Debian Flags for Position-Independent Executables

This article outlines the compile-time and linker flags enforced across the Debian archive to implement Position-Independent Executables (PIE). By ensuring binaries are loaded at random memory locations via Address Space Layout Randomization (ASLR), these flags mitigate attacks such as Return-Oriented Programming (ROP). The sections below cover the specific compiler options, the role of Debian's build system, and the underlying toolchain defaults.

The Core PIE Compilation and Linker Flags

To produce a position-independent binary, the build process requires two distinct flags:

  • -fPIE (or -fpie): Passed to the compiler (CFLAGS, CXXFLAGS). It instructs the compiler to generate position-independent machine code for an executable, similar to -fPIC used for shared libraries, but optimized to eliminate indirection overhead when accessing symbols known to be local to the executable.
  • -pie (or -Wl,-pie): Passed to the linker (LDFLAGS). It instructs the linker to produce a dynamically linked position-independent executable capable of being loaded at an arbitrary virtual memory address.

The Role of dpkg-buildflags

Debian orchestrates archive-wide hardening features via the dpkg-dev package through the dpkg-buildflags command. Within this framework, PIE is handled by the pie feature in the hardening area.

When building a Debian package, standard packaging scripts evaluate dpkg-buildflags --get CFLAGS and dpkg-buildflags --get LDFLAGS. For systems or targets where PIE must be explicitly passed, dpkg-buildflags injects -fPIE into CFLAGS, CXXFLAGS, OBJCFLAGS, and FFLAGS, while appending -pie to LDFLAGS.

If a package cannot compile with position independence (for instance, static bootloaders or low-level utilities), package maintainers explicitly opt out in debian/rules:

export DEB_BUILD_MAINT_OPTIONS = hardening=-pie

Disabling the feature causes dpkg-buildflags to emit -fno-PIE and -no-pie to counteract any default toolchain configurations.

GCC Default Configuration (--enable-default-pie)

Starting with Debian 9 (Stretch), Debian switched to enabling PIE by default directly within the GNU Compiler Collection (GCC) on most 64-bit and supported 32-bit architectures (including amd64, arm64, armhf, i386, ppc64el, and s390x).

During GCC's build configuration, Debian configures the compiler with:

--enable-default-pie

Because this setting is built into GCC's spec files:

  1. gcc automatically assumes -fPIE during the compilation step.
  2. gcc automatically passes -pie to the linker during the final link step.

Consequently, modern Debian packages do not always need explicit -fPIE or -pie flags displayed in their build logs, as the underlying native toolchain automatically enforces PIE unless explicitly instructed otherwise with -fno-PIE and -no-pie.

PIE relies on accompanying linker flags to provide comprehensive memory protection:

  • -Wl,-z,relro: Read-Only Relocations (RELRO). Marks sections of the ELF binary as read-only after the dynamic linker resolves relocations.
  • -Wl,-z,now: Immediate Binding (BINDNOW). Resolves all dynamic symbols at application launch time rather than lazily on demand. This allows the Global Offset Table (GOT) to be marked entirely read-only, preventing attackers from overwriting GOT pointers.