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TUTORIALS 》 A Linux system call in C without a standard library

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11 - Ramin Farajpour Cami, Iran, Iran ↗
LinkedinGithub 🔗
Senior Cyber Security Researcher, EDG (Engineering Development Group) of CCI (Center for Cyber Intelligence).


When we learn C, we are taught that main is the first function called in a C program. But in reality, main is simply a convention of the standard library.

root@raminfp:# cat main.c
#include <stdio.h>

int main(int argc, char* argv[])
{
    printf("hello world\n");

    return 0;
}

root@raminfp:# gcc main.c
root@raminfp:# a.out
hello world
root@raminfp:#

Now we decode a.out with gdb tools:

root@raminfp:# gdb a.out

Starting program: ~/a.out 

 [----------------------------------registers-----------------------------------]
RAX: 0x5555555546a0 (<main>:	push   rbp)
RBX: 0x0 
RCX: 0x0 
RDX: 0x7fffffffddf8 --> 0x7fffffffe1da ("LC_PAPER=fa_IR")
RSI: 0x7fffffffdde8 --> 0x7fffffffe18d ("/syscall/C_syscall_without_standard_library_linux/a.out")
RDI: 0x1 
RBP: 0x7fffffffdd00 --> 0x5555555546d0 (<__libc_csu_init>:	push   r15)
RSP: 0x7fffffffdd00 --> 0x5555555546d0 (<__libc_csu_init>:	push   r15)
RIP: 0x5555555546a4 (<main+4>:	sub    rsp,0x10)
R8 : 0x555555554740 (<__libc_csu_fini>:	repz ret)
R9 : 0x7ffff7de8bd0 (<_dl_fini>:	push   rbp)
R10: 0x10000000000 
R11: 0x7ffff7ffa19c (mov    ch,BYTE PTR [rdx])
R12: 0x555555554570 (<_start>:	xor    ebp,ebp)
R13: 0x7fffffffdde0 --> 0x1 
R14: 0x0 
R15: 0x0
EFLAGS: 0x246 (carry PARITY adjust ZERO sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x55555555469b <frame_dummy+43>:	jmp    0x5555555545e0 <register_tm_clones>
   0x5555555546a0 <main>:	push   rbp
   0x5555555546a1 <main+1>:	mov    rbp,rsp
=> 0x5555555546a4 <main+4>:	sub    rsp,0x10
   0x5555555546a8 <main+8>:	mov    DWORD PTR [rbp-0x4],edi
   0x5555555546ab <main+11>:	mov    QWORD PTR [rbp-0x10],rsi
   0x5555555546af <main+15>:	lea    rdi,[rip+0x9e]        # 0x555555554754
   0x5555555546b6 <main+22>:	call   0x555555554560
[------------------------------------stack-------------------------------------]
0000| 0x7fffffffdd00 --> 0x5555555546d0 (<__libc_csu_init>:	push   r15)
0008| 0x7fffffffdd08 --> 0x7ffff7a313f1 (<__libc_start_main+241>:	mov    edi,eax)
0016| 0x7fffffffdd10 --> 0x7ffff7dce798 --> 0x7ffff7a30d30 (<init_cacheinfo>:	push   r15)
0024| 0x7fffffffdd18 --> 0x7fffffffdde8 --> 0x7fffffffe18d ("/home/raminfp/Desktop/syscall/C_syscall_without_standard_library_linux/a.out")
0032| 0x7fffffffdd20 --> 0x1f7b9a888 
0040| 0x7fffffffdd28 --> 0x5555555546a0 (<main>:	push   rbp)
0048| 0x7fffffffdd30 --> 0x0 
0056| 0x7fffffffdd38 --> 0xdac473773e2a1848 
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value

Breakpoint 1, 0x00005555555546a4 in main ()

notice the output of gdb run command. The first function in reality is _start

0x555555554570 (<_start>:	xor    ebp,ebp)

Now if we try to compile our current code with -nostdlib gcc option, we will run into linker errors as shown below:

root@raminfp:# gcc -s -O2 -nostdlib main.c
/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000000310
/tmp/ccqHCAhy.o: In function `main':
main.c:(.text.startup+0xc): undefined reference to `puts'
collect2: error: ld returned 1 exit status

The linker is complaining about missing _start. We have a linker error on puts, which is a libc function. So how do we print "hello world" without puts?

The answer is Linux kernel exposes a bunch of syscalls (system-calls), which are functions(APIs) that user-space programs can use to interact with the OS. You find listd of syscall table: https://github.com/torvalds/ ... /syscalls/syscall_64.tbl

Lets find out which syscall uses puts. For that we can use tools strace.

root@raminfp:# cat puts.c

#include <stdio.h>

int main(int argc, char* argv[])
{
    puts("hello");

    return 0;
}

root@raminfp:# gcc puts.c
root@raminfp:# strace ./a.out > /dev/null

execve("./a.out", ["./a.out"], [/* 69 vars */]) = 0
brk(NULL)                               = 0x557f38db6000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap(NULL, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fda079d0000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=127890, ...}) = 0
mmap(NULL, 127890, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fda079b0000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\20\5\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1856752, ...}) = 0
mmap(NULL, 3959200, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fda073e8000
mprotect(0x7fda075a5000, 2097152, PROT_NONE) = 0
mmap(0x7fda077a5000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1bd000) = 0x7fda077a5000
mmap(0x7fda077ab000, 14752, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fda077ab000
close(3)                                = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fda079ae000
arch_prctl(ARCH_SET_FS, 0x7fda079ae700) = 0
mprotect(0x7fda077a5000, 16384, PROT_READ) = 0
mprotect(0x557f38ce9000, 4096, PROT_READ) = 0
mprotect(0x7fda079d3000, 4096, PROT_READ) = 0
munmap(0x7fda079b0000, 127890)          = 0
fstat(1, {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 3), ...}) = 0
ioctl(1, TCGETS, 0x7ffd9aaa7a40)        = -1 ENOTTY (Inappropriate ioctl for device)
brk(NULL)                               = 0x557f38db6000
brk(0x557f38dd8000)                     = 0x557f38dd8000
write(1, "hello world\n", 6)            = 6
exit_group(0)                           = ?
+++ exited with 0 +++

In output as shown above, we can see write(1, "hello world\n", 6). Which means it is fine to replace puts() to write() API as shown below.

root@raminfp:# whatis write
write (2)            - write to a file descriptor
write (1)            - send a message to another user

root@raminfp:# man 2 write
root@raminfp:# cat write.c 
#include <unistd.h>
#include <stdio.h>

int main(int argc, char* argv[])
{

    write(1, "hello world\n", 6);

    return 0;
}

root@raminfp:# gcc -s -O2 -nostdlib write.c 
write.c: In function ‘main’:
write.c:12:5: warning: ignoring return value of ‘write’, declared with attribute warn_unused_result [-Wunused-result]
     write(1, "hello world\n", 13);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000000310
/tmp/ccYMZ2gc.o: In function `main':
write.c:(.text.startup+0x16): undefined reference to `write'
collect2: error: ld returned 1 exit status

Oops! even the "write" function is a part of the standard library !

So we can use Calling convention https://en.wikipedia.org/wiki/Calling_convention for it: http://stackoverflow.com/questions/ ...

1. User-level applications use integer registers for passing the sequence %rdi, %rsi, %rdx, %rcx, %r8 and %r9. The kernel interface uses %rdi, %rsi, %rdx, %r10, %r8 and %r9.

2. A system-call is done via the syscall instruction. The kernel destroys registers %rcx and %r11.

3. The number of the syscall has to be passed in register %rax.

4. System-calls are limited to six arguments, no argument is passed directly on the stack.

5. Returning from the syscall, register %rax contains the result of the system-call. A value in the range between -4095 and -1 indicates an error, it is -errno.

6. Only values of class INTEGER or class MEMORY are passed to the kernel.

this will be our syscall wrapper (an Intel x86 syntax):
root@raminfp:# cat syscall.S
.intel_syntax noprefix

.text
    .globl syscall

    syscall:
        mov rax,rdi
        mov rdi,rsi
        mov rsi,rdx
        mov rdx,rcx
        mov r10,r8
        mov r8,r9
        syscall
        ret

rax = syscall number here for write is 1
rdi = param1
rsi = param2
rdx = param3
rcx = param4
r8  = param5
r9  = param6


Now we can use Assm(Assembly) and C for our new hello world program and compile the same:

root@raminfp:# cat assm_syscall.S
// Putting it all together, our _start function needs to:
// - zero rbp
// - put argc into rdi (1st parameter for main)
// - put the stack address of argv[0] into rsi (2nd param for main),
//   which will be interpreted as an array of char pointers.
// - align stack to 16-bytes
// - call main

.intel_syntax noprefix
.text
    .globl _start, syscall

    _start:
	// _start function
	
        xor rbp,rbp  /* xoring a value with itself = 0 */
        pop rdi      /* rdi = argc */
        	     /* the pop instruction already added 8 to rsp */
        mov rsi,rsp  /* rest of the stack as an array of char ptr */

        and rsp,-16
        call main    // call main function 

	// _EXIT
	// man 2 _EXIT
	mov rdi,rax /* syscall param 1 = rax (ret value of main) */
        mov rax,60 /* SYS_exit */
        syscall
	ret

    syscall:
        mov rax,rdi
        mov rdi,rsi
        mov rsi,rdx
        mov rdx,rcx
        mov r10,r8
        mov r8,r9
        syscall
        ret



root@raminfp:# cat assm_syscall.c
void* syscall(
    void* syscall_number,
    void* param1,
    void* param2,
    void* param3,
    void* param4,
    void* param5
);

typedef unsigned long int uintptr; /* size_t */
typedef long int intptr; /* ssize_t */

static
intptr write(int fd, void const* data, uintptr nbytes)
{
    return (intptr)
        syscall(
            (void*)1, /* SYS_write */
            (void*)(intptr)fd,
            (void*)data,
            (void*)nbytes,
            0, /* ignored */
            0  /* ignored */
        );
}

int main(int argc, char* argv[])
{
    write(1, "hello world\n", 13);

    return 0;
}


Now if we compile the two source file assm_syscall.S and assm_syscall.c files as shown below, we get the same output as any standard libc printf() (or write(1,...)) output as you can see below.

root@raminfp:# gcc -s -O2 -nostdlib assm_syscall.S assm_syscall.c
root@raminfp:#./a.out
hello world


So this is how you can breakdown standard libc APIs and write a C code without using the same. And if required you can write your own custom libraries based on this technique !

Here is the same I published in Github: C source code and Assembly linux system call


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