ld: Account for .tbss size on ARM EABI targets
A patch to the GNU linker (ld) ensures correct accounting for .tbss section size on ARM EABI targets, aligning behavior with LLVM's lld.
Torbjörn Svensson is pinging for feedback on a patch that addresses how the GNU linker accounts for the size of the .tbss section on ARM EABI targets. The patch ensures that .tbss occupies virtual memory address space if .tdata does, preventing potential overlaps with subsequent sections like .bss. This change aims to align binutils behavior with LLVM’s lld and includes new test cases for various linking scenarios.
- proposer
Pings for review of a patch that ensures .tbss size is accounted for on ARM EABI targets.
“Gentle ping! :)”
- proposer
Submits a patch to correctly handle .tbss section size on ARM EABI targets, aligning with LLVM lld.
“Bare-metal ARM EABI programs allocate TLS storage directly, so if .tdata consumses VMA, then .tbss must also do so. Otherwise, a following section such as .bss can overlap it. Keep the existing behavior for non-EABI targets, and add tests for static, executable, PIE, and dynamic links. This change aligns with LLVM lld behavior for .tbss.”
- reviewer
Questions the patch's premise, implying it might be addressing a problem in the loader rather than the linker itself, and asks why the linker should be altered.
“In other words, the loader is broken for TLS. Why break the linker as well?”
- proposer
Clarifies that the context is bare-metal execution where no loader is involved, and the issue pertains to the linker's behavior.
“What loader? I'm taking about bare-metal, i.e. there is nothing doing any loading. The CPU is executing straight”
Technical Tradeoffs
- Aligns binutils behavior with LLVM lld for consistency.
- Introduces a new linker configuration option (`tls_nobits_occupies_vma`).
In Details
This patch addresses an issue in the GNU linker (ld) concerning Thread-Local Storage (TLS) on ARM EABI targets, specifically for bare-metal environments. The problem arises because the .tbss section (uninitialized TLS data) needs to occupy virtual memory space if the .tdata section (initialized TLS data) does, to prevent subsequent sections like .bss from overlapping. The proposed solution adds a new build-time property tls_nobits_occupies_vma to ld.h and conditionally enables this behavior for ARM EABI targets in armelf.em, aligning with LLVM's lld behavior. It also includes…
- .tbss
- A section in an executable file that typically contains uninitialized Thread-Local Storage (TLS) data. Its size is accounted for but it does not occupy memory in the final image.
- ARM EABI
- The Application Binary Interface for the ARM architecture, defining conventions for program execution, data representation, and system calls.
- TLS
- Thread-Local Storage. A mechanism that provides data that is local to a thread. Each thread in a process gets its own copy of the data.
- VMA
- Virtual Memory Address. The address of a location in a computer's virtual memory space.
- bare-metal
- Refers to running code directly on hardware without an underlying operating system or loader, typically in embedded systems or low-level development.
- LLVM lld
- A fast, new linker from the LLVM project, designed to be a modern replacement for existing linkers.