The missing link: explaining ELF static linking, semantically

Abstract

Beneath the surface, software usually depends on complex linker behaviour to work as intended. Even linking hello _ world.c is surprisingly involved, and systems software such as libc and operating system kernels rely on a host of linker features. But linking is poorly understood by working programmers and has largely been neglected by language researchers. In this paper we survey the many use-cases that linkers support and the poorly specified linker speak by which they are controlled: metadata in object files, command-line options, and linker-script language. We provide the first validated formalisation of a realistic executable and linkable format (ELF), and capture aspects of the Application Binary Interfaces for four mainstream platforms (AArch64, AMD64, Power64, and IA32). Using these, we develop an executable specification of static linking, covering (among other things) enough to link small C programs (we use the example of bzip2) into a correctly running executable. We provide our specification in Lem and Isabelle/HOL forms. This is the first formal specification of mainstream linking. We have used the Isabelle/HOL version to prove a sample correctness property for one case of AMD64 ABI relocation, demonstrating that the specification supports formal proof, and as a first step towards the much more ambitious goal of verified linking. Our work should enable several novel strands of research, including linker-aware verified compilation and program analysis, and better languages for controlling linking.

QED at Large: A Survey of Engineering of Formally Verified Software

“…OS kernel subsystems and formats are additional formalization targets: Bishop et al (2006) and Bishop et al (2018) defined and validated executable specifications in HOL4 for the TCP/IP stack, and Kell et al (2016) formalized the ELF binary format in HOL4 for executables used in Unix-like operating systems such as Linux.…”

Understanding GCC builtins to develop better tools

“…The current paper demonstrates that GCC builtins are used more frequently than inline assembly, which provides even stronger incentives to implement support by C tools. Other studies focused on the role of linkers [20] and the preprocessor [13]. C projects are often built using Makefiles, whose feature usage has also been investigated [29].…”

An Analysis of x86-64 Inline Assembly in C Programs

“…Linker. Kell et al studied the semantic role of linkers in C [34]. As with inline assembly, linker features are used in C programs, but transcend the language.…”