Chapter 11. elfx32: ELF 32-bit Object Files for 64-bit Processors

Table of Contents

11.1. elfx32 Special Symbols and WRT

The elfx32 object format is the 32-bit version of the Executable and Linkable Object Format for 64-bit execution. Similar to elf64, it allows for use of 64-bit registers and instructions, but like elf32, limits pointers to 32 bits in size. As it shares many similarities with elf32 and elf64, only differences between these formats and elfx32 will be described in this chapter. For details on elf32, see Chapter 9; for details on elf64, see Chapter 10. Operating system support for elfx32 is currently less common than for elf64.

Yasm defaults to BITS 64 mode when outputting to the elfx32 object format.

elfx32 supports the same debug formats, sections, section attributes, and directives as elf32 and elf64. See Section 9.2 for more details on section attributes, and Section 9.3 for details on the additional directives ELF provides.

11.1. elfx32 Special Symbols and WRT

Due to the availability of RIP-relative addressing, elfx32 shared library handling and position-independent code is essentially identical to elf64.

As in elf64, elfx32 defines four special symbols which you can use as the right-hand side of the WRT operator to obtain PIC relocation types. They are ..gotpcrel,, ..plt and ..sym and have the same functionality as they do in elf64. Their functions are summarized here:

While RIP-relative addressing allows you to encode an instruction pointer relative data reference to foo with [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]. As in elf64, 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.
As in elf64, referring to an external or global symbol using wrt causes 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.
As in elf64, referring to a procedure name using wrt ..plt causes 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 CALL or JMP), since ELF contains no relocation type to refer to PLT entries absolutely.
As in elf64, referring to a symbol name using wrt ..sym causes 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.