[AArch64] Zig self-hosted linker places mutable C++ data in read-only `.text`, causing SIGSEGV with RTLD_GLOBAL plugins

[xiangguomin]

Environment

• hermetic_cc_toolchain: v4.1.0
• Zig version: 0.12.0
• Target: aarch64-linux-gnu
• Host: x86_64-linux
• Bazel compilation modes affected: all (fastbuild, dbg, opt)

Symptom

Any binary that loads a shared plugin via RTLD_GLOBAL (e.g. ROS2 pluginlib) crashes
with SIGSEGV at startup on AArch64. The crash occurs when the plugin writes to a mutable
C++ runtime object (e.g. std::locale::id counters) that the Zig linker placed in the
read-only .text section.

Root Cause

The Zig 0.12.0 self-hosted linker incorrectly emits mutable C++ runtime data objects into
.text instead of .bss/.data. This goes unnoticed for statically-linked executables,
but fails at runtime when a plugin is loaded with RTLD_GLOBAL because the dynamic linker
maps .text read-only.

Workaround

Force LLD for all AArch64 link steps by adding -fuse-ld=lld to all_link_actions in
toolchain/zig_cc_toolchain.bzl:

if ctx.attr.target.startswith("aarch64"):                                                                                                            
    link_flag_sets.append(                                                                                                                           
        flag_set(                                                                                                                                    
            actions = all_link_actions,                                                                                                              
            flag_groups = [flag_group(flags = ["-fuse-ld=lld"])],                                                                                    
        ),                                                                                                                                           
    )                                                                                                                                                
                                                                                                                                                     
LLD correctly places mutable objects in writable .bss/.data.                                                                                         
 
▎ Note: ctx.attr.target_cpu is always "k8" for Linux regardless of architecture.                                                                     
▎ Use ctx.attr.target (the full Zig target triple, e.g. "aarch64-linux-gnu.2.35") to
▎ detect AArch64.