Static build segfaults on x86_64

[Darren Tucker]

Bostjan Skufca wrote:
> Hello everyone,
> 
> I would like to ask you for advice on how to approach (or solve) this
> particular problem.
> 
> I use Slackware Linux and compile Openssh from source. I prefer to
> compile it statically so it doesn't get messed up if I update openssl
> libraries. Up until now this approach was working OK for me.

As an aside: you don't need to statically link the whole binary just to 
get the crypto libs.  If you build OpenSSL with just the static library 
(libcrypto.a) and configure OpenSSH --with-ssl-dir=thatdir then the 
linker will pick up the crypto functions from the static library and the 
remainder from the system dynamic libraries.

> Lately I have been challenged with Slackware64 installations and I
> have come across a problem with Openssh version (5.3p1, but result is
> the same with 5.2p1). What happens is that sshd daemon keeps accepting
> connections as long as no one disconnects. On the first DISconnection
> the server daemon dies with segfault message, of which a strace output
> I have included below. 

Unfortunately the strace does not show anything useful, but from your 
description it sounds like it's crashing in the SIGCHLD handler, 
although I don't know why it would show up only with static linking.

You might get something more useful from enabling sshd's debugging, eg

# /path/to/sshd -o LogLevel=debug3 -D

[...]
> How should I start solving this problem? I am proficient in PHP and
> other Untyped languages but I am only moderately familiar with C
> programming.

Did it generate a core dump?  if so, your first step is to feed it into 
gdb and generate a backtrace, which will tell you where it crashed.  You do:

$ gdb /path/to/sshd core
(gdb) bt

If it doesn't generate a core dump (eg because the ulimit prevents it) 
then you can attach gdb to the process you expect to crash, wait for it 
to crash then generate the backtrace

$ gdb /path/to/sshd [pid of sshd]
[wait for crash]
(gdb) bt

-- 
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.