Code sequences for TLS on ARM

Question

The ELF Handling For Thread-Local Storage document gives assembly sequences for the various models (local exec/initial exec/general dynamic) for various architectures. But not ARM -- is there anywhere I can see such code sequences for ARM? I'm working on a compiler and want to generate code that will operate properly with the platform linkers (both program and dynamic).

For clarity, let's assume an ARMv7 CPU and a pretty new kernel and glibc (say 3.13+ / 2.19+), but I'd also be interested in what has to change for older hw/sw if that's easy to explain.

Answer 1

I don't exactly understand what you want. However, the assembler sequences (for ARMv6+ and a capable kernel) are,

mrc p15, 0, rX, c13, c0, 2  @ get the user r/w register

This is called TPIDRURW in some ARM manuals. Your TLS tables/structure must be parented from this value (probably a pointer). Using the mcr is faster, but you can also call the helper (see below) if you don't set HWCAP_TLS in your ELF (which can be used on all ARM CPUs supported by Linux).

The intent of address 0xffff0fe8 seems to be that you can use those 4-bytes instead of using the above assembler directly with (rX == r0) as maybe it is different for some machine somewhere.

---

It is dependent on the CPU type. There is a helper in the vector page @0xffff0fe0 in entry-armv.S; it is in the process/thread structure if the hardware doesn't support it. Documentation is in kernel_user_helpers.txt

Usage example:

typedef void * (__kuser_get_tls_t)(void);
#define __kuser_get_tls (*(__kuser_get_tls_t *)0xffff0fe0)

void foo()
{
    void *tls = __kuser_get_tls();
    printf("TLS = %p\n", tls);
}

You do a syscall to set the TLS stuff. clone is a way to setup a thread context. The thread_info holds all register for a thread; it may share an mm (memory management or process memory view) with other task_struct. Ie, the thread_info has a tp_value for each created thread.

Here is a dicussion of the ARM implementation. ELF/nptl/glibc and Linux kernel are all involved (and/or search terms to investigate more). The syscall for get_tls() was probably too expensive and the current mainline has a vector page helper (mapped by all threads/processes).

Some glibc source, tls-macros.h, tlsdesc.c, etc. Most likely a full/concise answer will depend on the version of,

1. Your ARM CPU.
2. Your Linux kernel.
3. Your glibc.
4. Your compiler (and flags!).