arch_prctl (nr 158)

Linux Signature

int arch_prctl(int code, unsigned long addr);

Description

Sets or gets architecture-specific thread state. On x86-64, primarily used to set the FS and GS segment base registers for thread-local storage (TLS).

Current Implementation

• ARCH_SET_FS (0x1002): Writes addr to the IA32_FS_BASE MSR (0xC000_0100). This is how musl sets up its TLS pointer during C runtime initialisation. Returns 0 on success.
• All other codes: Returns -EINVAL (-22).

Source: osl/src/syscalls/misc.rs — sys_arch_prctl

Future Work

• Implement ARCH_GET_FS (0x1003) to read back the current FS base.
• Implement ARCH_SET_GS (0x1001) and ARCH_GET_GS (0x1004) for GS-based TLS.
• Save/restore FS_BASE across context switches if multiple user processes use TLS concurrently (currently each process sets it fresh via the trampoline, but preemption during a syscall could lose the value).