Add document on tracing system calls

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+<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>