1 <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 V1.68.1"><link rel="start" 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="surname">Tridgell</span></h3><div class="affiliation"><span class="orgname">Samba Team<br></span></div></div></div></div></div><p>
2 This file describes how to do a system call trace on Samba to work out
3 what its doing wrong. This is not for the faint of heart, but if you
4 are reading this then you are probably desperate.
6 Actually its not as bad as the the above makes it sound, just don't
7 expect the output to be very pretty :-)
9 Ok, down to business. One of the big advantages of unix systems is
10 that they nearly all come with a system trace utility that allows you
11 to monitor all system calls that a program is making. This is
12 extremely using for debugging and also helps when trying to work out
13 why something is slower than you expect. You can use system tracing
14 without any special compilation options.
16 The system trace utility is called different things on different
17 systems. On Linux systems its called strace. Under SunOS 4 its called
18 trace. Under SVR4 style systems (including solaris) its called
19 truss. Under many BSD systems its called ktrace.
21 The first thing you should do is read the man page for your native
22 system call tracer. In the discussion below I'll assume its called
23 strace as strace is the only portable system tracer (its available for
24 free for many unix types) and its also got some of the nicest
27 Next, try using strace on some simple commands. For example, <span><strong class="command">strace
28 ls</strong></span> or <span><strong class="command">strace echo hello</strong></span>.
30 You'll notice that it produces a LOT of output. It is showing you the
31 arguments to every system call that the program makes and the
32 result. Very little happens in a program without a system call so you
33 get lots of output. You'll also find that it produces a lot of
34 "preamble" stuff showing the loading of shared libraries etc. Ignore
35 this (unless its going wrong!)
37 For example, the only line that really matters in the <span><strong class="command">strace echo
38 hello</strong></span> output is:
39 </p><pre class="programlisting">
40 write(1, "hello\n", 6) = 6
41 </pre><p>all the rest is just setting up to run the program.</p><p>
42 Ok, now you're familiar with strace. To use it on Samba you need to
43 strace the running smbd daemon. The way I tend ot use it is to first
44 login from my Windows PC to the Samba server, then use smbstatus to
45 find which process ID that client is attached to, then as root I do
46 <span><strong class="command">strace -p PID</strong></span> to attach to that process. I normally redirect the
47 stderr output from this command to a file for later perusal. For
48 example, if I'm using a csh style shell:
49 </p><p><span><strong class="command">strace -f -p 3872 >& strace.out</strong></span></p><p>or with a sh style shell:</p><p><span><strong class="command">strace -f -p 3872 > strace.out 2>&1</strong></span></p><p>
50 Note the "-f" option. This is only available on some systems, and
51 allows you to trace not just the current process, but any children it
52 forks. This is great for finding printing problems caused by the
53 "print command" being wrong.
55 Once you are attached you then can do whatever it is on the client
56 that is causing problems and you will capture all the system calls
59 So how do you interpret the results? Generally I search through the
60 output for strings that I know will appear when the problem
61 happens. For example, if I am having touble with permissions on a file
62 I would search for that files name in the strace output and look at
63 the surrounding lines. Another trick is to match up file descriptor
64 numbers and "follow" what happens to an open file until it is closed.
66 Beyond this you will have to use your initiative. To give you an idea
67 of what you are looking for here is a piece of strace output that
68 shows that <code class="filename">/dev/null</code> is not world writeable, which
69 causes printing to fail with Samba:
70 </p><pre class="programlisting">
71 [pid 28268] open("/dev/null", O_RDWR) = -1 EACCES (Permission denied)
72 [pid 28268] open("/dev/null", O_WRONLY) = -1 EACCES (Permission denied)
74 The process is trying to first open <code class="filename">/dev/null</code> read-write
75 then read-only. Both fail. This means <code class="filename">/dev/null</code> has
76 incorrect permissions.
77 </p></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="pt04.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="pt04.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="devprinting.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Part IV. Debugging and tracing </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 15. Samba Printing Internals</td></tr></table></div></body></html>
projects / samba / blob