我有些困惑，试图使用ptrace + seccomp获取syscall的返回值。
man 4 bpf说：
FILTER MACHINE
A filter program is an array of instructions, with all branches forwardly
directed, terminated by a return instruction
男人2 ptrace说：
PTRACE_O_TRACESECCOMP
While this triggers a PTRACE_EVENT stop, it is
similar to a syscall-enter-stop, in that the tracee has not yet
entered the syscall that seccomp triggered on. The seccomp event
message data (from the SECCOMP_RET_DATA portion of the seccomp filter
rule) can be retrieved with PTRACE_GETEVENTMSG.
PTRACE_GETEVENTMSG
For PTRACE_EVENT_SECCOMP, this is the seccomp(2)
filter's SECCOMP_RET_DATA associated with the triggered rule.
男人2 seccomp说：
SECCOMP_RET_TRACE
The tracer will be notified of a
PTRACE_EVENT_SECCOMP and the SECCOMP_RET_DATA
portion of the filter's return value will be available to
the tracer via PTRACE_GETEVENTMSG
[...]
The seccomp check will not be run again after the tracer is notified.
事实证明，在
BPF_RET语句之后，BPF程序无法执行其他操作。因此，当Tracee在SECCOMP_RET_TRACE上中断时，它处于syscall-enter-stop状态，并且尚未进行syscall，因此，返回代码绝对无处可去。我希望在随后的调用PTRACE_SYSCALL之后，tracee将处于syscall-exit-stop状态，并且tracer将能够使用PTRACE_GETEVENTMSG获得syscall的结果。但这在我的示例中不起作用。#include <linux/filter.h>
#include <linux/seccomp.h>
#include <linux/unistd.h>
#include <stddef.h>
#include <stdio.h>
#include <sys/prctl.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <unistd.h>
int main(int argc, char **argv)
{
pid_t pid;
int status;
if (argc < 2) {
fprintf(stderr, "Usage: %s <prog> <arg1> ... <argN>\n", argv[0]);
return 1;
}
if ((pid = fork()) == 0) {
ptrace(PTRACE_TRACEME, 0, 0, 0);
struct sock_filter filter[] = {
BPF_STMT(BPF_LD | BPF_W | BPF_ABS, (offsetof(struct seccomp_data, nr))),
BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_open, 1, 2),
BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_openat, 0, 1),
BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_TRACE | SECCOMP_RET_DATA),
BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),
};
struct sock_fprog prog = {
.filter = filter,
.len = (unsigned short) (sizeof(filter)/sizeof(filter[0])),
};
if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1)
return 2;
if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog) == -1)
return 3;
kill(getpid(), SIGSTOP);
return execvp(argv[1], argv + 1);
} else {
waitpid(pid, &status, 0);
ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_TRACESECCOMP);
ptrace(PTRACE_CONT, pid, 0, 0);
int status = 0;
unsigned long ret_data = 0;
while(1) {
while (1) {
waitpid(pid, &status, 0);
fprintf(stderr, "status = %08x\n", status);
if (status >> 8 == (SIGTRAP | (PTRACE_EVENT_SECCOMP << 8)))
break;
if (WIFEXITED(status))
return 0;
ptrace(PTRACE_CONT, pid, 0, 0);
}
// restart stopped tracee
ptrace(PTRACE_SYSCALL, pid, 0, 0);
// wait for SIGTRAP, when tracee will be in the syscall-exit-stop state
waitpid(pid, &status, 0);
ptrace(PTRACE_GETEVENTMSG, pid, 0, &ret_data);
fprintf(stderr, "retdat = %lu\n", ret_data);
ptrace(PTRACE_CONT, pid, 0, 0);
}
return 0;
}
}
我能够得到syscall的返回码检查寄存器
// ptrace(PTRACE_GETEVENTMSG, pid, 0, &ret_data);
struct user_regs_struct regs;
ptrace(PTRACE_GETREGS, pid, 0, ®s);
fprintf(stderr, "retdat = %lu\n", regs.rax);
但我想知道如何按照文档中指定的方式进行操作。
最佳答案
如何使用SECCOMP_RET_DATA和PTRACE_GETEVENTMSG获取系统调用的返回码？
简单的答案是你做不到。甚至在进入系统调用之前都会发送seccomp事件。您还看不到任何结果，因为还没有任何系统调用。要获得一个，必须在收到seccomp事件后使用PTRACE_SYSCALL将过程旋转两次。
bool WaitForSyscallExit(const pid_t pid)
{
bool entered = false;
int status = 0;
while (true)
{
ptrace(PTRACE_SYSCALL, pid, 0, 0);
waitpid(pid, &status, 0);
if (WSTOPSIG(status) == SIGTRAP)
{
if (entered)
{
// If we had already entered before, then current SIGTRAP signal means exiting
break;
}
entered = true;
}
else if (WIFEXITED(status) || WIFSIGNALED(status) || WCOREDUMP(status))
{
std::cerr << "The child has unexpectedly exited." << std::endl;
return false;
}
}
return true;
}
使用
PTRACE_SYSCALL时，该过程将被停止两次（第一次进入系统调用后，第二次和最后一次退出后）。您只能在系统调用实际完成后才能获取结果，因此在第二个进程停止之后即可。是的，您只能通过手动读取寄存器来执行此操作，因为seccomp结构只能在此事件的seccomp跟踪处理程序中使用。甚至结构本身也不包含与系统调用结果相关的任何内容，手册页也没有提到获取结果值。