Chapter 14. Tracing samba system calls

Andrew Tridgell

Samba Team

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.

Actually its not as bad as the the above makes it sound, just don't expect the output to be very pretty :-)

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.

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.

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.

Next, try using strace on some simple commands. For example, strace ls or strace echo hello.

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!)

For example, the only line that really matters in the strace echo hello output is:

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

all the rest is just setting up to run the program.

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 strace -p PID 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:

strace -f -p 3872 >& strace.out

or with a sh style shell:

strace -f -p 3872 > strace.out 2>&1

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.

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.

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.

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 /dev/null is not world writeable, which causes printing to fail with Samba:

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

The process is trying to first open /dev/null read-write then read-only. Both fail. This means /dev/null has incorrect permissions.