S1 ( x )

/**
* @defgroup symmacros Macros to provide or require explicit symbols
* @{
Expand All @@ -52,30 +67,97 @@
char _sym[0]
#endif /* ASSEMBLY */

/** Require a symbol within this object file */
/** Require a symbol within this object file
*
* The symbol is referenced by a relocation in a discarded section, so
* if it is not available at link time the link will fail.
*/
#ifdef ASSEMBLY
#define REQUIRE_SYMBOL( _sym ) \
.equ __need_ # _sym, _sym
.section ".discard", "a", @progbits ; \
.extern _sym ; \
.long _sym ; \
.previous
#else /* ASSEMBLY */
#define REQUIRE_SYMBOL( _sym ) \
extern char _sym; \
static char * _C2 ( _C2 ( __require_, _sym ), _C2 ( _, __LINE__ ) ) \
__attribute__ (( section ( ".discard" ), used )) \
= &_sym
#endif

/** Request that a symbol be available at runtime
*
* The requested symbol is entered as undefined into the symbol table
* for this object, so the linker will pull in other object files as
* necessary to satisfy the reference. However, the undefined symbol
* is not referenced in any relocations, so the link can still succeed
* if no file contains it.
*
* A symbol passed to this macro may not be referenced anywhere
* else in the file. If you want to do that, see IMPORT_SYMBOL().
*/
#ifdef ASSEMBLY
#define REQUEST_SYMBOL( _sym ) \
.equ __need_ ## _sym, _sym
#else /* ASSEMBLY */
#define REQUEST_SYMBOL( _sym ) \
__asm__ ( ".equ\t__need_" #_sym ", " #_sym )
#endif /* ASSEMBLY */

/** Set up a symbol to be usable in another file by IMPORT_SYMBOL()
*
* The symbol must already be marked as global.
*/
#define EXPORT_SYMBOL( _sym ) PROVIDE_SYMBOL ( __export_ ## _sym )

/** Make a symbol usable to this file if available at link time
*
* If no file passed to the linker contains the symbol, it will have
* @c NULL value to future uses. Keep in mind that the symbol value is
* really the @e address of a variable or function; see the code
* snippet below.
*
* In C using IMPORT_SYMBOL, you must specify the declaration as the
* second argument, for instance
*
* @code
* IMPORT_SYMBOL ( my_func, int my_func ( int arg ) );
* IMPORT_SYMBOL ( my_var, int my_var );
*
* void use_imports ( void ) {
* if ( my_func && &my_var )
* my_var = my_func ( my_var );
* }
* @endcode
*
* GCC considers a weak declaration to override a strong one no matter
* which comes first, so it is safe to include a header file declaring
* the imported symbol normally, but providing the declaration to
* IMPORT_SYMBOL is still required.
*
* If no EXPORT_SYMBOL declaration exists for the imported symbol in
* another file, the behavior will be most likely be identical to that
* for an unavailable symbol.
*/
#ifdef ASSEMBLY
#define IMPORT_SYMBOL( _sym ) \
REQUEST_SYMBOL ( __export_ ## _sym ) ; \
.weak _sym
#else /* ASSEMBLY */
#define IMPORT_SYMBOL( _sym, _decl ) \
REQUEST_SYMBOL ( __export_ ## _sym ) ; \
extern _decl __attribute__ (( weak ))
#endif

/** @} */

/**
* @defgroup objmacros Macros to provide or require explicit objects
* @{
*/

/* Not quite sure why cpp requires two levels of macro call in order
* to actually expand OBJECT...
*/
#undef _H1
#define _H1( x, y ) x ## y
#undef _H2
#define _H2( x, y ) _H1 ( x, y )
#define PREFIX_OBJECT( _prefix ) _H2 ( _prefix, OBJECT )
#define PREFIX_OBJECT( _prefix ) _C2 ( _prefix, OBJECT )
#define OBJECT_SYMBOL PREFIX_OBJECT ( obj_ )

/** Always provide the symbol for the current object (defined by -DOBJECT) */
Expand All @@ -84,6 +166,9 @@ PROVIDE_SYMBOL ( OBJECT_SYMBOL );
/** Explicitly require another object */
#define REQUIRE_OBJECT( _obj ) REQUIRE_SYMBOL ( obj_ ## _obj )

/** Pull in another object if it exists */
#define REQUEST_OBJECT( _obj ) REQUEST_SYMBOL ( obj_ ## _obj )

/** @} */

/** Select file identifier for errno.h (if used) */
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