<chapter id="tracing">
<chapterinfo>
<author>
<firstname>Andrew</firstname><surname>Tridgell</surname>
<affiliation>
<orgname>Samba Team</orgname>
</affiliation>
</author>
</chapterinfo>
<title>Tracing samba system calls</title>
<para>
This file describes how to do a system call trace on Samba to work out
what its doing wrong. This is not for the faint of heart, but if you
are reading this then you are probably desperate.
</para>
<para>
Actually its not as bad as the the above makes it sound, just don't
expect the output to be very pretty :-)
</para>
<para>
Ok, down to business. One of the big advantages of unix systems is
that they nearly all come with a system trace utility that allows you
to monitor all system calls that a program is making. This is
extremely using for debugging and also helps when trying to work out
why something is slower than you expect. You can use system tracing
without any special compilation options.
</para>
<para>
The system trace utility is called different things on different
systems. On Linux systems its called strace. Under SunOS 4 its called
trace. Under SVR4 style systems (including solaris) its called
truss. Under many BSD systems its called ktrace.
</para>
<para>
The first thing you should do is read the man page for your native
system call tracer. In the discussion below I'll assume its called
strace as strace is the only portable system tracer (its available for
free for many unix types) and its also got some of the nicest
features.
</para>
<para>
Next, try using strace on some simple commands. For example, <command>strace
ls</command> or <command>strace echo hello</command>.
</para>
<para>
You'll notice that it produces a LOT of output. It is showing you the
arguments to every system call that the program makes and the
result. Very little happens in a program without a system call so you
get lots of output. You'll also find that it produces a lot of
"preamble" stuff showing the loading of shared libraries etc. Ignore
this (unless its going wrong!)
</para>
<para>
For example, the only line that really matters in the <command>strace echo
hello</command> output is:
</para>
<para><programlisting>
write(1, "hello\n", 6) = 6
</programlisting></para>
<para>all the rest is just setting up to run the program.</para>
<para>
Ok, now you're familiar with strace. To use it on Samba you need to
strace the running smbd daemon. The way I tend ot use it is to first
login from my Windows PC to the Samba server, then use smbstatus to
find which process ID that client is attached to, then as root I do
<command>strace -p PID</command> to attach to that process. I normally redirect the
stderr output from this command to a file for later perusal. For
example, if I'm using a csh style shell:
</para>
<para><command>strace -f -p 3872 >& strace.out</command></para>
<para>or with a sh style shell:</para>
<para><command>strace -f -p 3872 > strace.out 2>&1</command></para>
<para>
Note the "-f" option. This is only available on some systems, and
allows you to trace not just the current process, but any children it
forks. This is great for finding printing problems caused by the
"print command" being wrong.
</para>
<para>
Once you are attached you then can do whatever it is on the client
that is causing problems and you will capture all the system calls
that smbd makes.
</para>
<para>
So how do you interpret the results? Generally I search through the
output for strings that I know will appear when the problem
happens. For example, if I am having touble with permissions on a file
I would search for that files name in the strace output and look at
the surrounding lines. Another trick is to match up file descriptor
numbers and "follow" what happens to an open file until it is closed.
</para>
<para>
Beyond this you will have to use your initiative. To give you an idea
of what you are looking for here is a piece of strace output that
shows that <filename>/dev/null</filename> is not world writeable, which
causes printing to fail with Samba:
</para>
<para><programlisting>
[pid 28268] open("/dev/null", O_RDWR) = -1 EACCES (Permission denied)
[pid 28268] open("/dev/null", O_WRONLY) = -1 EACCES (Permission denied)
</programlisting></para>
<para>
The process is trying to first open <filename>/dev/null</filename> read-write
then read-only. Both fail. This means <filename>/dev/null</filename> has
incorrect permissions.
</para>
</chapter>