Position-independent code

In computing, position-independent code (PIC ) or position-independent executable (PIE) is a body of machine code that executes properly regardless of its memory address. PIC is commonly used for shared libraries, so that the same library code can be loaded at a location in each program's address space where it does not overlap with other memory in use by, for example, other shared libraries. PIC was also used on older computer systems that lacked a Memory Management Unit (MMU), so that the operating system could keep applications away from each other even within the single address space of an MMU-less system. Position-independent code can be executed at any memory address without modification. This differs from absolute code, which must be loaded at a specific location to function correctly, and load-time locatable (LTL) code, in which a linker or program loader modifies a program before execution, so it can be run only from a particular memory location. The latter terms are sometimes referred to as position-dependent code. Generating position-independent code is often the default behavior for compilers, but they may place restrictions on the use of some language features, such as disallowing use of absolute addresses (position-independent code has to use relative addressing).…

In computing, position-independent code (PIC ) or position-independent executable (PIE) is a body of machine code that executes properly regardless of its memory address. PIC is commonly used for shared libraries, so that the same library code can be loaded at a location in each program's address space where it does not overlap with other memory in use by, for example, other shared libraries. PIC was also used on older computer systems that lacked a Memory Management Unit (MMU), so that the operating system could keep applications away from each other even within the single address space of an MMU-less system. Position-independent code can be executed at any memory address without modification. This differs from absolute code, which must be loaded at a specific location to function correctly, and load-time locatable (LTL) code, in which a linker or program loader modifies a program before execution, so it can be run only from a particular memory location. The latter terms are sometimes referred to as position-dependent code. Generating position-independent code is often the default behavior for compilers, but they may place restrictions on the use of some language features, such as disallowing use of absolute addresses (position-independent code has to use relative addressing). Instructions that refer directly to specific memory addresses sometimes execute faster, and replacing them with equivalent relative-addressing instructions may result in slightly slower execution, although modern processors make the difference practically negligible.

Position-independent executables

Position-independent executables (PIE) are executable binaries made entirely from position-independent code. While some systems only run PIC executables, there are other reasons they are used. PIE binaries are used in some security-focused Linux distributions to allow PaX or Exec Shield to use address space layout randomization (ASLR) to prevent attackers from knowing where existing executable code is during a security attack using exploits that rely on knowing the offset of the executable code in the binary, such as return-to-libc attacks. (The official Linux kernel since 2.6.12 of 2005 has a weaker ASLR that also works with PIE. It is weak in that randomness is applied to whole ELF file units.) Apple's macOS and iOS fully support PIE executables as of versions 10.7 and 4.3, respectively; a warning is issued when non-PIE iOS executables are submitted for approval to Apple's App Store but there's no hard requirement yet and non-PIE applications are not rejected. OpenBSD has PIE enabled by default on most architectures since OpenBSD 5.3, released on 1 May 2013. Support for PIE in statically linked binaries, such as the executables in /bin and /sbin directories, was added near the end of 2014. openSUSE added PIE as a default in 2015–02. Beginning with Fedora 23, Fedora maintainers decided to build packages with PIE enabled as the default. Ubuntu 17.10 has PIE enabled by default across all architectures. Gentoo's new profiles now support PIE by default. Around July 2017, Debian enabled PIE by default. Android enabled support for PIEs in Jelly Bean and removed non-PIE linker support in Lollipop.

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