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<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Chapter�14.�Tracing samba system calls</title><link rel="stylesheet" href="../samba.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.74.0"><link rel="home" href="index.html" title="SAMBA Developers Guide"><link rel="up" href="pt04.html" title="Part�IV.�Debugging and tracing"><link rel="prev" href="pt04.html" title="Part�IV.�Debugging and tracing"><link rel="next" href="devprinting.html" title="Chapter�15.�Samba Printing Internals"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter�14.�Tracing samba system calls</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="pt04.html">Prev</a>�</td><th width="60%" align="center">Part�IV.�Debugging and tracing</th><td width="20%" align="right">�<a accesskey="n" href="devprinting.html">Next</a></td></tr></table><hr></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="tracing"></a>Chapter�14.�Tracing samba system calls</h2></div><div><div class="author"><h3 class="author"><span class="firstname">Andrew</span> <span class="orgname">Samba Team</span> <span class="surname">Tridgell</span></h3><div class="affiliation"><span class="orgname">Samba Team<br></span></div></div></div></div></div><p>

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.

</p><p>

Actually its not as bad as the the above makes it sound, just don't

expect the output to be very pretty :-)

</p><p>

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.

</p><p>

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.

</p><p>

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.

</p><p>

Next, try using strace on some simple commands. For example, <code class="literal">strace

ls</code> or <code class="literal">strace echo hello</code>.

</p><p>

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!)

</p><p>

For example, the only line that really matters in the <code class="literal">strace echo

hello</code> output is:

</p><pre class="programlisting">

write(1, "hello\n", 6) = 6

</pre><p>all the rest is just setting up to run the program.</p><p>

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

find which process ID that client is attached to, then as root I do

<code class="literal">strace -p PID</code> 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:

</p><p><code class="literal">strace -f -p 3872 >& strace.out</code></p><p>or with a sh style shell:</p><p><code class="literal">strace -f -p 3872 > strace.out 2>&1</code></p><p>

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.

</p><p>

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.

</p><p>

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.

</p><p>

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 <code class="filename">/dev/null</code> is not world writeable, which

causes printing to fail with Samba:

</p><pre class="programlisting">

[pid 28268] open("/dev/null", O_RDWR) = -1 EACCES (Permission denied)

[pid 28268] open("/dev/null", O_WRONLY) = -1 EACCES (Permission denied)

</pre><p>

The process is trying to first open <code class="filename">/dev/null</code> read-write

then read-only. Both fail. This means <code class="filename">/dev/null</code> has

incorrect permissions.

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