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Thread Local Storage

19 Jan 2020

This is an ongoing series of posts on ELF Binary Relocations and Thread Local Storage. This article covers only Thread Local Storage and assumes the reader has had a primer in ELF Relocations, if not please start with my previous article ELF Binaries and Relocation Entries

This is a the part illustrated 3 part series covering

In the last article we covered ELF Binary internals and how relocation entries are used to during link time to allow our programs to access symbols (variables). However, what if we want a different variable instance for each thread? This is where thread local storage (TLS) comes in.

In this article we will discuss how TLS works. Our outline:

As before, the examples in this article can be found in my tls-examples project. Please check it out.

Thread Local Storage

Did you know that in C you can prefix variables with __thread to create thread local variables?

Example

__thread int i;

A thread local variable is a variable that will have a unique instance per thread. Each time a new thread is created, the space required to store the thread local variables is allocated.

TLS variables are stored in dynamic TLS sections.

TLS Sections

In the previous article we saw how variables were stored in the .data and .bss sections. These are initialized once per program or library.

When we get to binaries that use TLS we will additionally have .tdata and .tbss sections.

These exist in a special TLS segment which is loaded per thread. In the next article we will discuss more about how this loading works.

TLS Data Structures

As we recall, to access data in .data and .bss sections simple code sequences with relocation entries are used. These sequences set and add registers to build pointers to our data. For example, the below sequence uses 2 relocations to compose a .bss section address into register r11.

Addr.   Machine Code    Assembly             Relocations
0000000c <get_x_addr>:
   c:   19 60 [00 00]   l.movhi r11,[0]      # c  R_OR1K_AHI16 .bss
  10:   44 00  48 00    l.jr r9
  14:   9d 6b [00 00]    l.addi r11,r11,[0]  # 14 R_OR1K_LO_16_IN_INSN .bss

With TLS the code sequences to access our data will also build pointers to our data, but they need to traverse the TLS data structures.

As the code sequence is read only and will be the same for each thread another level of indirection is needed, this is provided by the Thread Pointer (TP).

The Thread Pointer points into a data structure that allows us to locate TLS data sections. The TLS data structure includes:

These are illustrated as below:

  dtv[]   [ dtv[0], dtv[1], dtv[2], .... ]
            counter ^ |      \
               ----/ /        \________
              /     V                  V
/------TCB-------\/----TLS[1]----\   /----TLS[2]----\
| pthread tcbhead | tbss   tdata |   | tbss   tdata |
\----------------/\--------------/   \--------------/
          ^
          |
   TP-----/

Thread Pointer (TP)

The TP is unique to each thread. It provides the starting point to the TLS data structure.

Thread Control Block (TCB)

The TCB is the head of the TLS data structure. The TCB consists of:

For OpenRISC tcbhead_t is defined in sysdeps/or1k/nptl/tls.h as:

typedef struct {
  dtv_t *dtv;
} tcbhead_t

For x86_64 the tcbhead_t is defined in sysdeps/x86_64/nptl/tls.h as:

typedef struct
{
  void *tcb;            /* Pointer to the TCB.  Not necessarily the
                           thread descriptor used by libpthread.  */
  dtv_t *dtv;
  void *self;           /* Pointer to the thread descriptor.  */
  int multiple_threads;
  int gscope_flag;
  uintptr_t sysinfo;
  uintptr_t stack_guard;
  uintptr_t pointer_guard;
  unsigned long int vgetcpu_cache[2];
  /* Bit 0: X86_FEATURE_1_IBT.
     Bit 1: X86_FEATURE_1_SHSTK.
   */
  unsigned int feature_1;
  int __glibc_unused1;
  /* Reservation of some values for the TM ABI.  */
  void *__private_tm[4];
  /* GCC split stack support.  */
  void *__private_ss;
  /* The lowest address of shadow stack,  */
  unsigned long long int ssp_base;
  /* Must be kept even if it is no longer used by glibc since programs,
     like AddressSanitizer, depend on the size of tcbhead_t.  */
  __128bits __glibc_unused2[8][4] __attribute__ ((aligned (32)));

  void *__padding[8];
} tcbhead_t;

The x86_64 implementation includes many more fields including:

Dynamic Thread Vector (DTV)

The DTV is an array of pointers to each TLS data section. The first entry in the DTV array contains the generation counter. The generation counter is really just the array size. The DTV can be dynamically resized as more TLS modules are loaded.

The dtv_t type is a union as defined below:

typedef struct {
  void *val;     // Aligned pointer to data/bss
  void *to_free; // Unaligned pointer for free()
} dtv_pointer

typedef union {
  int counter;          // for entry 0
  dtv_pointer pointer;  // for all other entries
} dtv_t

Each dtv_t entry can be either a counter or a pointer. By convention the first entry, dtv[0] is a counter and the rest are pointers.

Thread Local Storage (TLS)

The initial set of TLS data sections is allocated contiguous with the TCB. Additional TLS data blocks will be allocated dynamically. There will be one entry for each loaded module, the first module being the current program. For dynamic libraries it is lazily initialized per thread.

Local (or TLS[1])

TLS[2]

The __tls_get_addr() function

The __tls_get_addr() function can be used at any time to traverse the TLS data structure and return a variable’s address. The function is given a pointer to an architecture specific argument tls_index.

For static builds the implementation is architecture dependant and defined in OpenRISC sysdeps/or1k/libc-tls.c as:

__tls_get_addr (tls_index *ti)
{
  dtv_t *dtv = THREAD_DTV ();
  return (char *) dtv[1].pointer.val + ti->ti_offset;
}

Note for for static builds the module index can be hard coded to 1 as there will always be only one module.

For dynamically linked programs the implementation is defined as part of the runtime dynamic linker in elf/dl-tls.c as:

void *
__tls_get_addr (GET_ADDR_ARGS)
{
  dtv_t *dtv = THREAD_DTV ();

  if (__glibc_unlikely (dtv[0].counter != GL(dl_tls_generation)))
    return update_get_addr (GET_ADDR_PARAM);

  void *p = dtv[GET_ADDR_MODULE].pointer.val;

  if (__glibc_unlikely (p == TLS_DTV_UNALLOCATED))
    return tls_get_addr_tail (GET_ADDR_PARAM, dtv, NULL);

  return (char *) p + GET_ADDR_OFFSET;
}

Here several macros are used so it’s a bit hard to follow but there are:

TLS Access Models

As one can imagine, traversing the TLS data structures when accessing each variable could be slow. For this reason there are different TLS access models that the compiler can choose to minimize variable access overhead.

Global Dynamic

The Global Dynamic (GD), sometimes called General Dynamic, access model is the slowest access model which will traverse the entire TLS data structure for each variable access. It is used for accessing variables in dynamic shared libraries.

Before Linking

Global Dynamic Object

Not counting relocations for the PLT and GOT entries; before linking the .text contains 1 placeholder for a GOT offset. This GOT entry will contain the arguments to __tls_get_addr.

After Linking

Global Dynamic Program

After linking there will be 2 relocation entries in the GOT to be resolved by the dynamic linker. These are R_TLS_DTPMOD, the TLS module index, and R_TLS_DTPOFF, the offset of the variable into the TLS module.

Example

File: tls-gd.c

extern __thread int x;

int* get_x_addr() {
  return &x;
}

Code Sequence (OpenRISC)

tls-gd.o:     file format elf32-or1k

Disassembly of section .text:

0000004c <get_x_addr>:
  4c:	18 60 [00 00] 	l.movhi r3,[0]          # 4c: R_OR1K_TLS_GD_HI16	x
  50:	9c 21 ff f8 	l.addi r1,r1,-8
  54:	a8 63 [00 00] 	l.ori r3,r3,[0]         # 54: R_OR1K_TLS_GD_LO16	x
  58:	d4 01 80 00 	l.sw 0(r1),r16
  5c:	d4 01 48 04 	l.sw 4(r1),r9
  60:	04 00 00 02 	l.jal 68 <get_x_addr+0x1c>
  64:	1a 00 [00 00] 	 l.movhi r16,[0]        # 64: R_OR1K_GOTPC_HI16	_GLOBAL_OFFSET_TABLE_-0x4
  68:	aa 10 [00 00] 	l.ori r16,r16,[0]       # 68: R_OR1K_GOTPC_LO16	_GLOBAL_OFFSET_TABLE_
  6c:	e2 10 48 00 	l.add r16,r16,r9
  70:	04 00 [00 00] 	l.jal [0]               # 70: R_OR1K_PLT26	__tls_get_addr
  74:	e0 63 80 00 	 l.add r3,r3,r16
  78:	85 21 00 04 	l.lwz r9,4(r1)
  7c:	86 01 00 00 	l.lwz r16,0(r1)
  80:	44 00 48 00 	l.jr r9
  84:	9c 21 00 08 	 l.addi r1,r1,8

Code Sequence (x86_64)

tls-gd.o:     file format elf64-x86-64

Disassembly of section .text:

0000000000000020 <get_x_addr>:
  20:	48 83 ec 08          	  sub    $0x8,%rsp
  24:	66 48 8d 3d [00 00 00 00] lea    [0](%rip),%rdi  # 28 R_X86_64_TLSGD	x-0x4
  2c:	66 66 48 e8 [00 00 00 00] callq  [0]             # 30 R_X86_64_PLT32	__tls_get_addr-0x4
  34:	48 83 c4 08          	  add    $0x8,%rsp
  38:	c3                   	  retq   

Local Dynamic

The Local Dynamic (LD) access model is an optimization for Global Dynamic where multiple variables may be accessed from the same TLS module. Instead of traversing the TLS data structure for each variable, the TLS data section address is loaded once by calling __tls_get_addr with an offset of 0. Next, variables can be accessed with individual offsets.

Local Dynamic is not supported on OpenRISC yet.

Before Linking

Local Dynamic Object

Not counting relocations for the PLT and GOT entries; before linking the .text contains 1 placeholder for a GOT offset and 2 placeholders for the TLS offsets. This GOT entry will contain the arguments to __tls_get_addr. The TLD offsets will be the offsets to our variables in the TLD data section.

After Linking

Local Dynamic Program

After linking there will be 1 relocation entry in the GOT to be resolved by the dynamic linker. This is R_TLS_DTPMOD, the TLS module index, the offset will be 0x0.

Example

File: tls-ld.c

static __thread int x;
static __thread int y;

int sum() {
  return x + y;
}

Code Sequence (x86_64)

tls-ld.o:     file format elf64-x86-64

Disassembly of section .text:

0000000000000030 <sum>:
  30:	48 83 ec 08          	sub    $0x8,%rsp
  34:	48 8d 3d [00 00 00 00] 	lea    [0](%rip),%rdi   # 37 R_X86_64_TLSLD	x-0x4
  3b:	e8 [00 00 00 00]       	callq  [0]              # 3c R_X86_64_PLT32	__tls_get_addr-0x4
  40:	8b 90 [00 00 00 00]    	mov    [0](%rax),%edx   # 42 R_X86_64_DTPOFF32	x
  46:	03 90 [00 00 00 00]    	add    [0](%rax),%edx   # 48 R_X86_64_DTPOFF32	y
  4c:	48 83 c4 08          	add    $0x8,%rsp
  50:	89 d0                	mov    %edx,%eax
  52:	c3                   	retq   

Initial Exec

The Initial Exec (IE) access model does not require traversing the TLS data structure. It requires that the compiler knows that offset from the TP to the variable can be computed during link time.

As Initial Exec does not require calling __tls_get_addr is is more efficient compared the GD and LD access.

Before Linking

Initial Exec Object

Text contains a placeholder for the got address of the offset. Not counting relocation entry for the GOT; before linking the .text contains 1 placeholder for a GOT offset. This GOT entry will contain the TP offset to the variable.

After Linking

Initial Exec Program

After linking there will be no remaining relocation entries. The .text section contains the actual GOT offset and the GOT entry will contain the TP offset to the variable.

Example

File: tls-ie.c

Initial exec C code will be the same as global dynamic, however IE access will be chosen when static compiling.

extern __thread int x;

int* get_x_addr() {
  return &x;
}

Code Sequence (OpenRISC)

00000038 <get_x_addr>:
  38:	9c 21 ff fc 	l.addi r1,r1,-4
  3c:	1a 20 [00 00] 	l.movhi r17,[0x0]   # 3c: R_OR1K_TLS_IE_AHI16	x
  40:	d4 01 48 00 	l.sw 0(r1),r9
  44:	04 00 00 02 	l.jal 4c <get_x_addr+0x14>
  48:	1a 60 [00 00] 	 l.movhi r19,[0x0]  # 48: R_OR1K_GOTPC_HI16	_GLOBAL_OFFSET_TABLE_-0x4
  4c:	aa 73 [00 00] 	l.ori r19,r19,[0x0] # 4c: R_OR1K_GOTPC_LO16	_GLOBAL_OFFSET_TABLE_
  50:	e2 73 48 00 	l.add r19,r19,r9
  54:	e2 31 98 00 	l.add r17,r17,r19
  58:	85 71 [00 00] 	l.lwz r11,[0](r17)  # 58: R_OR1K_TLS_IE_LO16	x
  5c:	85 21 00 00 	l.lwz r9,0(r1)
  60:	e1 6b 50 00 	l.add r11,r11,r10
  64:	44 00 48 00 	l.jr r9
  68:	9c 21 00 04 	 l.addi r1,r1,4

Code Sequence (x86_64)

0000000000000010 <get_x_addr>:
  10:	48 8b 05 [00 00 00 00] 	   mov    0x0(%rip),%rax   # 13: R_X86_64_GOTTPOFF	x-0x4
  17:	64 48 03 04 25 00 00 00 00 add    %fs:0x0,%rax
  20:	c3                         retq   

Local Exec

The Local Exec (LD) access model does not require traversing the TLS data structure or a GOT entry. It is chosen by the compiler when accessing file local variables in the current program.

The Local Exec access model is the most efficient.

Before Linking

Local Exec Object

Before linking the .text section contains one relocation entry for a TP offset.

After Linking

Local Exec Program

After linking the .text section contains the value of the TP offset.

Example

File: tls-le.c

In the Local Exec example the variable x is local, it is not extern.

static __thread int x;

int * get_x_addr() {
  return &x;
}

Code Sequence (OpenRISC)

00000010 <get_x_addr>:
  10:	19 60 [00 00] 	l.movhi r11,[0x0]    # 10: R_OR1K_TLS_LE_AHI16	.LANCHOR0
  14:	e1 6b 50 00 	l.add r11,r11,r10
  18:	44 00 48 00 	l.jr r9
  1c:	9d 6b [00 00] 	 l.addi r11,r11,[0]  # 1c: R_OR1K_TLS_LE_LO16	.LANCHOR0

Code Sequence (x86_64)

0000000000000010 <get_x_addr>:
  10:	64 48 8b 04 25 00 00 00 00  mov    %fs:0x0,%rax
  19:	48 05 [00 00 00 00]    	    add    $0x0,%rax  # 1b: R_X86_64_TPOFF32	x
  1f:	c3                   	    retq

Linker Relaxation

As some TLS access methods are more efficient than others we would like to choose the best method for each variable access. However, we sometimes don’t know where a variable will come from until link time.

On some architectures the linker will rewrite the TLS access code sequence to change to a more efficient access model, this is called relaxation.

One type of relaxation performed by the linker is GD to IE relaxation. During compile time GD relocation may be chosen for extern variables. However, during link time the variable may be found in the same module i.e. not a shared object which would require GD access. In this case the access model can be changed to IE.

That’s pretty cool.

The architecture I work on OpenRISC does not support any of this yet, it requires changes to the compiler and linker. The compiler needs to be updated to mark sections of the output .text that can be rewritten (often with added NOP codes). The linker needs to be updated to know how to identify the relaxation opportunity and perform it.

Summary

In this article we have covered how TLS variables are accessed per thread via the TLS data structure. Also, we saw how different TLS access models provide varying levels of efficiency.

In the next article we will look more into how this is implemented in GCC, the linker and the GLIBC runtime dynamic linker.

Further Reading