AArch64 (ARM64) Support

Overview

Reli supports AArch64 Linux as an experimental platform. This enables profiling on ARM-based servers (e.g., AWS Graviton) and Apple Silicon Macs running Linux VMs or Docker containers. Both NTS and ZTS targets are supported.

Implementation differences from x86_64

ptrace register access

AArch64 Linux does not support PTRACE_GETREGS or PTRACE_PEEKUSER. Instead, reli uses PTRACE_GETREGSET with an iovec structure to read registers:

• General-purpose registers: PTRACE_GETREGSET with NT_PRSTATUS reads the user_pt_regs struct (X0-X30, SP, PC, PSTATE).
• Thread pointer (TLS): PTRACE_GETREGSET with NT_ARM_TLS (0x401) reads the TPIDR_EL0 register, which is the AArch64 equivalent of x86_64's FS_BASE.

DWARF register numbering

The DWARF register numbers differ between architectures:

Role	x86_64	AArch64
Instruction pointer	16 (RIP)	32 (PC)
Stack pointer	7 (RSP)	31 (SP)
Frame pointer	6 (RBP)	29 (X29/FP)
Return address	16 (RIP)	30 (X30/LR)
Callee-saved	RBX, RBP, R12-R15	X19-X30

TLS Variant I vs Variant II

This is the most significant architectural difference for ZTS support.

• x86_64 (TLS Variant II): FS_BASE points directly to struct pthread. The _thread_db_pthread_dtvp offset (from libpthread debug symbols) gives the position of the DTV pointer within the struct. So DTV = *(FS_BASE + dtvp_offset).

• AArch64 (TLS Variant I): TPIDR_EL0 points to tcbhead_t, which is located after struct pthread in memory (glibc defines THREAD_SELF as (struct pthread *)tp - 1). The DTV pointer is the first field of tcbhead_t at offset 0. So DTV = *(TPIDR_EL0 + 0).

The _thread_db_pthread_dtvp offset cannot be used directly with TPIDR_EL0 on AArch64 because it's an offset within struct pthread, not from the thread pointer.

.eh_frame differences

AArch64 ELF binaries use different CIE (Common Information Entry) defaults:

Field	x86_64	AArch64
Code alignment factor	1	4
Data alignment factor	-8	-8
Return address register	16 (RIP)	30 (LR)

The frame pointer unwinding layout is identical on both architectures: [FP+0] = saved FP, [FP+8] = return address.