c - How to detect if the current process is being run by GDB?

Question

The standard way would be the following:

if (ptrace(PTRACE_TRACEME, 0, NULL, 0) == -1)
  printf("traced!
");

In this case ptrace returns an error if the current process is traced (i.e. running it with gdb or attaching to it).

But there is a serious problem with this: if the call returns successfully, gdb may not attach to it later. Which is a problem since I'm not trying to implement anti-debug stuff. My purpose is to emit an 'int 3' when a contition is met (i.e. an assert fails) and gdb is running (otherwise I get a SIGTRAP which stops the application).

Disabling SIGTRAP and emitting an 'int 3' every time is not a good sollution because the application I'm testing might be using SIGTRAP for some other purpose (in which case I'm still screwed, so it wouldn't matter but it's the principle of the thing :))

Thanks

Answer 1

On windows there is an API IsDebuggerPresent to check if process is under debugging. At linux, we can check this with another way (Not so efficient).

Check "/proc/self/status" for "TracerPid" attribute.

Example code:

#include <sys/stat.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <ctype.h>

bool debuggerIsAttached()
{
    char buf[4096];

    const int status_fd = ::open("/proc/self/status", O_RDONLY);
    if (status_fd == -1)
        return false;

    const ssize_t num_read = ::read(status_fd, buf, sizeof(buf) - 1);
    ::close(status_fd);

    if (num_read <= 0)
        return false;

    buf[num_read] = '';
    constexpr char tracerPidString[] = "TracerPid:";
    const auto tracer_pid_ptr = ::strstr(buf, tracerPidString);
    if (!tracer_pid_ptr)
        return false;

    for (const char* characterPtr = tracer_pid_ptr + sizeof(tracerPidString) - 1; characterPtr <= buf + num_read; ++characterPtr)
    {
        if (::isspace(*characterPtr))
            continue;
        else
            return ::isdigit(*characterPtr) != 0 && *characterPtr != '0';
    }

    return false;
}