Playing with ptrace, Part II

In Part II of his series on ptrace, Pradeep tackles the more advanced topics of setting breakpoints and injecting code into running processes.

In Part I of this article [LJ, November 2002], we saw how ptrace can be used to trace system calls and change system call arguments. In this article, we investigate advanced techniques like setting breakpoints and injecting code into running programs. Debuggers use these methods to set up breakpoints and execute debugging handlers. As with Part I, all code in this article is i386 architecture-specific.

Attaching to a Running Process

In Part I, we ran the process to be traced as a child after calling ptrace(PTRACE_TRACEME, ..). If you simply wanted to see how the process is making system calls and trace the program, this would be sufficient. If you want to trace or debug a process already running, then ptrace(PTRACE_ATTACH, ..) should be used.

When a ptrace(PTRACE_ATTACH, ..) is called with the pid to be traced, it is roughly equivalent to the process calling ptrace(PTRACE_TRACEME, ..) and becoming a child of the tracing process. The traced process is sent a SIGSTOP, so we can examine and modify the process as usual. After we are done with modifications or tracing, we can let the traced process continue on its own by calling ptrace(PTRACE_DETACH, ..).

The following is the code for a small example tracing program:

int main()
{   int i;
    for(i = 0;i < 10; ++i) {
        printf("My counter: %d\n", i);
        sleep(2);
    }
    return 0;
}

Save the program as dummy2.c. Compile and run it:

gcc -o dummy2 dummy2.c
./dummy2 &
Now, we can attach to dummy2 by using the code below:
#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <linux/user.h>   /* For user_regs_struct
                             etc. */
int main(int argc, char *argv[])
{   pid_t traced_process;
    struct user_regs_struct regs;
    long ins;
    if(argc != 2) {
        printf("Usage: %s <pid to be traced>\n",
               argv[0], argv[1]);
        exit(1);
    }
    traced_process = atoi(argv[1]);
    ptrace(PTRACE_ATTACH, traced_process,
           NULL, NULL);
    wait(NULL);
    ptrace(PTRACE_GETREGS, traced_process,
           NULL, &regs);
    ins = ptrace(PTRACE_PEEKTEXT, traced_process,
                 regs.eip, NULL);
    printf("EIP: %lx Instruction executed: %lx\n",
           regs.eip, ins);
    ptrace(PTRACE_DETACH, traced_process,
           NULL, NULL);
    return 0;
}
The above program simply attaches to a process, waits for it to stop, examines its eip (instruction pointer) and detaches.

To inject code use ptrace(PTRACE_POKETEXT, ..) and ptrace(PTRACE_POKEDATA, ..) after the traced process has stopped.

Setting Breakpoints

How do debuggers set breakpoints? Generally, they replace the instruction to be executed with a trap instruction, so that when the traced program stops, the tracing program, the debugger, can examine it. It will replace the original instruction once the tracing program continues the traced process. Here's an example:

#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <linux/user.h>
const int long_size = sizeof(long);
void getdata(pid_t child, long addr,
             char *str, int len)
{   char *laddr;
    int i, j;
    union u {
            long val;
            char chars[long_size];
    }data;
    i = 0;
    j = len / long_size;
    laddr = str;
    while(i < j) {
        data.val = ptrace(PTRACE_PEEKDATA, child,
                          addr + i * 4, NULL);
        memcpy(laddr, data.chars, long_size);
        ++i;
        laddr += long_size;
    }
    j = len % long_size;
    if(j != 0) {
        data.val = ptrace(PTRACE_PEEKDATA, child,
                          addr + i * 4, NULL);
        memcpy(laddr, data.chars, j);
    }
    str[len] = '\0';
}
void putdata(pid_t child, long addr,
             char *str, int len)
{   char *laddr;
    int i, j;
    union u {
            long val;
            char chars[long_size];
    }data;
    i = 0;
    j = len / long_size;
    laddr = str;
    while(i < j) {
        memcpy(data.chars, laddr, long_size);
        ptrace(PTRACE_POKEDATA, child,
               addr + i * 4, data.val);
        ++i;
        laddr += long_size;
    }
    j = len % long_size;
    if(j != 0) {
        memcpy(data.chars, laddr, j);
        ptrace(PTRACE_POKEDATA, child,
               addr + i * 4, data.val);
    }
}
int main(int argc, char *argv[])
{   pid_t traced_process;
    struct user_regs_struct regs, newregs;
    long ins;
    /* int 0x80, int3 */
    char code[] = {0xcd,0x80,0xcc,0};
    char backup[4];
    if(argc != 2) {
        printf("Usage: %s <pid to be traced>\n",
               argv[0], argv[1]);
        exit(1);
    }
    traced_process = atoi(argv[1]);
    ptrace(PTRACE_ATTACH, traced_process,
           NULL, NULL);
    wait(NULL);
    ptrace(PTRACE_GETREGS, traced_process,
           NULL, &regs);
    /* Copy instructions into a backup variable */
    getdata(traced_process, regs.eip, backup, 3);
    /* Put the breakpoint */
    putdata(traced_process, regs.eip, code, 3);
    /* Let the process continue and execute
       the int 3 instruction */
    ptrace(PTRACE_CONT, traced_process, NULL, NULL);
    wait(NULL);
    printf("The process stopped, putting back "
           "the original instructions\n");
    printf("Press <enter> to continue\n");
    getchar();
    putdata(traced_process, regs.eip, backup, 3);
    /* Setting the eip back to the original
       instruction to let the process continue */
    ptrace(PTRACE_SETREGS, traced_process,
           NULL, &regs);
    ptrace(PTRACE_DETACH, traced_process,
           NULL, NULL);
    return 0;
}

Here we replace the three bytes with the code for a trap instruction, and when the process stops, we replace the original instructions and reset the eip to original location. Figures 1-4 clarify how the instruction stream looks when above program is executed.

Figure 1. After the Process Is Stopped

Figure 2. After the Trap Instruction Bytes Are Set

Figure 3. Trap Is Hit and Control Is Given to the Tracing Program

Figure 4. After the Original Instructions Are Replaced and eip Is Reset to the Original Location

Now that we have a clear idea of how breakpoints are set, let's inject some code bytes into a running program. These code bytes will print “hello world”.

The following program is a simple “hello world” program with modifications to fit our needs. Compile the following program with:

gcc -o hello hello.c
void main()
{
__asm__("
         jmp forward
backward:
         popl   %esi      # Get the address of
                          # hello world string
         movl   $4, %eax  # Do write system call
         movl   $2, %ebx
         movl   %esi, %ecx
         movl   $12, %edx
         int    $0x80
         int3             # Breakpoint. Here the
                          # program will stop and
                          # give control back to
                          # the parent
forward:
         call   backward
         .string \"Hello World\\n\""
       );
}

The jumping backward and forward here is required to find the address of the “hello world” string.

We can get the machine code for the above assembly from GDB. Fire up GDB and disassemble the program:

(gdb) disassemble main
Dump of assembler code for function main:
0x80483e0 <main>:       push   %ebp
0x80483e1 <main+1>:     mov    %esp,%ebp
0x80483e3 <main+3>:     jmp    0x80483fa <forward>
End of assembler dump.
(gdb) disassemble forward
Dump of assembler code for function forward:
0x80483fa <forward>:    call   0x80483e5 <backward>
0x80483ff <forward+5>:  dec    %eax
0x8048400 <forward+6>:  gs
0x8048401 <forward+7>:  insb   (%dx),%es:(%edi)
0x8048402 <forward+8>:  insb   (%dx),%es:(%edi)
0x8048403 <forward+9>:  outsl  %ds:(%esi),(%dx)
0x8048404 <forward+10>: and    %dl,0x6f(%edi)
0x8048407 <forward+13>: jb     0x8048475
0x8048409 <forward+15>: or     %fs:(%eax),%al
0x804840c <forward+18>: mov    %ebp,%esp
0x804840e <forward+20>: pop    %ebp
0x804840f <forward+21>: ret
End of assembler dump.
(gdb) disassemble backward
Dump of assembler code for function backward:
0x80483e5 <backward>:   pop    %esi
0x80483e6 <backward+1>: mov    $0x4,%eax
0x80483eb <backward+6>: mov    $0x2,%ebx
0x80483f0 <backward+11>:        mov    %esi,%ecx
0x80483f2 <backward+13>:        mov    $0xc,%edx
0x80483f7 <backward+18>:        int    $0x80
0x80483f9 <backward+20>:        int3
End of assembler dump.

We need to take the machine code bytes from main+3 to backward+20, which is a total of 41 bytes. The machine code can be seen with the x command in GDB:

(gdb) x/40bx main+3
<main+3>: eb 15 5e b8 04 00 00 00
<backward+6>: bb 02 00 00 00 89 f1 ba
<backward+14>: 0c 00 00 00 cd 80 cc
<forward+1>: e6 ff ff ff 48 65 6c 6c
<forward+9>: 6f 20 57 6f 72 6c 64 0a
Now we have the instruction bytes to be executed. Why wait? We can inject them using the same method as in the previous example. The following is the source code; only the main function is given here:
int main(int argc, char *argv[])
{   pid_t traced_process;
    struct user_regs_struct regs, newregs;
    long ins;
    int len = 41;
    char insertcode[] =
"\xeb\x15\x5e\xb8\x04\x00"
        "\x00\x00\xbb\x02\x00\x00\x00\x89\xf1\xba"
        "\x0c\x00\x00\x00\xcd\x80\xcc\xe8\xe6\xff"
        "\xff\xff\x48\x65\x6c\x6c\x6f\x20\x57\x6f"
        "\x72\x6c\x64\x0a\x00";
    char backup[len];
    if(argc != 2) {
        printf("Usage: %s <pid to be traced>\n",
               argv[0], argv[1]);
        exit(1);
    }
    traced_process = atoi(argv[1]);
    ptrace(PTRACE_ATTACH, traced_process,
           NULL, NULL);
    wait(NULL);
    ptrace(PTRACE_GETREGS, traced_process,
           NULL, &regs);
    getdata(traced_process, regs.eip, backup, len);
    putdata(traced_process, regs.eip,
            insertcode, len);
    ptrace(PTRACE_SETREGS, traced_process,
           NULL, &regs);
    ptrace(PTRACE_CONT, traced_process,
           NULL, NULL);
    wait(NULL);
    printf("The process stopped, Putting back "
           "the original instructions\n");
    putdata(traced_process, regs.eip, backup, len);
    ptrace(PTRACE_SETREGS, traced_process,
           NULL, &regs);
    printf("Letting it continue with "
           "original flow\n");
    ptrace(PTRACE_DETACH, traced_process,
           NULL, NULL);
    return 0;
}

______________________

Comments

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

Code injection doubts

sanmk's picture

Hi Pradeep,
Nice article...but I did not completely understand the code injection part.

The example you have explained inserts the code for printing "hello world" into a running process.
1. I did not exactly understand why you did the jump forward and backward steps.
Can you please elaborate on that?

2. I wrote a normal C program to print hello world:

#include
int main()
{
printf("hello world\n");
return 0;
}

I generated the byte code for this program using gdb. I replaced the contents of
char insertcode[] array with this new bytecode and ran the program.
As you might have guessed, it didn't work . What is the difference between your and my implementation?

I carried out this experiment so as to be able to inject code without having to learn assembly language programming. How do I inject the code of normal C program, without having to use assembly coding?

hello.c didn't work for me (amd64)

Anonymous's picture

I coded it into:
void main()
{
__asm__(
"jmp forward\n"
"backward:\n"
"popq %rsi\n"
"movl $4, %eax\n"
"movl $2, %ebx\n"
"movl %esi, %ecx\n"
"movl $12, %edx\n"
"int $0x80\n"
"int3\n"
"forward:\n"
"call backward\n"
".string \"Hello World\\n\"\n"

);
}

the instruction set you are

code_ninja's picture

the instruction set you are using is specific to intel's architecture, amd's architecture may differ and these instruction set will not run on amd. check out amd's manual for its instruction set

amd64 is not an x86

Anonymous's picture

he did say all sample code is for x86 only

White Paper
Linux Management with Red Hat Satellite: Measuring Business Impact and ROI

Linux has become a key foundation for supporting today's rapidly growing IT environments. Linux is being used to deploy business applications and databases, trading on its reputation as a low-cost operating environment. For many IT organizations, Linux is a mainstay for deploying Web servers and has evolved from handling basic file, print, and utility workloads to running mission-critical applications and databases, physically, virtually, and in the cloud. As Linux grows in importance in terms of value to the business, managing Linux environments to high standards of service quality — availability, security, and performance — becomes an essential requirement for business success.

Learn More

Sponsored by Red Hat

White Paper
Private PaaS for the Agile Enterprise

If you already use virtualized infrastructure, you are well on your way to leveraging the power of the cloud. Virtualization offers the promise of limitless resources, but how do you manage that scalability when your DevOps team doesn’t scale? In today’s hypercompetitive markets, fast results can make a difference between leading the pack vs. obsolescence. Organizations need more benefits from cloud computing than just raw resources. They need agility, flexibility, convenience, ROI, and control.

Stackato private Platform-as-a-Service technology from ActiveState extends your private cloud infrastructure by creating a private PaaS to provide on-demand availability, flexibility, control, and ultimately, faster time-to-market for your enterprise.

Learn More

Sponsored by ActiveState