Table of Contents
elf64 object format is the 64-bit version of the
Executable and Linkable Object Format. As it shares many similarities with
elf32, only differences between
elf64 will be described in this chapter. For details on
elf32, see Chapter 9.
Yasm defaults to
BITS 64 mode when outputting to the
elf64 object format.
elf64 supports the same debug formats as
elf32, however, the
stabs debug format is
limited to 32-bit addresses, so
dwarf2 (see Chapter 19) is the recommended
elf64 also supports the exact same sections, section
attributes, and directives as
elf32. See Section 9.2 for more
details on section attributes, and Section 9.3 for details on the additional directives
The primary difference
than 64-bit support in general) is the differences in shared library handling and
position-independent code. As
BITS 64 enables the use of
RIP-relative addressing, most variable accesses can be
relative to RIP, allowing easy relocation of the shared library to a different memory
While RIP-relative addressing is available, it does not handle all possible variable
access modes, so special symbols are still required, as in
elf32. And as with
elf64 output format makes use of
WRT for utilizing the PIC-specific relocation types.
elf64 defines four 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 here:
[rel foo], it’s sometimes necessary to encode a RIP-relative reference to a linker-generated symbol pointer for symbol foo; this is done using
wrt ..gotpcrel, e.g.
[rel foo wrt ..gotpcrel]. Unlike in
elf32, this relocation, combined with RIP-relative addressing, makes it possible to load an address from the ((global offset table)) using a single instruction. Note that since RIP-relative references are limited to a signed 32-bit displacement, the GOT size accessible through this method is limited to 2 GB.
elf32, referring to an external or global symbol using
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.
elf32, referring to a procedure name using
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.
elf32, referring to a symbol name using
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.