1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
"http://www.w3.org/TR/REC-html40/loose.dtd">
<html>
<head>
<meta content="text/html; charset=iso-8859-1" http-equiv="Content-Type">
<title>Kernel debugging</title>
</head>
<body alink="#FF0000" vlink="#55188A" link="#0000EF" bgcolor="#FFFFFF" text="#000099">
<table border="0">
<tr align="left">
<td valign="top">
<table border="0">
<tr align="left"><td valign="top" >
<img width="120" height="171" src="/uml-small.png">
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/index.html">Site Home Page</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/uses.html">What it's good for</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/case-studies.html">Case Studies</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/kernel.html">Kernel Capabilities</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/dl-sf.html">Downloading it</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/run.html">Running it</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/compile.html">Compiling</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/install.html">Installation</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/fs_making.html">Building filesystems</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/faq.html">Troubles</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/contrib.html">User Contributions</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/links.html">Related Links</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/todo.html">The ToDo list</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/projects.html">Projects</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/diary.html">Diary</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/thanks.html">Thanks</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/contacts.html">Contacts</a></font>
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">Tutorials<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/UserModeLinux-HOWTO.html">The HOWTO (html)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/UserModeLinux-HOWTO.txt">The HOWTO (text)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/hostfs.html">Host file access</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/input.html">Device inputs</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/shared_fs.html">Sharing filesystems</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/fs.html">Creating filesystems</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/networking.html">Virtual Networking</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/mconsole.html">Management Console</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/debugging.html">Kernel Debugging</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/gprof.html">gprof and gcov</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/xtut.html">Running X</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/trouble.html">Diagnosing problems</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/config.html">Configuration</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slack_readme.html">Installing Slackware</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/arch-port.html">Porting UML</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/iomem.html">IO memory emulation</a></font>
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">How you can help<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/help-gen.html">Overview</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/help-doc.html">Documentation</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/help-userspace.html">Utilities</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/help-kernel-v1.html">Kernel bugs</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/projects.html">Kernel projects</a></font>
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">Screenshots<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/net.html">A virtual network</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/x.html">An X session</a></font>
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">Transcripts<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/login.html">A login session</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/debug-session.html">A debugging session</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slackinst.html">Slackware installation</a></font>
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">Reference<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/switches.html">Kernel switches</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slack_readme.html">Slackware README</a></font>
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">Papers<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/als2000/index.html">ALS 2000 paper (html)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/als2000.tex">ALS 2000 paper (TeX)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slides/als2000/slides.html">ALS 2000 slides</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slides/lca2001/lca.html">LCA 2001 slides</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slides/ols2001/index.html">OLS 2001 paper (html)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/ols2001.tex">OLS 2001 paper (TeX)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/als2001/index.html">ALS 2001 paper (html)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/als2001.tex">ALS 2001 paper (TeX)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slides/ists2002/umlsec.htm">UML security (html)</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/slides/lca2002/lca2002.htm">LCA 2002 (html)</a></font>
</td></tr>
<tr align="left"><td valign="top" bgcolor="#e0e0e0">Fun and Games<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/http://user-mode-linux.sourceforge.net/cgi-bin/hangman">Kernel Hangman</a></font>
<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/sdotm.html">Disaster of the Month</a></font>
</td></tr>
</table>
</td>
<td valign="top" align="left">
<center>
<h3>Kernel debugging</h3>
</center>
<b>Note:</b> The interface that makes debugging, as described here,
possible is present in 2.4.0-test6 kernels and later.
<p>
Since the user-mode kernel runs as a normal Linux process, it is
possible to debug it with gdb almost like any other process. It is
slightly different because the kernel's threads are already being
ptraced for system call interception, so gdb can't ptrace them.
However, a mechanism has been added to work around that problem.
<p>
In order to debug the kernel, you need build it from source. See
<a href="compile.html">this page</a>
for information on doing that.
<p>
<a name="Starting the kernel under gdb"/><table width="100%" bgcolor="#e0e0e0">
<tr>
<td>
<b>
<font color="black">Starting the kernel under gdb</font>
</b>
</td>
</tr>
</table>
<blockquote head="Starting the kernel under gdb">
You can have the kernel running under the control of gdb from the
beginning by putting 'debug' on the command line. You will get an
xterm with gdb running inside it. The kernel will send some commands
to gdb which will leave it stopped at the beginning of start_kernel.
At this point, you can get things going with 'next', 'step', or 'cont'.
<p>
There is a transcript of a debugging session <a href="debug-session.html">here</a>, with breakpoints being set in the
scheduler and in an interrupt handler.
</blockquote>
<a name="Attaching gdb to the kernel"/><table width="100%" bgcolor="#e0e0e0">
<tr>
<td>
<b>
<font color="black">Attaching gdb to the kernel</font>
</b>
</td>
</tr>
</table>
<blockquote head="Attaching gdb to the kernel">
If you don't have the kernel running under gdb, you can attach gdb to
it later by sending the tracing thread a SIGUSR1. The first line of
the console output identifies its pid:
<blockquote>
<tt>
<font color="#000000">tracing thread pid = 20093</font>
</tt>
</blockquote>
When you send it the signal:
<blockquote>
<tt>
<font color="#000000">kill -USR1 20093</font>
</tt>
</blockquote>
you will get an xterm with gdb running in it.
<p>
If you have the mconsole compiled into UML, then the mconsole client
can be used to start gdb:
<blockquote>
<tt>
<font color="#000000">(mconsole) config gdb=xterm</font>
</tt>
</blockquote>
will fire up an xterm with gdb running in it.
</blockquote>
<a name="Debugging modules"/><table width="100%" bgcolor="#e0e0e0">
<tr>
<td>
<b>
<font color="black">Debugging modules</font>
</b>
</td>
</tr>
</table>
<blockquote head="Debugging modules">
gdb has support for debugging code which is dynamically loaded into
the process. This support is what is needed to debug kernel modules
under UML.
<br>
Boot the kernel under the debugger and load the module with insmod or
modprobe. With gdb, do:
<blockquote>
<tt>
<font color="#000000">(gdb) p module_list</font>
</tt>
</blockquote>
This is a list of modules that have been loaded into the kernel, with
the most recently loaded module first. Normally, the module you want
is at module_list. If it's not, walk down the next links, looking at
the name fields until find the module you want to debug. Take the
address of that structure, and add module.size_of_struct (which in
2.4.10 kernels is 96 (0x60)) to it. Gdb can make this hard addition for
you :-):
<blockquote>
<tt>
<font color="#000000">printf "%#x\n", (int)module_list module_list->size_of_struct
</font>
</tt>
</blockquote>
The offset from the module start occasionally changes (before 2.4.0, it
was module.size_of_struct + 4), so it's a good idea to check the init and
cleanup addresses once in a while, as describe below.
Now do:
<blockquote>
<tt>
<font color="#000000">
(gdb) add-symbol-file /path/to/module/on/host that_address
</font>
</tt>
</blockquote>
Tell gdb you really want to do it, and you're in business.
<br>
If there's any doubt that you got the offset right, like breakpoints
appear not to work, or they're appearing in the wrong place, you can
check it by looking at the module structure. The init and cleanup
fields should look like:
<tt>
<pre>
<font size="-1">
init = 0x588066b0 <init_hostfs>, cleanup = 0x588066c0 <exit_hostfs>
</font>
</pre>
</tt>
with no offsets on the symbol names. If the names are right, but they
are offset, then the offset tells you how much you need to add to the
address you gave to add-symbol-file.
<p>
When you want to load in a new version of the module, you need to get
gdb to forget about the old one. The only way I've found to do that
is to tell gdb to forget about all symbols that it knows about:
<blockquote>
<tt>
<font color="#000000">(gdb) symbol-file</font>
</tt>
</blockquote>
Then reload the symbols from the kernel binary:
<blockquote>
<tt>
<font color="#000000">(gdb) symbol-file /path/to/kernel</font>
</tt>
</blockquote>
and repeat the process above. You'll also need to re-enable breakpoints.
They were disabled when you dumped all the symbols because gdb
couldn't figure out where they should go.
</blockquote>
<a name="Using alternate debuggers"/><table width="100%" bgcolor="#e0e0e0">
<tr>
<td>
<b>
<font color="black">Using alternate debuggers</font>
</b>
</td>
</tr>
</table>
<blockquote head="Using alternate debuggers">
UML has support for attaching to an already running debugger rather than
starting gdb itself. This is present in <a href="http://sourceforge.net/cvs/?group_id=429">CVS</a> as of
17 Apr 2001. I sent it to Alan for inclusion in
the ac tree, and it will be in my 2.4.4 release.
<p>
This is useful when gdb is a subprocess of some UI, such as emacs or ddd. It
can also be used to run debuggers other than gdb on UML. Below is an example
of using strace as an alternate debugger.
<p>
To do this, you need to get the pid of the debugger and pass it in with the
'gdb-pid=<pid>' switch along with the 'debug' switch.
<p>
If you are using gdb under some UI, then tell it to 'att 1', and you'll find
yourself attached to UML.
<p>
If you are using something other than gdb as your debugger, then you'll need
to get it to do the equivalent of 'att 1' if it doesn't do it automatically.
<p>
An example of an alternate debugger is strace. You can strace the actual
kernel as follows:
<ul>
<li>
Run the following in a shell
<blockquote>
<tt>
<font color="#000000">sh -c 'echo pid=$$; echo -n hit return; read x; exec strace -p 1 -o
strace.out'</font>
</tt>
</blockquote>
</li>
<li>
Run UML with 'debug' and 'gdb-pid=<pid>' with the pid printed out by the
previous command
</li>
<li>
Hit return in the shell, and UML will start running, and strace output will
start accumulating in the output file.
</li>
</ul>
Note that this is different from running
<blockquote>
<tt>
<font color="#000000">strace ./linux</font>
</tt>
</blockquote>
That will strace only the main UML thread, the tracing thread, which doesn't
do any of the actual kernel work. It just oversees the virtual machine. In
contrast, using strace as described above will show you the low-level activity
of the virtual machine.
</blockquote>
</td>
</tr>
</table>
<center>
<font size="-1">
Hosted at </font>
<a href="http://sourceforge.net">
<IMG alt="SourceForge Logo" border="0" height="31" width="88" src="http://sourceforge.net/sflogo.php?group_id=429">
</a>
</center>
</body>
</html>
|