The ELF specification contains enough features to allow position-independent code (PIC) to be written, which makes ELF shared libraries very flexible. However, it also means Yasm has to be able to generate a variety of strange relocation types in ELF object files, if it is to be an assembler which can write PIC.
Since ELF does not support segment-base references, the
WRT operator is not used
for its normal purpose; therefore Yasm’s
format makes use of
WRT for a different purpose, namely the
PIC-specific relocation types.
elf32 defines five special symbols which you can use as
the right-hand side of the
WRT operator to obtain PIC
relocation types. They are
..sym. Their functions are summarized
wrt ..gotpcwill end up giving the distance from the beginning of the current section to the global offset table. (((
_GLOBAL_OFFSET_TABLE_)) is the standard symbol name used to refer to the GOT.) So you would then need to add
$$to the result to get the real address of the GOT.
wrt ..gotoffwill give the distance from the beginning of the GOT to the specified location, so that adding on the address of the GOT would give the real address of the location you wanted.
wrt ..gotcauses the linker to build an entry in the GOT containing the address of the symbol, and the reference gives the distance from the beginning of the GOT to the entry; so you can add on the address of the GOT, load from the resulting address, and end up with the address of the symbol.
wrt ..pltcauses the linker to build a procedure linkage table entry for the symbol, and the reference gives the address of the PLT entry. You can only use this in contexts which would generate a PC-relative relocation normally (i.e. as the destination for
JMP), since ELF contains no relocation type to refer to PLT entries absolutely.
wrt ..symcauses Yasm to write an ordinary relocation, but instead of making the relocation relative to the start of the section and then adding on the offset to the symbol, it will write a relocation record aimed directly at the symbol in question. The distinction is a necessary one due to a peculiarity of the dynamic linker.