openssh unix dev - Jun 2005 - Error when allocating PTY

hi everybody,

I'm working on an boot image and actually try to get an ssh server working
on clients, booting that
image over the net. I want to authenticate with my public key on the client,
which seems to work fine.
The only problem I have is that the sshd couldn't open an console. I've
allready tested some things,
but didn't fix it, yet. in den dev directory all tty's and pty's ()
exists but I suppose my problem
has somethinkg to do with a missing kernel configuration.
Here is the corresponding debug output of sshd:

debug1: channel 0: new [server-session]
debug1: session_new: init
debug1: session_new: session 0
debug1: session_open: channel 0
debug1: session_open: session 0: link with channel 0
debug1: server_input_channel_open: confirm session
debug1: server_input_channel_req: channel 0 request pty-req reply 0
debug1: session_by_channel: session 0 channel 0
debug1: session_input_channel_req: session 0 req pty-req
debug1: Allocating pty.
debug1: Received SIGCHLD.
openpty: Exec format error
session_pty_req: session 0 alloc failed
debug1: server_input_channel_req: channel 0 request shell reply 0
debug1: session_by_channel: session 0 channel 0
debug1: session_input_channel_req: session 0 req shell
debug1: Received SIGCHLD.

Perhaps some of you can give me a hint or even had the same problem and can tell
me how to fix it. Am I right that "openpty" is a syscall? I've
taken a look into the System.map from my kernel
but didn't find it.

regards,
Martin

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Martin Kiepfer wrote:
> I'm working on an boot image and actually try to get an ssh server
> working on clients, booting that  image over the net.
Using what OS and what libraries?  Configured OpenSSH with what options? 
  You mentioned System.map so I'm guessing the kernel is Linux but the 
rest is not specified.
> Here is the corresponding debug output of sshd:
[...]
> debug1: Allocating pty.
> debug1: Received SIGCHLD.
> openpty: Exec format error
Some platforms use a privileged helper program to set permissions and 
perform other operations on ptys that require privileges.  I suspect 
this is also the case for yours, and that the helper is missing or broken.

 > Am I right that "openpty" is a syscall?

It's usually a library function (probably in libc but possibly elsewhere).

-- 
Darren Tucker (dtucker at zip.com.au)
GPG key 8FF4FA69 / D9A3 86E9 7EEE AF4B B2D4  37C9 C982 80C7 8FF4 FA69
     Good judgement comes with experience. Unfortunately, the experience
usually comes from bad judgement.

On Wed, 8 Jun 2005, Martin Kiepfer wrote:
> hi everybody,
> 
> I'm working on an boot image and actually try to get an ssh server
working on clients, booting that
> image over the net. I want to authenticate with my public key on the
client, which seems to work fine.
> The only problem I have is that the sshd couldn't open an console.
I've allready tested some things,
> but didn't fix it, yet. in den dev directory all tty's and
pty's () exists but I suppose my problem
> has somethinkg to do with a missing kernel configuration.
> Here is the corresponding debug output of sshd:
> 
> debug1: channel 0: new [server-session]
> debug1: session_new: init
> debug1: session_new: session 0
> debug1: session_open: channel 0
> debug1: session_open: session 0: link with channel 0
> debug1: server_input_channel_open: confirm session
> debug1: server_input_channel_req: channel 0 request pty-req reply 0
> debug1: session_by_channel: session 0 channel 0
> debug1: session_input_channel_req: session 0 req pty-req
> debug1: Allocating pty.
> debug1: Received SIGCHLD.
> openpty: Exec format error
> session_pty_req: session 0 alloc failed
> debug1: server_input_channel_req: channel 0 request shell reply 0
> debug1: session_by_channel: session 0 channel 0
> debug1: session_input_channel_req: session 0 req shell
> debug1: Received SIGCHLD.
> 
> Perhaps some of you can give me a hint or even had the same problem and can
tell me how to fix it. Am I right that "openpty" is a syscall?
I've taken a look into the System.map from my kernel
> but didn't find it.
> 
The matter is that openpty is not a syscall but is a library function. Your
LIBC needs to exec some SUID helper to do chown/chmod on a pty, but it
didn't
find it. You can find that helper under /usr/libexec directory or, in some 
systems, under /lib directory, and you should copy that helper to the same
place in your boot image in order to fix your problem.
-- 

    Sincerely Your, Dan.