1.. _readme:
2
3Linux kernel release 5.x <http://kernel.org/>
4=============================================
5
6These are the release notes for Linux version 5.  Read them carefully,
7as they tell you what this is all about, explain how to install the
8kernel, and what to do if something goes wrong.
9
10What is Linux?
11--------------
12
13  Linux is a clone of the operating system Unix, written from scratch by
14  Linus Torvalds with assistance from a loosely-knit team of hackers across
15  the Net. It aims towards POSIX and Single UNIX Specification compliance.
16
17  It has all the features you would expect in a modern fully-fledged Unix,
18  including true multitasking, virtual memory, shared libraries, demand
19  loading, shared copy-on-write executables, proper memory management,
20  and multistack networking including IPv4 and IPv6.
21
22  It is distributed under the GNU General Public License v2 - see the
23  accompanying COPYING file for more details.
24
25On what hardware does it run?
26-----------------------------
27
28  Although originally developed first for 32-bit x86-based PCs (386 or higher),
29  today Linux also runs on (at least) the Compaq Alpha AXP, Sun SPARC and
30  UltraSPARC, Motorola 68000, PowerPC, PowerPC64, ARM, Hitachi SuperH, Cell,
31  IBM S/390, MIPS, HP PA-RISC, Intel IA-64, DEC VAX, AMD x86-64 Xtensa, and
32  ARC architectures.
33
34  Linux is easily portable to most general-purpose 32- or 64-bit architectures
35  as long as they have a paged memory management unit (PMMU) and a port of the
36  GNU C compiler (gcc) (part of The GNU Compiler Collection, GCC). Linux has
37  also been ported to a number of architectures without a PMMU, although
38  functionality is then obviously somewhat limited.
39  Linux has also been ported to itself. You can now run the kernel as a
40  userspace application - this is called UserMode Linux (UML).
41
42Documentation
43-------------
44
45 - There is a lot of documentation available both in electronic form on
46   the Internet and in books, both Linux-specific and pertaining to
47   general UNIX questions.  I'd recommend looking into the documentation
48   subdirectories on any Linux FTP site for the LDP (Linux Documentation
49   Project) books.  This README is not meant to be documentation on the
50   system: there are much better sources available.
51
52 - There are various README files in the Documentation/ subdirectory:
53   these typically contain kernel-specific installation notes for some
54   drivers for example. Please read the
55   :ref:`Documentation/process/changes.rst <changes>` file, as it
56   contains information about the problems, which may result by upgrading
57   your kernel.
58
59Installing the kernel source
60----------------------------
61
62 - If you install the full sources, put the kernel tarball in a
63   directory where you have permissions (e.g. your home directory) and
64   unpack it::
65
66     xz -cd linux-5.x.tar.xz | tar xvf -
67
68   Replace "X" with the version number of the latest kernel.
69
70   Do NOT use the /usr/src/linux area! This area has a (usually
71   incomplete) set of kernel headers that are used by the library header
72   files.  They should match the library, and not get messed up by
73   whatever the kernel-du-jour happens to be.
74
75 - You can also upgrade between 5.x releases by patching.  Patches are
76   distributed in the xz format.  To install by patching, get all the
77   newer patch files, enter the top level directory of the kernel source
78   (linux-5.x) and execute::
79
80     xz -cd ../patch-5.x.xz | patch -p1
81
82   Replace "x" for all versions bigger than the version "x" of your current
83   source tree, **in_order**, and you should be ok.  You may want to remove
84   the backup files (some-file-name~ or some-file-name.orig), and make sure
85   that there are no failed patches (some-file-name# or some-file-name.rej).
86   If there are, either you or I have made a mistake.
87
88   Unlike patches for the 5.x kernels, patches for the 5.x.y kernels
89   (also known as the -stable kernels) are not incremental but instead apply
90   directly to the base 5.x kernel.  For example, if your base kernel is 5.0
91   and you want to apply the 5.0.3 patch, you must not first apply the 5.0.1
92   and 5.0.2 patches. Similarly, if you are running kernel version 5.0.2 and
93   want to jump to 5.0.3, you must first reverse the 5.0.2 patch (that is,
94   patch -R) **before** applying the 5.0.3 patch. You can read more on this in
95   :ref:`Documentation/process/applying-patches.rst <applying_patches>`.
96
97   Alternatively, the script patch-kernel can be used to automate this
98   process.  It determines the current kernel version and applies any
99   patches found::
100
101     linux/scripts/patch-kernel linux
102
103   The first argument in the command above is the location of the
104   kernel source.  Patches are applied from the current directory, but
105   an alternative directory can be specified as the second argument.
106
107 - Make sure you have no stale .o files and dependencies lying around::
108
109     cd linux
110     make mrproper
111
112   You should now have the sources correctly installed.
113
114Software requirements
115---------------------
116
117   Compiling and running the 5.x kernels requires up-to-date
118   versions of various software packages.  Consult
119   :ref:`Documentation/process/changes.rst <changes>` for the minimum version numbers
120   required and how to get updates for these packages.  Beware that using
121   excessively old versions of these packages can cause indirect
122   errors that are very difficult to track down, so don't assume that
123   you can just update packages when obvious problems arise during
124   build or operation.
125
126Build directory for the kernel
127------------------------------
128
129   When compiling the kernel, all output files will per default be
130   stored together with the kernel source code.
131   Using the option ``make O=output/dir`` allows you to specify an alternate
132   place for the output files (including .config).
133   Example::
134
135     kernel source code: /usr/src/linux-5.x
136     build directory:    /home/name/build/kernel
137
138   To configure and build the kernel, use::
139
140     cd /usr/src/linux-5.x
141     make O=/home/name/build/kernel menuconfig
142     make O=/home/name/build/kernel
143     sudo make O=/home/name/build/kernel modules_install install
144
145   Please note: If the ``O=output/dir`` option is used, then it must be
146   used for all invocations of make.
147
148Configuring the kernel
149----------------------
150
151   Do not skip this step even if you are only upgrading one minor
152   version.  New configuration options are added in each release, and
153   odd problems will turn up if the configuration files are not set up
154   as expected.  If you want to carry your existing configuration to a
155   new version with minimal work, use ``make oldconfig``, which will
156   only ask you for the answers to new questions.
157
158 - Alternative configuration commands are::
159
160     "make config"      Plain text interface.
161
162     "make menuconfig"  Text based color menus, radiolists & dialogs.
163
164     "make nconfig"     Enhanced text based color menus.
165
166     "make xconfig"     Qt based configuration tool.
167
168     "make gconfig"     GTK+ based configuration tool.
169
170     "make oldconfig"   Default all questions based on the contents of
171                        your existing ./.config file and asking about
172                        new config symbols.
173
174     "make olddefconfig"
175                        Like above, but sets new symbols to their default
176                        values without prompting.
177
178     "make defconfig"   Create a ./.config file by using the default
179                        symbol values from either arch/$ARCH/defconfig
180                        or arch/$ARCH/configs/${PLATFORM}_defconfig,
181                        depending on the architecture.
182
183     "make ${PLATFORM}_defconfig"
184                        Create a ./.config file by using the default
185                        symbol values from
186                        arch/$ARCH/configs/${PLATFORM}_defconfig.
187                        Use "make help" to get a list of all available
188                        platforms of your architecture.
189
190     "make allyesconfig"
191                        Create a ./.config file by setting symbol
192                        values to 'y' as much as possible.
193
194     "make allmodconfig"
195                        Create a ./.config file by setting symbol
196                        values to 'm' as much as possible.
197
198     "make allnoconfig" Create a ./.config file by setting symbol
199                        values to 'n' as much as possible.
200
201     "make randconfig"  Create a ./.config file by setting symbol
202                        values to random values.
203
204     "make localmodconfig" Create a config based on current config and
205                           loaded modules (lsmod). Disables any module
206                           option that is not needed for the loaded modules.
207
208                           To create a localmodconfig for another machine,
209                           store the lsmod of that machine into a file
210                           and pass it in as a LSMOD parameter.
211
212                           Also, you can preserve modules in certain folders
213                           or kconfig files by specifying their paths in
214                           parameter LMC_KEEP.
215
216                   target$ lsmod > /tmp/mylsmod
217                   target$ scp /tmp/mylsmod host:/tmp
218
219                   host$ make LSMOD=/tmp/mylsmod \
220                           LMC_KEEP="drivers/usb:drivers/gpu:fs" \
221                           localmodconfig
222
223                           The above also works when cross compiling.
224
225     "make localyesconfig" Similar to localmodconfig, except it will convert
226                           all module options to built in (=y) options. You can
227                           also preserve modules by LMC_KEEP.
228
229     "make kvmconfig"   Enable additional options for kvm guest kernel support.
230
231     "make xenconfig"   Enable additional options for xen dom0 guest kernel
232                        support.
233
234     "make tinyconfig"  Configure the tiniest possible kernel.
235
236   You can find more information on using the Linux kernel config tools
237   in Documentation/kbuild/kconfig.rst.
238
239 - NOTES on ``make config``:
240
241    - Having unnecessary drivers will make the kernel bigger, and can
242      under some circumstances lead to problems: probing for a
243      nonexistent controller card may confuse your other controllers.
244
245    - A kernel with math-emulation compiled in will still use the
246      coprocessor if one is present: the math emulation will just
247      never get used in that case.  The kernel will be slightly larger,
248      but will work on different machines regardless of whether they
249      have a math coprocessor or not.
250
251    - The "kernel hacking" configuration details usually result in a
252      bigger or slower kernel (or both), and can even make the kernel
253      less stable by configuring some routines to actively try to
254      break bad code to find kernel problems (kmalloc()).  Thus you
255      should probably answer 'n' to the questions for "development",
256      "experimental", or "debugging" features.
257
258Compiling the kernel
259--------------------
260
261 - Make sure you have at least gcc 4.9 available.
262   For more information, refer to :ref:`Documentation/process/changes.rst <changes>`.
263
264   Please note that you can still run a.out user programs with this kernel.
265
266 - Do a ``make`` to create a compressed kernel image. It is also
267   possible to do ``make install`` if you have lilo installed to suit the
268   kernel makefiles, but you may want to check your particular lilo setup first.
269
270   To do the actual install, you have to be root, but none of the normal
271   build should require that. Don't take the name of root in vain.
272
273 - If you configured any of the parts of the kernel as ``modules``, you
274   will also have to do ``make modules_install``.
275
276 - Verbose kernel compile/build output:
277
278   Normally, the kernel build system runs in a fairly quiet mode (but not
279   totally silent).  However, sometimes you or other kernel developers need
280   to see compile, link, or other commands exactly as they are executed.
281   For this, use "verbose" build mode.  This is done by passing
282   ``V=1`` to the ``make`` command, e.g.::
283
284     make V=1 all
285
286   To have the build system also tell the reason for the rebuild of each
287   target, use ``V=2``.  The default is ``V=0``.
288
289 - Keep a backup kernel handy in case something goes wrong.  This is
290   especially true for the development releases, since each new release
291   contains new code which has not been debugged.  Make sure you keep a
292   backup of the modules corresponding to that kernel, as well.  If you
293   are installing a new kernel with the same version number as your
294   working kernel, make a backup of your modules directory before you
295   do a ``make modules_install``.
296
297   Alternatively, before compiling, use the kernel config option
298   "LOCALVERSION" to append a unique suffix to the regular kernel version.
299   LOCALVERSION can be set in the "General Setup" menu.
300
301 - In order to boot your new kernel, you'll need to copy the kernel
302   image (e.g. .../linux/arch/x86/boot/bzImage after compilation)
303   to the place where your regular bootable kernel is found.
304
305 - Booting a kernel directly from a floppy without the assistance of a
306   bootloader such as LILO, is no longer supported.
307
308   If you boot Linux from the hard drive, chances are you use LILO, which
309   uses the kernel image as specified in the file /etc/lilo.conf.  The
310   kernel image file is usually /vmlinuz, /boot/vmlinuz, /bzImage or
311   /boot/bzImage.  To use the new kernel, save a copy of the old image
312   and copy the new image over the old one.  Then, you MUST RERUN LILO
313   to update the loading map! If you don't, you won't be able to boot
314   the new kernel image.
315
316   Reinstalling LILO is usually a matter of running /sbin/lilo.
317   You may wish to edit /etc/lilo.conf to specify an entry for your
318   old kernel image (say, /vmlinux.old) in case the new one does not
319   work.  See the LILO docs for more information.
320
321   After reinstalling LILO, you should be all set.  Shutdown the system,
322   reboot, and enjoy!
323
324   If you ever need to change the default root device, video mode,
325   etc. in the kernel image, use your bootloader's boot options
326   where appropriate.  No need to recompile the kernel to change
327   these parameters.
328
329 - Reboot with the new kernel and enjoy.
330
331If something goes wrong
332-----------------------
333
334 - If you have problems that seem to be due to kernel bugs, please check
335   the file MAINTAINERS to see if there is a particular person associated
336   with the part of the kernel that you are having trouble with. If there
337   isn't anyone listed there, then the second best thing is to mail
338   them to me (torvalds@linux-foundation.org), and possibly to any other
339   relevant mailing-list or to the newsgroup.
340
341 - In all bug-reports, *please* tell what kernel you are talking about,
342   how to duplicate the problem, and what your setup is (use your common
343   sense).  If the problem is new, tell me so, and if the problem is
344   old, please try to tell me when you first noticed it.
345
346 - If the bug results in a message like::
347
348     unable to handle kernel paging request at address C0000010
349     Oops: 0002
350     EIP:   0010:XXXXXXXX
351     eax: xxxxxxxx   ebx: xxxxxxxx   ecx: xxxxxxxx   edx: xxxxxxxx
352     esi: xxxxxxxx   edi: xxxxxxxx   ebp: xxxxxxxx
353     ds: xxxx  es: xxxx  fs: xxxx  gs: xxxx
354     Pid: xx, process nr: xx
355     xx xx xx xx xx xx xx xx xx xx
356
357   or similar kernel debugging information on your screen or in your
358   system log, please duplicate it *exactly*.  The dump may look
359   incomprehensible to you, but it does contain information that may
360   help debugging the problem.  The text above the dump is also
361   important: it tells something about why the kernel dumped code (in
362   the above example, it's due to a bad kernel pointer). More information
363   on making sense of the dump is in Documentation/admin-guide/bug-hunting.rst
364
365 - If you compiled the kernel with CONFIG_KALLSYMS you can send the dump
366   as is, otherwise you will have to use the ``ksymoops`` program to make
367   sense of the dump (but compiling with CONFIG_KALLSYMS is usually preferred).
368   This utility can be downloaded from
369   https://www.kernel.org/pub/linux/utils/kernel/ksymoops/ .
370   Alternatively, you can do the dump lookup by hand:
371
372 - In debugging dumps like the above, it helps enormously if you can
373   look up what the EIP value means.  The hex value as such doesn't help
374   me or anybody else very much: it will depend on your particular
375   kernel setup.  What you should do is take the hex value from the EIP
376   line (ignore the ``0010:``), and look it up in the kernel namelist to
377   see which kernel function contains the offending address.
378
379   To find out the kernel function name, you'll need to find the system
380   binary associated with the kernel that exhibited the symptom.  This is
381   the file 'linux/vmlinux'.  To extract the namelist and match it against
382   the EIP from the kernel crash, do::
383
384     nm vmlinux | sort | less
385
386   This will give you a list of kernel addresses sorted in ascending
387   order, from which it is simple to find the function that contains the
388   offending address.  Note that the address given by the kernel
389   debugging messages will not necessarily match exactly with the
390   function addresses (in fact, that is very unlikely), so you can't
391   just 'grep' the list: the list will, however, give you the starting
392   point of each kernel function, so by looking for the function that
393   has a starting address lower than the one you are searching for but
394   is followed by a function with a higher address you will find the one
395   you want.  In fact, it may be a good idea to include a bit of
396   "context" in your problem report, giving a few lines around the
397   interesting one.
398
399   If you for some reason cannot do the above (you have a pre-compiled
400   kernel image or similar), telling me as much about your setup as
401   possible will help.  Please read the :ref:`admin-guide/reporting-bugs.rst <reportingbugs>`
402   document for details.
403
404 - Alternatively, you can use gdb on a running kernel. (read-only; i.e. you
405   cannot change values or set break points.) To do this, first compile the
406   kernel with -g; edit arch/x86/Makefile appropriately, then do a ``make
407   clean``. You'll also need to enable CONFIG_PROC_FS (via ``make config``).
408
409   After you've rebooted with the new kernel, do ``gdb vmlinux /proc/kcore``.
410   You can now use all the usual gdb commands. The command to look up the
411   point where your system crashed is ``l *0xXXXXXXXX``. (Replace the XXXes
412   with the EIP value.)
413
414   gdb'ing a non-running kernel currently fails because ``gdb`` (wrongly)
415   disregards the starting offset for which the kernel is compiled.
416