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[build] Formalise mechanism for accessing absolute symbols

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In a position-dependent executable, where all addresses are fixed
at link time, we can use the standard technique as documented by
GNU ld to get the value of an absolute symbol, e.g.:

    extern char _my_symbol[];

    printf ( "Absolute symbol value is %x\n", ( ( int ) _my_symbol ) );

This technique may not work in a position-independent executable.
When dynamic relocations are applied, the runtime addresses will no
longer be equal to the link-time addresses.  If the code to obtain the
address of _my_symbol uses PC-relative addressing, then it will
calculate the runtime "address" of the absolute symbol, which will no
longer be equal the the link-time "address" (i.e. the correct value)
of the absolute symbol.

Define macros ABS_SYMBOL(), ABS_VALUE_INIT(), and ABS_VALUE() that
provide access to the correct values of absolute symbols even in
position-independent code, and use these macros wherever absolute
symbols are accessed.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown
2025-05-09 14:25:59 +01:00
parent 1d58d928fe
commit 134d76379e
9 changed files with 112 additions and 20 deletions
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8
src/arch/x86/interface/pxe/pxe_call.c
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@@ -55,12 +55,12 @@ extern void pxe_int_1a ( void );
static int int_1a_hooked = 0;
/** Real-mode code segment size */
extern char _text16_memsz[];
#define _text16_memsz ( ( size_t ) _text16_memsz )
extern size_t ABS_SYMBOL ( _text16_memsz );
#define _text16_memsz ABS_VALUE ( _text16_memsz )
/** Real-mode data segment size */
extern char _data16_memsz[];
#define _data16_memsz ( ( size_t ) _data16_memsz )
extern size_t ABS_SYMBOL (_data16_memsz );
#define _data16_memsz ABS_VALUE ( _data16_memsz )
/** PXENV_UNDI_TRANSMIT API call profiler */
static struct profiler pxe_api_tx_profiler __profiler =