Re: Invalid message (magic 0x6175472) [DEBUG output]

[Cameron Kerr]

On 1/02/2005, at 3:16 PM, Cameron Kerr wrote:

> Debian GNU/Linux Testing on x86, kernel 2.4.29 (and priors), Screen 
> version 4.00.02 (FAU) 5-Dec-03
>
> Hello, I'm writing a system that puts a lot of windows (about 12) into 
> a screen session. Sometimes, (ie, not all the time), I find that 
> inside the screen session, I get the following messages
>
> Unknown user /dev/pts/11 tried to send a command!
> Invalid message (magic 0x6175472)
>
> The magic in the last message changes, from what I can tell. I'm sure 
> it was 0x000000 at one time, so it seems to be some packet parsing 
> problem or similar. As for the Unknown user line, I don't know why 
> that happens, nor do I get know if its always pts/11.
>
> Anyway, the end result is that some of the windows don't show up 
> inside the screen session.
>
> Thinking it may have something to do with too many windows coming in 
> too close together and dropping off the connection queue, I increased 
> the queue length to listen() to 20, with no difference.
>
> The message is output from this piece of code in ReceiveMsg
>
>   if (m.protocol_revision != MSG_REVISION)
>     {
>       Msg(0, "Invalid message (magic 0x%08x).",
>           m.protocol_revision);
>       return;
>     }
>
> So it seems that the message is unaligned. I suspect this is due to 
> the first message, and thus there is something left in the buffer.
>
> The first message, regarding the 'Unknown user' message, is produced 
> by the following code.
>
> #ifdef MULTIUSER
>   user = *FindUserPtr(mp->m.attach.auser);
>   if (user == 0)
>     {
>       Msg(0, "Unknown user %s tried to send a command!",
>           mp->m.attach.auser);
>       return;
>     }
> #else
>
> I'm building it with -DDEBUG now, to have a closer look under the hood.

Here is what I found when compiled with -DDEBUG

Knock - knock!
Ha, there was someone knocking on my fifo??
secopen(/var/run/screen/S-cameron/974.cameron-lab99, 0x800, 0000)
Deq event fd 5 type 1 queued 1
New event fd 5 type 1 queued 0
*** RecMsg: type 8 tty /dev/pts/12
display: /dev/pts/12 display not found
but window /dev/pts/12 (backfacing)found.
FindUserPtr /dev/pts/11 not found, id -1
Msg('Unknown user /dev/pts/11 tried to send a command!') (0);

Knock - knock!
Ha, there was someone knocking on my fifo??
secopen(/var/run/screen/S-cameron/974.cameron-lab99, 0x800, 0000)
Deq event fd 5 type 1 queued 1
New event fd 5 type 1 queued 0
Msg('Invalid message (magic 0x61757472).') (0);


It would seem that the two bugs may be unrelated, but I think both may 
be race-conditions (recall they don't happen all of the time).

Hmm, that's interesting, I just noticed that the magic value in 
question always(?) seems to be 0x61757472 or 0x0000000

--
Cameron Kerr
address@hidden; http://humbledown.org