@Enchantner, Thank to your question. I met with similar problem recently. It inspired me a lot. After fixing the error according @Jester, there is still segment error. So I spent some time digging into it. Finally, I worked it out.
The ptrace
word length for peek/poke is 64 bit, not 32, on x86-64 Linux. But the code is using unsigned
(32-bit), which will cast to long type, 64-bit, in the ptrace(2)
call.
For example, a POKETEXT of 0xaabbccdd
will actually write 0x00000000aabbccdd
to memory, assuming it happens to be zero-extended by the caller. Necessarily, it will mess up memory.
So it will work to change the type to long
, which is 64-bit in the x86-64 System V ABI.
#include <stdio.h>
#include <stdarg.h>
#include <stdlib.h>
#include <signal.h>
#include <syscall.h>
#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/reg.h>
#include <sys/user.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
void procmsg(const char* format, ...)
{
va_list ap;
fprintf(stdout, "[%d] ", getpid());
va_start(ap, format);
vfprintf(stdout, format, ap);
va_end(ap);
}
void run_target(const char* programname)
{
procmsg("target started. will run '%s'\n", programname);
/* Allow tracing of this process */
if (ptrace(PTRACE_TRACEME, 0, 0, 0) < 0) {
perror("ptrace");
return;
}
/* Replace this process's image with the given program */
execl(programname, programname, (char *)NULL);
}
void run_debugger(pid_t child_pid)
{
int wait_status;
struct user_regs_struct regs;
procmsg("debugger started\n");
/* Wait for child to stop on its first instruction */
wait(&wait_status);
/* Obtain and show child's instruction pointer */
ptrace(PTRACE_GETREGS, child_pid, 0, ®s);
procmsg("Child started. RIP = 0x%08x\n", regs.rip);
long addr = 0x004000cb;//0x004000da;
long data = ptrace(PTRACE_PEEKTEXT, child_pid, (void*)addr, 0);
procmsg("Original data at 0x%08x: 0x%08x\n", addr, data);
/* Write the trap instruction 'int 3' into the address */
long data_with_trap = (data & 0xFFFFFFFFFFFFFF00) | 0xCC;
ptrace(PTRACE_POKETEXT, child_pid, (void*)addr, (void*)data_with_trap);
/* See what's there again... */
long readback_data = ptrace(PTRACE_PEEKTEXT, child_pid, (void*)addr, 0);
procmsg("After trap, data at 0x%08x: 0x%08x\n", addr, readback_data);
/* Let the child run to the breakpoint and wait for it to
** reach it
*/
ptrace(PTRACE_CONT, child_pid, 0, 0);
wait(&wait_status);
if (WIFSTOPPED(wait_status)) {
procmsg("Child got a signal: %s\n", strsignal(WSTOPSIG(wait_status)));
}
else {
perror("wait");
return;
}
/* See where the child is now */
ptrace(PTRACE_GETREGS, child_pid, 0, ®s);
procmsg("Child stopped at RIP = 0x%08x\n", regs.rip);
/* Remove the breakpoint by restoring the previous data
** at the target address, and unwind the EIP back by 1 to
** let the CPU execute the original instruction that was
** there.
*/
ptrace(PTRACE_POKETEXT, child_pid, (void*)addr, (void*)data);
/* See what's there again... */
readback_data = ptrace(PTRACE_PEEKTEXT, child_pid, (void*)addr, 0);
procmsg("After restore, data at 0x%08x: 0x%08x\n", addr, readback_data);
regs.rip -= 1;
ptrace(PTRACE_SETREGS, child_pid, 0, ®s);
/* The child can continue running now */
ptrace(PTRACE_CONT, child_pid, 0, 0);
wait(&wait_status);
if (WIFEXITED(wait_status)) {
procmsg("Child exited\n");
} else if(WIFSIGNALED(wait_status)) {
procmsg("signal !!!\n");
}
else {
procmsg("Unexpected signal. %s \n", strsignal(WSTOPSIG(wait_status)));
}
}
int main(int argc, char** argv)
{
pid_t child_pid;
if (argc < 2) {
fprintf(stderr, "Expected a program name as argument\n");
return -1;
}
child_pid = fork();
if (child_pid == 0)
run_target(argv[1]);
else if (child_pid > 0)
run_debugger(child_pid);
else {
perror("fork");
return -1;
}
return 0;
}
In addition, you must modify the long addr = 0x004000cb;//0x004000da;
according your compiled object file. You see it's different in my Ubuntu system with the guide.
________________divider______________________________
It's my inference above all and shows indeed that it solve the problem. But I wanna test more to confirm the conclusion. This is my test code.
the objdump -d hello2
output
hello2: file format elf64-x86-64
Disassembly of section .text:
00000000004000b0 <.text>:
4000b0: b8 01 00 00 00 mov $0x1,%eax
4000b5: bf 01 00 00 00 mov $0x1,%edi
4000ba: 48 be f4 00 60 00 00 movabs $0x6000f4,%rsi
4000c1: 00 00 00
4000c4: ba 08 00 00 00 mov $0x8,%edx
4000c9: 0f 05 syscall
4000cb: b8 01 00 00 00 mov $0x1,%eax
4000d0: bf 01 00 00 00 mov $0x1,%edi
4000d5: 48 be fc 00 60 00 00 movabs $0x6000fc,%rsi
4000dc: 00 00 00
4000df: ba 07 00 00 00 mov $0x7,%edx
4000e4: 0f 05 syscall
4000e6: b8 3c 00 00 00 mov $0x3c,%eax
4000eb: bf 00 00 00 00 mov $0x0,%edi
4000f0: 0f 05 syscall
The debugger.c content
#include <stdio.h>
#include <stdarg.h>
#include <stdlib.h>
#include <signal.h>
#include <syscall.h>
#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/reg.h>
#include <sys/user.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
void procmsg(const char* format, ...)
{
va_list ap;
fprintf(stdout, "[%d] ", getpid());
va_start(ap, format);
vfprintf(stdout, format, ap);
va_end(ap);
}
void run_target(const char* programname)
{
procmsg("target started. will run '%s'\n", programname);
/* Allow tracing of this process */
if (ptrace(PTRACE_TRACEME, 0, 0, 0) < 0) {
perror("ptrace");
return;
}
/* Replace this process's image with the given program */
execl(programname, programname, (char *)NULL);
}
void run_debugger(pid_t child_pid)
{
int wait_status;
struct user_regs_struct regs;
procmsg("debugger started\n");
/* Wait for child to stop on its first instruction */
wait(&wait_status);
/* Obtain and show child's instruction pointer */
ptrace(PTRACE_GETREGS, child_pid, 0, ®s);
procmsg("Child started. RIP = 0x%016x\n", regs.rip);
long addr = 0x004000cb;//0x004000da;
unsigned data = ptrace(PTRACE_PEEKTEXT, child_pid, (void*)addr, 0);
procmsg("Original data at 0x%016x: 0x%016x\n", addr, data);
//test
unsigned data_u = data;
procmsg("test data_u: 0x%08x\n", data_u);
/* Write the trap instruction 'int 3' into the address */
long data_with_trap = (data & 0xFFFFFFFFFFFFFF00) | 0xCC;
ptrace(PTRACE_POKETEXT, child_pid, (void*)addr, (void*)data_with_trap);
/* See what's there again... */
long readback_data = ptrace(PTRACE_PEEKTEXT, child_pid, (void*)addr, 0);
procmsg("After trap,set breakpoint, data at 0x%016x: 0x%016x\n", addr, readback_data);
/* Let the child run to the breakpoint and wait for it to
** reach it
*/
ptrace(PTRACE_CONT, child_pid, 0, 0);
//wait to breakpoint
wait(&wait_status);
if (WIFSTOPPED(wait_status)) {
procmsg("Child got a signal: %s\n", strsignal(WSTOPSIG(wait_status)));
}
else {
perror("wait");
return;
}
/* See where the child is now */
ptrace(PTRACE_GETREGS, child_pid, 0, ®s);
procmsg("Child stopped at RIP = 0x%016x\n", regs.rip);
/* Remove the breakpoint by restoring the previous data
** at the target address, and unwind the EIP back by 1 to
** let the CPU execute the original instruction that was
** there.
*/
int result = ptrace(PTRACE_POKETEXT, child_pid, (void*)addr, (void*)data);
procmsg("poketext origin back result %d\n", result);
/* See what's there again... */
readback_data = ptrace(PTRACE_PEEKTEXT, child_pid, (void*)addr, 0);
procmsg("After restore, data at 0x%llx: 0x%llx\n", addr, readback_data);
int offset = 0;
for(offset = 1; offset < 25; offset++) {
readback_data = ptrace(PTRACE_PEEKTEXT, child_pid, (void*)(addr+offset), 0);
procmsg("After restore, data at 0x%llx: 0x%llx\n", addr+offset, readback_data);
}
regs.rip -= 1;
ptrace(PTRACE_SETREGS, child_pid, 0, ®s);
/* The child can continue running now */
ptrace(PTRACE_CONT, child_pid, 0, 0);
wait(&wait_status);
if (WIFEXITED(wait_status)) {
procmsg("Child exited\n");
} else if(WIFSIGNALED(wait_status)) {
procmsg("signal !!!\n");
}
else {
procmsg("Unexpected signal. %s \n", strsignal(WSTOPSIG(wait_status)));
}
}
int main(int argc, char** argv)
{
pid_t child_pid;
if (argc < 2) {
fprintf(stderr, "Expected a program name as argument\n");
return -1;
}
child_pid = fork();
if (child_pid == 0)
run_target(argv[1]);
else if (child_pid > 0)
run_debugger(child_pid);
else {
perror("fork");
return -1;
}
return 0;
}
The ./debugger.o hello2
output
[20832] debugger started
[20833] target started. will run 'hello2'
[20832] Child started. RIP = 0x00000000004000b0
[20832] Original data at 0x00000000004000cb: 0x00000000000001b8
[20832] test data_u: 0x000001b8
[20832] After trap,set breakpoint, data at 0x00000000004000cb: 0x00000000000001cc
Hello,
[20832] Child got a signal: Trace/breakpoint trap
[20832] Child stopped at RIP = 0x00000000004000cc
[20832] poketext origin back result 0
[20832] After restore, data at 0x4000cb: 0x1b8
[20832] After restore, data at 0x4000cc: 0x1
[20832] After restore, data at 0x4000cd: 0x0
[20832] After restore, data at 0x4000ce: 0x4800000000000000
[20832] After restore, data at 0x4000cf: 0xbe48000000000000
[20832] After restore, data at 0x4000d0: 0xfcbe480000000000
[20832] After restore, data at 0x4000d1: 0xfcbe4800000000
[20832] After restore, data at 0x4000d2: 0x6000fcbe48000000
[20832] After restore, data at 0x4000d3: 0x6000fcbe480000
[20832] After restore, data at 0x4000d4: 0x6000fcbe4800
[20832] After restore, data at 0x4000d5: 0x6000fcbe48
[20832] After restore, data at 0x4000d6: 0x6000fcbe
[20832] After restore, data at 0x4000d7: 0x6000fc
[20832] After restore, data at 0x4000d8: 0xba00000000006000
[20832] After restore, data at 0x4000d9: 0x7ba000000000060
[20832] After restore, data at 0x4000da: 0x7ba0000000000
[20832] After restore, data at 0x4000db: 0x7ba00000000
[20832] After restore, data at 0x4000dc: 0x7ba000000
[20832] After restore, data at 0x4000dd: 0xf00000007ba0000
[20832] After restore, data at 0x4000de: 0x50f00000007ba00
[20832] After restore, data at 0x4000df: 0xb8050f00000007ba
[20832] After restore, data at 0x4000e0: 0x3cb8050f00000007
[20832] After restore, data at 0x4000e1: 0x3cb8050f000000
[20832] After restore, data at 0x4000e2: 0x3cb8050f0000
[20832] After restore, data at 0x4000e3: 0x3cb8050f00
[20832] Unexpected signal. Segmentation fault
Clearly, I just make long to unsigned and write back and print in a loop.
......
unsigned data = ptrace(PTRACE_PEEKTEXT, child_pid, (void*)addr, 0);
procmsg("After restore, data at 0x%llx: 0x%llx\n", addr, readback_data);
......
int offset = 0;
for(offset = 1; offset < 25; offset++) {
readback_data = ptrace(PTRACE_PEEKTEXT, child_pid, (void*)(addr+offset), 0);
procmsg("After restore, data at 0x%llx: 0x%llx\n", addr+offset, readback_data);
}
.....
At last, the output,
[20832] After restore, data at 0x4000d0: 0xfcbe480000000000
,
shows the instruction in memory
4000d0: bf 01 00 00 00 mov $0x1,%edi
is written to zero. That causes the Segmentation fault
as expected.
I am satisfied with the answer by now. ~~