Program startup process in userspace

Introduction

Despite the linux-insides described mostly Linux kernel related stuff, I have decided to write this one part which mostly related to userspace.

There is already fourth part of System calls chapter which describes what does the Linux kernel do when we want to start a program. In this part I want to explore what happens when we run a program on a Linux machine from userspace perspective.

I don't know how about you, but in my university I learn that a C program starts executing from the function which is called main. And that's partly true. Whenever we are starting to write new program, we start our program from the following lines of code:

int main(int argc, char *argv[]) {
    // Entry point is here
}

But if you are interested in low-level programming, you may already know that the main function isn't the actual entry point of a program. You will believe it's true after you look at this simple program in debugger:

int main(int argc, char *argv[]) {
    return 0;
}

Let's compile this and run in gdb:

$ gcc -ggdb program.c -o program
$ gdb ./program
The target architecture is assumed to be i386:x86-64:intel
Reading symbols from ./program...done.

Let's execute gdb info subcommand with files argument. The info files prints information about debugging targets and memory spaces occupied by different sections.

(gdb) info files
Symbols from "/home/alex/program".
Local exec file:
    `/home/alex/program', file type elf64-x86-64.
    Entry point: 0x400430
    0x0000000000400238 - 0x0000000000400254 is .interp
    0x0000000000400254 - 0x0000000000400274 is .note.ABI-tag
    0x0000000000400274 - 0x0000000000400298 is .note.gnu.build-id
    0x0000000000400298 - 0x00000000004002b4 is .gnu.hash
    0x00000000004002b8 - 0x0000000000400318 is .dynsym
    0x0000000000400318 - 0x0000000000400357 is .dynstr
    0x0000000000400358 - 0x0000000000400360 is .gnu.version
    0x0000000000400360 - 0x0000000000400380 is .gnu.version_r
    0x0000000000400380 - 0x0000000000400398 is .rela.dyn
    0x0000000000400398 - 0x00000000004003c8 is .rela.plt
    0x00000000004003c8 - 0x00000000004003e2 is .init
    0x00000000004003f0 - 0x0000000000400420 is .plt
    0x0000000000400420 - 0x0000000000400428 is .plt.got
    0x0000000000400430 - 0x00000000004005e2 is .text
    0x00000000004005e4 - 0x00000000004005ed is .fini
    0x00000000004005f0 - 0x0000000000400610 is .rodata
    0x0000000000400610 - 0x0000000000400644 is .eh_frame_hdr
    0x0000000000400648 - 0x000000000040073c is .eh_frame
    0x0000000000600e10 - 0x0000000000600e18 is .init_array
    0x0000000000600e18 - 0x0000000000600e20 is .fini_array
    0x0000000000600e20 - 0x0000000000600e28 is .jcr
    0x0000000000600e28 - 0x0000000000600ff8 is .dynamic
    0x0000000000600ff8 - 0x0000000000601000 is .got
    0x0000000000601000 - 0x0000000000601028 is .got.plt
    0x0000000000601028 - 0x0000000000601034 is .data
    0x0000000000601034 - 0x0000000000601038 is .bss

Note on Entry point: 0x400430 line. Now we know the actual address of entry point of our program. Let's put a breakpoint by this address, run our program and see what happens:

(gdb) break *0x400430
Breakpoint 1 at 0x400430
(gdb) run
Starting program: /home/alex/program 

Breakpoint 1, 0x0000000000400430 in _start ()

Interesting. We don't see execution of the main function here, but we have seen that another function is called. This function is _start and as our debugger shows us, it is the actual entry point of our program. Where is this function from? Who does call main and when is it called? I will try to answer all these questions in the following post.

How the kernel starts a new program

First of all, let's take a look at the following simple C program:

// program.c

#include <stdlib.h>
#include <stdio.h>

static int x = 1;

int y = 2;

int main(int argc, char *argv[]) {
    int z = 3;

    printf("x + y + z = %d\n", x + y + z);

    return EXIT_SUCCESS;
}

We can be sure that this program works as we expect. Let's compile it:

$ gcc -Wall program.c -o sum

and run:

$ ./sum
x + y + z = 6

Ok, everything looks pretty good up to now. You may already know that there is a special family of functions - exec*. As we read in the man page:

The exec() family of functions replaces the current process image with a new process image.

All the exec* functions are simple frontends to the execve system call. If you have read the fourth part of the chapter which describes system calls, you may know that the execve system call is defined in the files/exec.c source code file and looks like:

SYSCALL_DEFINE3(execve,
        const char __user *, filename,
        const char __user *const __user *, argv,
        const char __user *const __user *, envp)
{
    return do_execve(getname(filename), argv, envp);
}

It takes an executable file name, set of command line arguments, and set of enviroment variables. As you may guess, everything is done by the do_execve function. I will not describe the implementation of the do_execve function in detail because you can read about this in here. But in short words, the do_execve function does many checks like filename is valid, limit of launched processes is not exceed in our system and etc. After all of these checks, this function parses our executable file which is represented in ELF format, creates memory descriptor for newly executed executable file and fills it with the appropriate values like area for the stack, heap and etc. When the setup of new binary image is done, the start_thread function will set up one new process. This function is architecture-specific and for the x86_64 architecture, its definition will be located in the arch/x86/kernel/process_64.c source code file.

The start_thread function sets new value to segment registers and program execution address. From this point, our new process is ready to start. Once the context switch will be done, control will be returned to userspace with new values of registers and the new executable will be started to execute.

That's all from the kernel side. The Linux kernel prepares the binary image for execution and its execution starts right after the context switch and returns controll to userspace when it is finished. But it does not answer our questions like where does _start come from and others. Let's try to answer these questions in the next paragraph.

How does a program start in userspace

In the previous paragraph we saw how an executable file is prepared to run by the Linux kernel. Let's look at the same, but from userspace side. We already know that the entry point of each program is its _start function. But where is this function from? It may came from a library. But if you remember correctly we didn't link our program with any libraries during compilation of our program:

$ gcc -Wall program.c -o sum

You may guess that _start comes from the stanard libray and that's true. If you try to compile our program again and pass the -v option to gcc which will enable verbose mode, you will see a long output. The full output is not interesting for us, let's look at the following steps:

First of all, our program should be compiled with gcc:

$ gcc -v -ggdb program.c -o sum
...
...
...
/usr/libexec/gcc/x86_64-redhat-linux/6.1.1/cc1 -quiet -v program.c -quiet -dumpbase program.c -mtune=generic -march=x86-64 -auxbase test -ggdb -version -o /tmp/ccvUWZkF.s
...
...
...

The cc1 compiler will compile our C source code and an produce assembly named /tmp/ccvUWZkF.s file. After this we can see that our assembly file will be compiled into object file with the GNU as assembler:

$ gcc -v -ggdb program.c -o sum
...
...
...
as -v --64 -o /tmp/cc79wZSU.o /tmp/ccvUWZkF.s
...
...
...

In the end our object file will be linked by collect2:

$ gcc -v -ggdb program.c -o sum
...
...
...
/usr/libexec/gcc/x86_64-redhat-linux/6.1.1/collect2 -plugin /usr/libexec/gcc/x86_64-redhat-linux/6.1.1/liblto_plugin.so -plugin-opt=/usr/libexec/gcc/x86_64-redhat-linux/6.1.1/lto-wrapper -plugin-opt=-fresolution=/tmp/ccLEGYra.res -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lc -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s --build-id --no-add-needed --eh-frame-hdr --hash-style=gnu -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o test /usr/lib/gcc/x86_64-redhat-linux/6.1.1/../../../../lib64/crt1.o /usr/lib/gcc/x86_64-redhat-linux/6.1.1/../../../../lib64/crti.o /usr/lib/gcc/x86_64-redhat-linux/6.1.1/crtbegin.o -L/usr/lib/gcc/x86_64-redhat-linux/6.1.1 -L/usr/lib/gcc/x86_64-redhat-linux/6.1.1/../../../../lib64 -L/lib/../lib64 -L/usr/lib/../lib64 -L. -L/usr/lib/gcc/x86_64-redhat-linux/6.1.1/../../.. /tmp/cc79wZSU.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-redhat-linux/6.1.1/crtend.o /usr/lib/gcc/x86_64-redhat-linux/6.1.1/../../../../lib64/crtn.o
...
...
...

Yes, we can see a long set of command line options which are passed to the linker. Let's go from another way. We know that our program depends on stdlib:

$ ldd program
    linux-vdso.so.1 (0x00007ffc9afd2000)
    libc.so.6 => /lib64/libc.so.6 (0x00007f56b389b000)
    /lib64/ld-linux-x86-64.so.2 (0x0000556198231000)

as we use some stuff from there like printf and etc. But not only. That's why we will get an error when we pass -nostdlib option to the compiler:

$ gcc -nostdlib program.c -o program
/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 000000000040017c
/tmp/cc02msGW.o: In function `main':
/home/alex/program.c:11: undefined reference to `printf'
collect2: error: ld returned 1 exit status

Besides other errors, we also see that _start symbol is undefined. So now we are sure that the _start function comes from standard library. But even if we link it with the standard library, it will not be compiled successfully anyway:

$ gcc -nostdlib -lc -ggdb program.c -o program
/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000400350

Ok, the compiler does not complain about undefined reference of standard library functions anymore as we linked our program with /usr/lib64/libc.so.6, but the _start symbol isn't resolved yet. Let's return to the verbose output of gcc and look at the parameters of collect2. The most important thing that we may see is that our program is linked not only with the standard library, but also with some object files. The first object file is: /lib64/crt1.o. And if we look inside this object file with objdump, we will see the _start symbol:

$ objdump -d /lib64/crt1.o 

/lib64/crt1.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <_start>:
   0:    31 ed                    xor    %ebp,%ebp
   2:    49 89 d1                 mov    %rdx,%r9
   5:    5e                       pop    %rsi
   6:    48 89 e2                 mov    %rsp,%rdx
   9:    48 83 e4 f0              and    $0xfffffffffffffff0,%rsp
   d:    50                       push   %rax
   e:    54                       push   %rsp
   f:    49 c7 c0 00 00 00 00     mov    $0x0,%r8
  16:    48 c7 c1 00 00 00 00     mov    $0x0,%rcx
  1d:    48 c7 c7 00 00 00 00     mov    $0x0,%rdi
  24:    e8 00 00 00 00           callq  29 <_start+0x29>
  29:    f4                       hlt

As crt1.o is a shared object file, we see only stubs here instead of real calls. Let's look at the source code of the _start function. As this function is architecture specific, implementation for _start will be located in the sysdeps/x86_64/start.S assembly file.

The _start starts from the clearing of ebp register as ABI suggests.

xorl %ebp, %ebp

And after this we put the address of termination function to the r9 register:

mov %RDX_LP, %R9_LP

As described in the ELF specification:

After the dynamic linker has built the process image and performed the relocations, each shared object gets the opportunity to execute some initialization code. ... Similarly, shared objects may have termination functions, which are executed with the atexit (BA_OS) mechanism after the base process begins its termination sequence.

So we need to put the address of the termination function to the r9 register as it will be passed to __libc_start_main in future as sixth argument. Note that the address of the termination function initially is located in the rdx register. Other registers besides rdx and rsp contain unspecified values. Actually the main point of the _start function is to call __libc_start_main. So the next action is to prepare for this function.

The signature of the __libc_start_main function is located in the csu/libc-start.c source code file. Let's look on it:

STATIC int LIBC_START_MAIN (int (*main) (int, char **, char **),
                             int argc,
                            char **argv,
                             __typeof (main) init,
                            void (*fini) (void),
                            void (*rtld_fini) (void),
                            void *stack_end)

It takes the address of the main function of a program, argc and argv. init and fini functions are constructor and destructor of the program. The rtld_fini is the termination function which will be called after the program will be exited to terminate and free its dynamic section. The last parameter of the __libc_start_main is a pointer to the stack of the program. Before we can call the __libc_start_main function, all of these parameters must be prepared and passed to it. Let's return to the sysdeps/x86_64/start.S assembly file and continue to see what happens before the __libc_start_main function will be called from there.

We can get all the arguments we need for __libc_start_main function from the stack. As _start is called, our stack looks like:

+-----------------+
|       NULL      |
+-----------------+ 
|       envp      |
+-----------------+ 
|       NULL      |
+------------------
|       argv      | <- rsp
+------------------
|       argc      |
+-----------------+

After we cleared ebp register and saved the address of the termination function in the r9 register, we pop an element from the stack to the rsi register, so after this rsp will point to the argv array and rsi will contain count of command line arguemnts passed to the program:

+-----------------+
|       NULL      |
+-----------------+ 
|       envp      |
+-----------------+ 
|       NULL      |
+------------------
|       argv      | <- rsp
+-----------------+

After this we move the address of the argv array to the rdx register

popq %rsi
mov %RSP_LP, %RDX_LP

From this moment we have argccand argv. We still need to put pointers to the construtor, destructor in appropriate registers and pass pointer to the stack. At the first following three lines we align stack to 16 bytes boundary as suggested in ABI and push rax which contains garbage:

and  $~15, %RSP_LP
pushq %rax

pushq %rsp
mov $__libc_csu_fini, %R8_LP
mov $__libc_csu_init, %RCX_LP
mov $main, %RDI_LP

After stack aligning we push the address of the stack, move the addresses of contstructor and destructor to the r8 and rcx registers and address of the main symbol to the rdi. From this moment we can call the __libc_start_main function from the csu/libc-start.c.

Before we look at the __libc_start_main function, let's add the /lib64/crt1.o and try to compile our program again:

$ gcc -nostdlib /lib64/crt1.o -lc -ggdb program.c -o program
/lib64/crt1.o: In function `_start':
(.text+0x12): undefined reference to `__libc_csu_fini'
/lib64/crt1.o: In function `_start':
(.text+0x19): undefined reference to `__libc_csu_init'
collect2: error: ld returned 1 exit status

Now we see another error that both __libc_csu_fini and __libc_csu_init functions are not found. We know that the addresses of these two functions are passed to the __libc_start_main as parameters and also these functions are constructor and destructor of our programs. But what do constructor and destructor in terms of C program means? We already saw the quote from the ELF specification:

After the dynamic linker has built the process image and performed the relocations, each shared object gets the opportunity to execute some initialization code. ... Similarly, shared objects may have termination functions, which are executed with the atexit (BA_OS) mechanism after the base process begins its termination sequence.

So the linker creates two special sections besides usual sections like .text, .data and others:

  • .init
  • .fini

We can find them with the readelf util:

$ readelf -e test | grep init
  [11] .init             PROGBITS         00000000004003c8  000003c8

$ readelf -e test | grep fini
  [15] .fini             PROGBITS         0000000000400504  00000504

Both of these sections will be placed at the start and end of the binary image and contain routines which are called constructor and destructor respectively. The main point of these routines is to do some initialization/finalization like initialization of global variables, such as errno, allocation and deallocation of memory for system routines and etc., before the actual code of a program is executed.

You may infer from the names of these functions, they will be called before the main function and after the main function. Definitions of .init and .fini sections are located in the /lib64/crti.o and if we add this object file:

$ gcc -nostdlib /lib64/crt1.o /lib64/crti.o  -lc -ggdb program.c -o program

we will not get any errors. But let's try to run our program and see what happens:

$ ./program
Segmentation fault (core dumped)

Yeah, we got segmentation fault. Let's look inside of the lib64/crti.o with objdump:

$ objdump -D /lib64/crti.o

/lib64/crti.o:     file format elf64-x86-64


Disassembly of section .init:

0000000000000000 <_init>:
   0:    48 83 ec 08              sub    $0x8,%rsp
   4:    48 8b 05 00 00 00 00     mov    0x0(%rip),%rax        # b <_init+0xb>
   b:    48 85 c0                 test   %rax,%rax
   e:    74 05                    je     15 <_init+0x15>
  10:    e8 00 00 00 00           callq  15 <_init+0x15>

Disassembly of section .fini:

0000000000000000 <_fini>:
   0:    48 83 ec 08              sub    $0x8,%rsp

As I wrote above, the /lib64/crti.o object file contains definition of the .init and .fini section, but also we can see here the stub for function. Let's look at the source code which is placed in the sysdeps/x86_64/crti.S source code file:

    .section .init,"ax",@progbits
    .p2align 2
    .globl _init
    .type _init, @function
_init:
    subq $8, %rsp
    movq PREINIT_FUNCTION@GOTPCREL(%rip), %rax
    testq %rax, %rax
    je .Lno_weak_fn
    call *%rax
.Lno_weak_fn:
    call PREINIT_FUNCTION

It contains the definition of the .init section and assembly code does 16-byte stack alignment and next we move address of the PREINIT_FUNCTION and if it is zero we don't call it:

00000000004003c8 <_init>:
  4003c8:       48 83 ec 08             sub    $0x8,%rsp
  4003cc:       48 8b 05 25 0c 20 00    mov    0x200c25(%rip),%rax        # 600ff8 <_DYNAMIC+0x1d0>
  4003d3:       48 85 c0                test   %rax,%rax
  4003d6:       74 05                   je     4003dd <_init+0x15>
  4003d8:       e8 43 00 00 00          callq  400420 <__libc_start_main@plt+0x10>
  4003dd:       48 83 c4 08             add    $0x8,%rsp
  4003e1:       c3                      retq

where the PREINIT_FUNCTION is the __gmon_start__ which does setup for profiling. You may note that we have no return instruction in the sysdeps/x86_64/crti.S. Actually that's why we got a segmentation fault. Prolog of _init and _fini is placed in the sysdeps/x86_64/crtn.S assembly file:

.section .init,"ax",@progbits
addq $8, %rsp
ret

.section .fini,"ax",@progbits
addq $8, %rsp
ret

and if we will add it to the compilation, our program will be successfully compiled and run!

$ gcc -nostdlib /lib64/crt1.o /lib64/crti.o /lib64/crtn.o  -lc -ggdb program.c -o program

$ ./program
x + y + z = 6

Conclusion

Now let's return to the _start function and try to go through a full chain of calls before the main of our program will be called.

The _start is always placed at the beginning of the .text section in our programs by the linked which is used default ld script:

$ ld --verbose | grep ENTRY
ENTRY(_start)

The _start function is defined in the sysdeps/x86_64/start.S assembly file and does preparation like getting argc/argv from the stack, stack preparation and etc., before the __libc_start_main function will be called. The __libc_start_main function from the csu/libc-start.c source code file does a registration of the constructor and destructor of application which are will be called before main and after it, starts up threading, does some security related actions like setting stack canary if need, calls initialization related routines and in the end it calls main function of our application and exits with its result:

result = main (argc, argv, __environ MAIN_AUXVEC_PARAM);
exit (result);

That's all.

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