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Code Sequences for TLS on ARM

The ELF Handling For Thread-Local Storage document provides assembly sequences for various models (local exec / initial exec / general dynamic) for various architectures. But not ARM - are there any such code sequences for ARM anywhere? I am working on a compiler and I want to generate code that will work correctly with the linkers of the platform (both software and dynamic). A.

For clarity, suppose the ARMv7 processor and a fairly new kernel and glibc (say 3.13 + / 2.19 +), but I'm also interested in what should change for older hw / sw, if this is easy to explain.

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I do not quite understand what you want. However, assembler sequences (for ARMv6 + and capable kernel):

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 should be allocated from this value (probably a pointer). Using mcr is faster, but you can also call an assistant (see below) if you do not install HWCAP_TLS in your ELF (which can be used on all ARM processors supported by Linux).

The purpose of the 0xffff0fe8 address seems to be that you can use these 4 bytes instead of using the above assembler directly with ( rX == r0 ), as it may be different from somewhere in the machine.

It depends on the type of CPU. There is a helper helper on the @ 0xffff0fe0 vector page in entry-armv.S ; it is in the process / thread structure if the hardware does not support it. The 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 make a system call to install TLS stuff. clone is a way to set up a thread context. thread_info contains the entire register for the thread; it can share a millimeter (memory or process memory view) with another task_struct . That is, the thread_info thread has a tp_value for each thread created.

Here is a description of the implementation of ARM. The ELF / nptl / glibc and Linux kernel are involved (and / or search terms to explore more). Syscall for get_tls() was probably too expensive, and the current trunk has an auxiliary vector page (displayed by all threads / processes).

Some sources are glibc, tls-macros.h , tlsdesc.c , etc. Most likely, the full / short answer will depend on the version,

  • Your processor is ARM.
  • Linux kernel
  • Your glibc.
  • Your compiler (and flags!).
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