我正在将mprotect调用注入到跟踪的进程中：

static int inject_mprotect(pid_t child, void *addr, size_t len, int prot)
{
    // Machine code:
    //  int $0x80       (system call)
    //  int3            (trap)
    char code[] = {0xcd,0x80,0xcc,0};
    char orig_code[3];
    struct user_regs_struct regs;
    struct user_regs_struct orig_regs;

    // Take a copy of current state
    __check_ptrace(PTRACE_GETREGS, child, NULL, &orig_regs);
    getdata(child, INSTRUCTION_POINTER(regs), orig_code, 3);

    // Inject the code, update registers
    putdata(child, INSTRUCTION_POINTER(regs), code, 3);
    __check_ptrace(PTRACE_GETREGS, child, NULL, &regs);
    XAX_REGISTER(regs) = MPROTECT_SYSCALL;
    MPROTECT_ARG_START(regs) = (unsigned long)addr;
    MPROTECT_ARG_LEN(regs) = len;
    MPROTECT_ARG_PROT(regs) = prot;   
    __check_ptrace(PTRACE_SETREGS, child, NULL, &regs);

    // Snip

但是,呼叫失败,返回-14(EFAULT).我浏览了mprotect源(内核3.13),看不到为什么我的系统调用会返回此值.

如果我跟踪注入的呼叫并打印出寄存器,则会看到以下内容：

SIGTRAP: eip: 0x34646ef8d4, syscall 10, rc = -38
PARENT 10 MPROTECT(start: 0x00007f45b9611000, len: 4096, prot: 0)
EIP: 0x00000034646ef8d4 AX: 0xffffffffffffffda  BX: 0x0000000000000005  CX: 0xffffffffffffffff
 DX: 0x0000000000000000 DI: 0x00007f45b9611000  BP: 0x00007fffcb93bc20  SI: 0x0000000000001000
 R8: 0x0000000000000000 R9: 0x0000000000000000  R10: 0x0000000000000000
SIGTRAP: eip: 0x34646ef8d4, syscall 10, rc = -14 Bad address (trap after system call exit)

为了验证系统调用格式,我向子级添加了mprotect调用,并转储了其参数和寄存器：

SIGTRAP: eip: 0x34646ef927, syscall 10, rc = -38
CHILD  10 MPROTECT(start: 0x00007f45b9611000, len: 4096, prot: 0)
EIP: 0x00000034646ef927 AX: 0xffffffffffffffda  BX: 0x0000000000000005  CX: 0xffffffffffffffff
 DX: 0x0000000000000000 DI: 0x00007f45b9611000  BP: 0x00007fffcb93bc20  SI: 0x0000000000001000
 R8: 0x000000000000004e R9: 0x746f72706d206c6c  R10: 0x00007fffcb93b9a0
SIGTRAP (child return): eip: 0x34646ef927, syscall 10, rc = 0

孩子的电话成功.因此,考虑到我要使用相同的参数进行相同的系统调用(10),为什么在子级调用成功的同时,注入的调用却因EFAULT而失败？

调用之间的唯一区别是regs.r8,regs.r9和regs.r10中有一些垃圾,但是基于this table of system calls on X86_64,我不认为这些寄存器的内容会影响系统调用.

解决方法:

问题与this question有关：i386和x86_64对系统调用使用不同的调用约定.您的示例代码使用int 0x80(i386变体),但syscall_number = 10(mprotect的64位系统调用号).根据this list,在32位环境中,syscall 10内核会响应取消链接,该链接可以返回EFAULT(错误地址).

在64位平台上,以一致的方式使用32位或64位变体可以解决此问题.