[User-mode-linux-devel] News from the trenches

[Jeff D. <jd...@ka...>]

I've got ptrace implemented enough that strace now works.  gdb doesn't yet 
because I haven't put in code to deal with hitting breakpoints.

Having strace working gave me the ability to figure out why it was impossible 
to telnet into a uml.  It turned out that in this code, where the lock was 
being turned off, and *arg == 0, get_user was setting val so something very 
non-zero, so the pty stayed locked, and the open of the slave side failed:

/* Set the lock flag on a pty */
static int pty_set_lock(struct tty_struct *tty, int * arg)
{
	int val;
	if (get_user(val,arg))
		return -EFAULT;
	if (val)
		set_bit(TTY_PTY_LOCK, &tty->flags);
	else
		clear_bit(TTY_PTY_LOCK, &tty->flags);
	return 0;
}

get_user looked like this:

#define get_user(x, ptr) \
({ __typeof__(*(ptr)) val; \
   memcpy(&val, (ptr), sizeof(*(ptr))); \
   (x) = (__typeof__(*(ptr))) val; \
   0; \
}) 

Spotted the bug?

I stared at it for a while before figuring out that the get_user val was 
hiding the pty_set_lock val.  Ironically, the compiler had been trying to tell 
me for ages about this.  It had always complained about val maybe not being 
used and I had looked at it and decided that it didn't know what it was 
talking about.  I think one a Henry Spencer's 10 Commandments is "Study the 
pronouncements of the compiler with care".  I should have.

I fixed another bug in the network driver which fixed the problem with 
outgoing tcp connections freezing.  Now you can telnet out for as long as you 
want.

X also works.  Displaying remotely to your native X server probably has worked 
for a long time.  You can also run X locally.  Use Xnest displaying to your 
native server as the local X server and have your X apps display to :0.

				Jeff