ptrace 在 Linux 中是如何工作的？

Question

ptrace 系统调用允许父进程检查附加的子进程。例如，在 Linux 中，strace（通过ptrace 系统调用实现）可以检查子进程调用的系统调用。

当附加的子进程调用系统调用时，可以通知 ptracing 父进程。但是这到底是怎么发生的呢？我想知道这个机制背后的技术细节。

提前谢谢你。

Answer 1

当附加的子进程调用系统调用时，可以通知 ptracing 父进程。但这究竟是怎么发生的呢？

父进程使用PTRACE_ATTACH 调用ptrace，他的子进程使用PTRACE_TRACEME 选项调用ptrace。这对将通过在它们的task_struct（kernel/ptrace.c: sys_ptrace 中填充一些字段来连接两个进程，子进程将在struct task_struct 的ptrace 字段中具有PT_PTRACED 标志，并将ptracer 进程的pid 作为父进程和ptrace_entry 列表- __ptrace_link;父母将在ptraced列表中记录孩子的pid。

然后 strace 将调用带有 PTRACE_SYSCALL 标志的 ptrace 以将自己注册为系统调用调试器，在子进程的 struct thread_info 中设置 thread_flag TIF_SYSCALL_TRACE（通过 set_tsk_thread_flag(child, TIF_SYSCALL_TRACE); 之类的东西）。 arch/x86/include/asm/thread_info.h:

 67 /*
 68  * thread information flags
 69  * - these are process state flags that various assembly files
 70  *   may need to access   ...*/

 75 #define TIF_SYSCALL_TRACE       0       /* syscall trace active */
 99 #define _TIF_SYSCALL_TRACE      (1 << TIF_SYSCALL_TRACE)

在每个系统调用进入或退出时，特定于体系结构的系统调用入口代码将检查这个_TIF_SYSCALL_TRACE flag（直接在系统调用的汇编器实现中，例如x86 arch/x86/kernel/entry_32.S：ENTRY(system_call) 中的jnz syscall_trace_entry 和@987654327 中的类似代码@)，如果设置了，ptracer 将收到信号 (SIGTRAP) 通知，并且 child 将被暂时停止。这通常在 syscall_trace_enter 和 syscall_trace_leave 中完成：

1457 long syscall_trace_enter(struct pt_regs *regs)

1483         if ((ret || test_thread_flag(TIF_SYSCALL_TRACE)) &&
1484             tracehook_report_syscall_entry(regs))
1485                 ret = -1L;

1507 void syscall_trace_leave(struct pt_regs *regs)

1531         if (step || test_thread_flag(TIF_SYSCALL_TRACE))
1532                 tracehook_report_syscall_exit(regs, step);

tracehook_report_syscall_* 是这里的实际工作人员，他们会调用ptrace_report_syscall。 include/linux/tracehook.h:

 80 /**
 81  * tracehook_report_syscall_entry - task is about to attempt a system call
 82  * @regs:               user register state of current task
 83  *
 84  * This will be called if %TIF_SYSCALL_TRACE has been set, when the
 85  * current task has just entered the kernel for a system call.
 86  * Full user register state is available here.  Changing the values
 87  * in @regs can affect the system call number and arguments to be tried.
 88  * It is safe to block here, preventing the system call from beginning.
 89  *
 90  * Returns zero normally, or nonzero if the calling arch code should abort
 91  * the system call.  That must prevent normal entry so no system call is
 92  * made.  If @task ever returns to user mode after this, its register state
 93  * is unspecified, but should be something harmless like an %ENOSYS error
 94  * return.  It should preserve enough information so that syscall_rollback()
 95  * can work (see asm-generic/syscall.h).
 96  *
 97  * Called without locks, just after entering kernel mode.
 98  */
 99 static inline __must_check int tracehook_report_syscall_entry(
100         struct pt_regs *regs)
101 {
102         return ptrace_report_syscall(regs);
103 }
104 
105 /**
106  * tracehook_report_syscall_exit - task has just finished a system call
107  * @regs:               user register state of current task
108  * @step:               nonzero if simulating single-step or block-step
109  *
110  * This will be called if %TIF_SYSCALL_TRACE has been set, when the
111  * current task has just finished an attempted system call.  Full
112  * user register state is available here.  It is safe to block here,
113  * preventing signals from being processed.
114  *
115  * If @step is nonzero, this report is also in lieu of the normal
116  * trap that would follow the system call instruction because
117  * user_enable_block_step() or user_enable_single_step() was used.
118  * In this case, %TIF_SYSCALL_TRACE might not be set.
119  *
120  * Called without locks, just before checking for pending signals.
121  */
122 static inline void tracehook_report_syscall_exit(struct pt_regs *regs, int step)
123 {
...
130 
131         ptrace_report_syscall(regs);
132 }

和ptrace_report_syscall generates SIGTRAP 用于调试器或通过ptrace_notify/ptrace_do_notify 进行跟踪：

 55 /*
 56  * ptrace report for syscall entry and exit looks identical.
 57  */
 58 static inline int ptrace_report_syscall(struct pt_regs *regs)
 59 {
 60         int ptrace = current->ptrace;
 61 
 62         if (!(ptrace & PT_PTRACED))
 63                 return 0;
 64 
 65         ptrace_notify(SIGTRAP | ((ptrace & PT_TRACESYSGOOD) ? 0x80 : 0));
 66 
 67         /*
 68          * this isn't the same as continuing with a signal, but it will do
 69          * for normal use.  strace only continues with a signal if the
 70          * stopping signal is not SIGTRAP.  -brl
 71          */
 72         if (current->exit_code) {
 73                 send_sig(current->exit_code, current, 1);
 74                 current->exit_code = 0;
 75         }
 76 
 77         return fatal_signal_pending(current);
 78 }

ptrace_notify 在kernel/signal.c 中实现，它停止子进程并将 sig_info 传递给 ptracer：

1961 static void ptrace_do_notify(int signr, int exit_code, int why)
1962 {
1963         siginfo_t info;
1964 
1965         memset(&info, 0, sizeof info);
1966         info.si_signo = signr;
1967         info.si_code = exit_code;
1968         info.si_pid = task_pid_vnr(current);
1969         info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
1970 
1971         /* Let the debugger run.  */
1972         ptrace_stop(exit_code, why, 1, &info);
1973 }
1974 
1975 void ptrace_notify(int exit_code)
1976 {
1977         BUG_ON((exit_code & (0x7f | ~0xffff)) != SIGTRAP);
1978         if (unlikely(current->task_works))
1979                 task_work_run();
1980 
1981         spin_lock_irq(&current->sighand->siglock);
1982         ptrace_do_notify(SIGTRAP, exit_code, CLD_TRAPPED);
1983         spin_unlock_irq(&current->sighand->siglock);
1984 }

ptrace_stop 在同一个 signal.c 文件中，3.13 的第 1839 行。