我试图将嵌入式 Linux 系统的 init 进程强制到exec()
我自己的 init 程序 (systemd),以便我可以在将外部文件系统写入系统闪存之前对其进行测试(并冒着设备变砖的风险)。使用 GDB,我可以运行命令gdb --pid=1
,然后在该 shell 类型中运行call execl("/lib/systemd/systemd", "systemd", 0)
(它完全按照我需要的方式工作),但我没有足够的空间将 GDB 放在系统的闪存上。
我想知道ptrace()
调用 GDB 的命令到底使用了什么call
,以便我可以在自己的简单 C 程序中实现它。
我试图strace
找出ptrace()
GDB 使用的调用内容,但生成的文件有 172,031 行长。我也尝试查看其源代码,但文件太多,无法找到我要查找的内容。
该设备运行的是 Linux 内核版本 3.10.0,配置可在此处获取:https://pastebin.com/rk0Zux62
答案1
这是一个可以完成此操作的 C 程序。请注意一些已知问题:
- 可能应该使用 memcpy 而不是严格的别名违规
- 使用自己的环境变量而不是旧的tracee的环境变量
- 如果被追踪者不进行任何系统调用,这将永远无法做任何事情
- 不检查被跟踪者何时停止以确保它确实是系统调用停止而不是信号停止或其他什么
- 应该在 syscall-exit-stop 而不是 syscall-enter-stop 中重新设置 IP
- 不对 execve 参数进行任何健全性检查(这样做对于 execveat 来说是一个很好的机会)
- 完全不可移植(
CONFIG_ARM_THUMB
在许多其他事情中进行硬编码) - 使进程处于一种状态,如果任何系统调用无法正常工作,进程可能会崩溃
编译它-fno-strict-aliasing
,然后运行它./a.out 1 /lib/systemd/systemd systemd
。
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ptrace.h>
#include <linux/ptrace.h>
#include <sys/wait.h>
#include <sys/user.h>
#include <sys/syscall.h>
#include <sys/mman.h>
#define CONFIG_ARM_THUMB
#ifdef CONFIG_ARM_THUMB
#define thumb_mode(regs) \
(((regs)->ARM_cpsr & PSR_T_BIT))
#else
#define thumb_mode(regs) (0)
#endif
extern char **environ;
static pid_t pid;
/* The length of a string, plus the null terminator, rounded up to the nearest sizeof(long). */
size_t str_size(char *str) {
size_t len = strlen(str);
return len + sizeof(long) - len % sizeof(long);
}
void must_poke(long addr, long data) {
if(ptrace(PTRACE_POKEDATA, pid, (void*)addr, (void*)data)) {
perror("ptrace(PTRACE_POKEDATA, ...)");
exit(1);
}
}
void must_poke_multi(long addr, long* data, size_t len) {
size_t i;
for(i = 0; i < len; ++i) {
must_poke(addr + i * sizeof(long), data[i]);
}
}
long must_poke_string(long addr, char* str) {
size_t len = str_size(str);
size_t longs_len = len / sizeof(long);
char *more_nulls_str = malloc(len);
memset(more_nulls_str + len - sizeof(long), 0, sizeof(long)); /* initialize the bit we might not copy over */
strcpy(more_nulls_str, str);
must_poke_multi(addr, (long*)more_nulls_str, longs_len);
free(more_nulls_str);
return addr + len;
}
int main(int argc, char** argv) {
struct user_regs regs;
int i, envc;
unsigned long mmap_base;
size_t mmap_string_offset, mmap_argv_offset, mmap_envp_offset;
size_t mmap_len = 2 * sizeof(char*); /* for the NULLs at the end of argv and envp */
if(argc < 3) {
fprintf(stderr, "Usage: %s <pid> <executable image> <args...>\n", argv[0]);
return 1;
}
pid = strtol(argv[1], NULL, 10);
/* for the image name */
mmap_len += str_size(argv[2]);
for(i = 3; i < argc; ++i) {
/* for the pointer in argv plus the string itself */
mmap_len += sizeof(char*) + str_size(argv[i]);
}
for(i = 0; environ[i]; ++i) {
/* for the pointer in envp plus the string itself */
mmap_len += sizeof(char*) + str_size(environ[i]);
}
envc = i;
if(ptrace(PTRACE_ATTACH, pid, 0, 0)) {
perror("ptrace(PTRACE_ATTACH, ...)");
return 1;
}
if(waitid(P_PID, pid, NULL, WSTOPPED)) {
perror("waitid");
return 1;
}
/* Stop at whatever syscall happens to be next */
if(ptrace(PTRACE_SYSCALL, pid, 0, 0)) {
perror("ptrace(PTRACE_SYSCALL, ...)");
return 1;
}
printf("Waiting for the target process to make a syscall...\n");
if(waitid(P_PID, pid, NULL, WSTOPPED)) {
perror("waitid");
return 1;
}
printf("Target made a syscall. Proceeding with injection.\n");
if(ptrace(PTRACE_GETREGS, pid, 0, ®s)) {
perror("ptrace(PTRACE_GETREGS, ...)");
return 1;
}
/* End up back on the syscall instruction so we can use it again */
regs.ARM_pc -= (thumb_mode(®s) ? 2 : 4);
/* mmap some space for the exec parameters */
regs.ARM_r0 = (long)0;
regs.ARM_r1 = (long)mmap_len;
regs.ARM_r2 = (long)(PROT_READ|PROT_WRITE);
regs.ARM_r3 = (long)(MAP_PRIVATE|MAP_ANONYMOUS);
regs.ARM_r4 = (long)-1;
regs.ARM_r5 = (long)0;
if(ptrace(PTRACE_SETREGS, pid, 0, ®s)) {
perror("ptrace(PTRACE_SETREGS, ...)");
return 1;
}
if(ptrace(PTRACE_SET_SYSCALL, pid, 0, SYS_mmap2)) {
perror("ptrace(PTRACE_SET_SYSCALL, ...)");
return 1;
}
/* jump to the end of the syscall */
if(ptrace(PTRACE_SYSCALL, pid, 0, 0)) {
perror("ptrace(PTRACE_SYSCALL, ...)");
return 1;
}
if(waitid(P_PID, pid, NULL, WSTOPPED)) {
perror("waitid");
return 1;
}
/* make sure it worked and get the memory address */
if(ptrace(PTRACE_GETREGS, pid, 0, ®s)) {
perror("ptrace(PTRACE_GETREGS, ...)");
return 1;
}
if(regs.ARM_r0 > -4096UL) {
errno = -regs.ARM_r0;
perror("traced process: mmap");
return 1;
}
mmap_base = regs.ARM_r0;
/* set up the execve args in memory */
mmap_argv_offset = must_poke_string(mmap_base, argv[2]);
mmap_string_offset = mmap_argv_offset + (argc - 2) * sizeof(char*); /* don't forget the null pointer */
for(i = 0; i < argc - 3; ++i) {
must_poke(mmap_argv_offset + i * sizeof(char*), mmap_string_offset);
mmap_string_offset = must_poke_string(mmap_string_offset, argv[i + 3]);
}
must_poke(mmap_argv_offset + (argc - 3) * sizeof(char*), 0);
mmap_envp_offset = mmap_string_offset;
mmap_string_offset = mmap_envp_offset + (envc + 1) * sizeof(char*); /* don't forget the null pointer */
for(i = 0; i < envc; ++i) {
must_poke(mmap_envp_offset + i * sizeof(char*), mmap_string_offset);
mmap_string_offset = must_poke_string(mmap_string_offset, environ[i]);
}
must_poke(mmap_envp_offset + envc * sizeof(char*), 0);
/* jump to the start of the next syscall (same PC since we reset it) */
if(ptrace(PTRACE_SYSCALL, pid, 0, 0)) {
perror("ptrace(PTRACE_SYSCALL, ...)");
return 1;
}
if(waitid(P_PID, pid, NULL, WSTOPPED)) {
perror("waitid");
return 1;
}
if(ptrace(PTRACE_GETREGS, pid, 0, ®s)) {
perror("ptrace(PTRACE_GETREGS, ...)");
return 1;
}
/* call execve */
regs.ARM_r0 = (long)mmap_base;
regs.ARM_r1 = (long)mmap_argv_offset;
regs.ARM_r2 = (long)mmap_envp_offset;
if(ptrace(PTRACE_SETREGS, pid, 0, ®s)) {
perror("ptrace(PTRACE_SETREGS, ...)");
return 1;
}
if(ptrace(PTRACE_SET_SYSCALL, pid, 0, SYS_execve)) {
perror("ptrace(PTRACE_SET_SYSCALL, ...)");
return 1;
}
/* and done. */
if(ptrace(PTRACE_DETACH, pid, 0, 0)) {
perror("ptrace(PTRACE_DETACH, ...)");
return 1;
}
return 0;
}
我通过 qemu-system-arm 使用 3.2.0-4 内核和 wheezy 用户区开发并测试了它https://people.debian.org/~aurel32/qemu/armel/