Linux Threads Through a Magnifier: Local Threads

Source code for this article is here.

Threads are everywhere. Even now, when you browse this page, threads are involved in the process. Most likely, you have more than one tab opened in the browser and each one has at least one thread associated with it. The server supplying this page runs several threads in order to serve multiple connections simultaneously. There may be unnumbered examples for threads, but let us concentrate on one specific implementation thereof. Namely, Linux implementation of threads.

It is hard to believe, that earlier Linux kernels did not support threads. Instead, all the "threading" was performed entirely in user space by a pthread (POSIX thread) library chosen for specific program. This reminds me of my attempt to implement multitasking in DOS when I was in college - possible, but full of headache.

Modern kernels, on the contrary, have full support for threads, which, from kernel's point of view are so-called "Light-weight Processes". They are usually organized in thread groups, which, in turn, represent processes as we know them. As a matter of fact, the getpid libc function (and sys_getpid system call) return an identifier of a thread group.

Let me reiterate - the best explanation is an explanation by example. In this article, I am going to cover the process of thread creation on 64 bit Linux running on PC using FASM (flat assembler).

Clone, Fork, Exec...

There are several system calls involved in process manipulations. The most known one is sys_fork. This system call "splits" a running process in two - parent and child. While they both continue execution from the instruction immediately following the sys_fork invocation, they have different PID (process ID) or, as we now know - different TGID (thread group ID) as well as each one gets a different return value from sys_fork. The return value is a child TGID for the parent process and 0 for the child. In case of error, fork returns -1 and sets errno appropriately, while sys_fork returns a negative error code. 

Exec does not return at all. Well, it formally has a return type of int, but getting a return value means, that the function failed. Exec* libc function or sys_execve system call are used in order to launch a new process. For example, if your application has to start another application, but you do not want or cannot, for any reason, execute system() function, then your application has to fork and the child process calls exec, thus, being replaced in memory by the new process. The execution of the new process starts normally from its entry point.

Clone - this is the function we are interested in. Clone is a libc wrapper for sys_clone Linux system call and is declared in the sched.h header as follows:

int clone(int (*fn)(void*), void *child_stack, int flags, void *arg, ...);

I encourage you to read the man page for clone libc function at http://linux.die.net/man/2/clone or with "man clone" :-) 

sys_clone

We are not going to deal with clone function here. There are lots of good resources on the internet which provide good examples for it. Instead, we are going to examine the sys_clone Linux system call.

First of all, let us take a look at the definition of the sys_clone in arch/x86/kernel/process.c:

long sys_clone(unsigned long clone_flags, unsigned long newsp,

               void __user *parent_tid, void __user *child_tid, struct pt_regs *regs)

Although, the definition looks quite complicated, in reality, it only needs clone_flags and newsp to be specified. 

But there is a strange thing - it does not take a pointer to the thread function as a parameter. That is normal - sys_clone only performs the action suggested by its name - clones the process. But how about the libc's clone? - you may ask. As I have mentioned above, libc's clone is a wrapper and what is does in addition to calling sys_clone is setting its return address in the cloned process to the address of the thread function. But let us examine it in more detail.

clone_flags - this value tells the kernel about how we want our process to be cloned. In our case, as we want to create a thread, rather then a separate process, we should use the following or'ed values:

CLONE_VM  (0x100) - tells the kernel to let the original process and the clone in the same memory space;

CLONE_FS (0x200) - both get the same file system information;

CLONE_FILES (0x400) - share file descriptors;

CLONE_SIGHAND (0x800) - both processes share the same signal handlers;

CLONE_THREAD (0x10000) - this tells the kernel, that both processes would belong to the same thread group (be threads within the same process);

SIGCHLD (0x11) - this is not a flag, this is the number of the SIGCHLD signal, which would be sent to the original process (thread) when the thread is terminated (used by wait functions).

newsp - the value of the stack pointer for the cloned process (new thread). This value may be NULL in which case, both threads are using the same stack. However, if the new thread attempts to write to the stack, then, due to the copy-on-write mechanism, it gets new memory pages, thus, leaving the stack of the original thread untouched.

Stack Allocation

Due to the fact, that in most cases, you would want to allocate a new stack for a new thread, I cannot leave this aspect uncovered in this article. To make things easier, let us implement a small function, which would receive the size of the requested  stack in bytes and return a pointer to the allocated memory region.

Important note:

As Linux follows AMD64 calling convention when running in 64 bits, function parameters and system call arguments are passed via the following registers:

Function call: arguments 1 - 6 via RDI, RSI, RDX, RCX, R8, R9; additional arguments are passed on stack.

System call: arguments 1 - 6 via RDI, RSI, RDX, R10, R8, R9; additional arguments are passed on stack.

C declaration:

void* map_stack(unsigned long stack_size);

Implementation:

PROT_READ     = 1

PROT_WRITE    = 2

MAP_PRIVATE   = 0x002

MAP_ANON      = 0x020

MAP_GROWSDOWN = 0x100

SYS_MMAP      = 9

map_stack:

   push  rdi rsi rdx r10 r8 r9                 ;Save registers

   mov   rsi, rdi                              ;Requested size

   xor   rdi, rdi                              ;Preferred address (may be NULL)   

   mov   rdx, PROT_READ or PROT_WRITE          ;Memory protection

   mov   r10, MAP_PRIVATE or MAP_ANON or MAP_GROWSDOWN ;Allocation attributes

   xor   r8, r8                                ;File descriptor (-1)

   dec   r8     

   xor   r9, r9                                ;Offset - irrelevant, so 0

   mov   rax, SYS_MMAP                         ;Set system call number

   syscall                                     ;Execute system call

   pop   r9 r8 r10 rdx rsi rdi                 ;Restore registers

   ret 

Calling this function would be as easy as:

mov  rdi, size

call map_stack

This function returns either a negative error code as provided by sys_mmap or the address of the allocated memory region. As we specified MAP_GROWSDOWN attribute, the obtained address would point to the top of the allocated region instead of pointing to its bottom, thus, making it perfect to specify as a new stack pointer.

Creation of Thread

In this section, we will implement a trivial create_thread function. It would allocate stack (of default size = 0x1000 bytes) for a new thread, invoke sys_clone and to either the instruction following call create_thread or to the thread function, depending on the return value of sys_clone.

C declaration:

long create_thread(void(*thread_func)(void*), void* param);

As you may see, the return type of the thread_func is void, unlike the real clone function. I will show you why a bit later.

Implementation:

create_thread:

   mov   r14, rdi    ;Save the address of the thread_func

   mov   r15, rsi    ;Save thread parameter

   mov   rdi, 0x1000 ;Requested stack size

   call  map_stack   ;Allocate stack

   mov   rsi, rax    ;Set newsp

   mov   rdi, CLONE_VM or CLONE_FS or CLONE_THREAD or CLONE_SIGHAND or SIGCHLD ;Set clone_flags

   xor   r10, r10    ;parent_tid

   xor   r8, r8      ;child_tid

   xor   r9, r9      ;regs

   mov   rax, SYS_CLONE

   syscall           ;Execute system call

   or    rax, 0      ;Check sys_clone return value

   jnz   .parent     ;If not 0, then it is the ID of the new thread

   push  r14         ;Otherwise, set new return address (thread_func)

   mov   rdi, r15    ;Set argument for the thread_func

   ret               ;Return to thread_func

.parent:

   ret               ;Return to parent (main thread)

Exiting Thread

Everyone who has ever searched the Web for Assembly programming tutorial for Linux is familiar with sys_exit system call. On 64 bit Intel platform it is call number 60. However, they all (tutorials) miss the point. Although, sys_exit works perfectly with single threaded hello-world-like applications, the situation is different with multithreaded ones. In general, sys_exit terminates thread, not a process, which, in case of a process with a single thread, is definitely enough, but may lead to strange artifacts (or even zombies) if, for example, a thread continues to print to stdout after you have terminated the main thread.

Now, the promised explanation on the the thread_func return type. In our case (as in most cases) the thread_func does not return by means of using the ret instruction. It just can't as there is no return address on the stack and even if you put one - returning would not terminate the thread. Instead, you should implement something like this exit_thread function.

C declaration:

void exit_thread(long result);

Implementation:

SYS_EXIT = 60

exit_thread:

                         ; Result is already in RDI

   mov   rax, SYS_EXIT   ; Set system call number

   syscall               ; Execute system call

Exiting Process

By exiting process we usually mean total termination of the running process. Linux gracefully provides us with a system call which terminates a group of threads (process) - sys_exit_group (call number 231). The function for terminating the process is as simple as this:

C declaration:

void exit_process(long result);

Implementation:

SYS_EXIT_GROUP = 231

exit_process:

                             ; Result is already in RDI

   mov   rax, SYS_EXIT_GROUP ; Set system call number

   syscall                   ; Execute system call

Attached Source Code

The source code attached to this article (which may be found here) contains a trivial example of the application that creates thread with the method described above. In addition, it contains the list of system call numbers for both 32 and 64 bit platforms.

Note for Nerds:

The attached code is for demonstration purpose only and may not contain such important elements as checking for errors, etc.

32 bit Systems

If you decide to convert the code given above to run on 32 bit systems, that would be quite easy. First of all - change register names to appropriate 32 bit ones. 

Second thing is to remember how parameters are passed to system calls in 32 bit kernels. They are still passed through registers, but the registers are different. Parameters 1st through 5th are passed through EBX, ECX, EDX, ESI, EDI. The system call number is placed as usual in EAX, the same register is used to store return value upon system call's completion.

Third - use int 0x80 instead of syscall instruction.

Forth - remember to change function prologues due to a different calling convention. While 64 bit systems use AMD64 ABI, 32 bit systems use cdecl passing arguments on stack by default.

Hope this article was interesting and helpful.

See you at the next (remote threads in Linux - stay tuned).