Accessing another process virtual memory in Linux (debugging)

Question

How does gdb access another process virtual memory on Linux? Is it all done via /proc?

Answer 1

How does gdb access another process virtual memory on Linux? Is it all done via /proc?

On Linux for reading memory:

1) If the number of bytes to read is fewer than 3 * sizeof (long) or the filesystem /proc is unavailable or reading from /proc/PID/mem is unsuccessful then ptrace is used with PTRACE_PEEKTEXT to read data.

These are these conditions in the function linux_proc_xfer_partial():

  /* Don't bother for one word.  */
  if (len < 3 * sizeof (long))
    return 0;

  /* We could keep this file open and cache it - possibly one per
     thread.  That requires some juggling, but is even faster.  */
  xsnprintf (filename, sizeof filename, "/proc/%d/mem",
         ptid_get_pid (inferior_ptid));
  fd = gdb_open_cloexec (filename, O_RDONLY | O_LARGEFILE, 0);
  if (fd == -1)
    return 0;

2) If the number of bytes to read is greater or equal to 3 * sizeof (long) and /proc is available then pread64 or (lseek() and read() are used:

static LONGEST
linux_proc_xfer_partial (struct target_ops *ops, enum target_object object,
             const char *annex, gdb_byte *readbuf,
             const gdb_byte *writebuf,
             ULONGEST offset, LONGEST len)
{
  .....

  /* If pread64 is available, use it.  It's faster if the kernel
     supports it (only one syscall), and it's 64-bit safe even on
     32-bit platforms (for instance, SPARC debugging a SPARC64
     application).  */
#ifdef HAVE_PREAD64
  if (pread64 (fd, readbuf, len, offset) != len)
#else
  if (lseek (fd, offset, SEEK_SET) == -1 || read (fd, readbuf, len) != len)
#endif
    ret = 0;
  else
    ret = len;

  close (fd);
  return ret;
}

On Linux for writing memory:

1) ptrace with PTRACE_POKETEXT or PTRACE_POKEDATA is used.

---

As for your second question:

where can I find information about ... setting hardware watchpoints

gdb, Internals Watchpoint:s http://sourceware.org/gdb/wiki/Internals%20Watchpoints

Reference:

• http://linux.die.net/man/2/ptrace
• http://www.alexonlinux.com/how-debugger-works