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 kvm_guest.config"   Enable additional options for kvm guest kernel
230                               support.
231
232     "make xen.config"   Enable additional options for xen dom0 guest kernel
233                         support.
234
235     "make tinyconfig"  Configure the tiniest possible kernel.
236
237   You can find more information on using the Linux kernel config tools
238   in Documentation/kbuild/kconfig.rst.
239
240 - NOTES on ``make config``:
241
242    - Having unnecessary drivers will make the kernel bigger, and can
243      under some circumstances lead to problems: probing for a
244      nonexistent controller card may confuse your other controllers.
245
246    - A kernel with math-emulation compiled in will still use the
247      coprocessor if one is present: the math emulation will just
248      never get used in that case.  The kernel will be slightly larger,
249      but will work on different machines regardless of whether they
250      have a math coprocessor or not.
251
252    - The "kernel hacking" configuration details usually result in a
253      bigger or slower kernel (or both), and can even make the kernel
254      less stable by configuring some routines to actively try to
255      break bad code to find kernel problems (kmalloc()).  Thus you
256      should probably answer 'n' to the questions for "development",
257      "experimental", or "debugging" features.
258
259Compiling the kernel
260--------------------
261
262 - Make sure you have at least gcc 4.9 available.
263   For more information, refer to :ref:`Documentation/process/changes.rst <changes>`.
264
265   Please note that you can still run a.out user programs with this kernel.
266
267 - Do a ``make`` to create a compressed kernel image. It is also
268   possible to do ``make install`` if you have lilo installed to suit the
269   kernel makefiles, but you may want to check your particular lilo setup first.
270
271   To do the actual install, you have to be root, but none of the normal
272   build should require that. Don't take the name of root in vain.
273
274 - If you configured any of the parts of the kernel as ``modules``, you
275   will also have to do ``make modules_install``.
276
277 - Verbose kernel compile/build output:
278
279   Normally, the kernel build system runs in a fairly quiet mode (but not
280   totally silent).  However, sometimes you or other kernel developers need
281   to see compile, link, or other commands exactly as they are executed.
282   For this, use "verbose" build mode.  This is done by passing
283   ``V=1`` to the ``make`` command, e.g.::
284
285     make V=1 all
286
287   To have the build system also tell the reason for the rebuild of each
288   target, use ``V=2``.  The default is ``V=0``.
289
290 - Keep a backup kernel handy in case something goes wrong.  This is
291   especially true for the development releases, since each new release
292   contains new code which has not been debugged.  Make sure you keep a
293   backup of the modules corresponding to that kernel, as well.  If you
294   are installing a new kernel with the same version number as your
295   working kernel, make a backup of your modules directory before you
296   do a ``make modules_install``.
297
298   Alternatively, before compiling, use the kernel config option
299   "LOCALVERSION" to append a unique suffix to the regular kernel version.
300   LOCALVERSION can be set in the "General Setup" menu.
301
302 - In order to boot your new kernel, you'll need to copy the kernel
303   image (e.g. .../linux/arch/x86/boot/bzImage after compilation)
304   to the place where your regular bootable kernel is found.
305
306 - Booting a kernel directly from a floppy without the assistance of a
307   bootloader such as LILO, is no longer supported.
308
309   If you boot Linux from the hard drive, chances are you use LILO, which
310   uses the kernel image as specified in the file /etc/lilo.conf.  The
311   kernel image file is usually /vmlinuz, /boot/vmlinuz, /bzImage or
312   /boot/bzImage.  To use the new kernel, save a copy of the old image
313   and copy the new image over the old one.  Then, you MUST RERUN LILO
314   to update the loading map! If you don't, you won't be able to boot
315   the new kernel image.
316
317   Reinstalling LILO is usually a matter of running /sbin/lilo.
318   You may wish to edit /etc/lilo.conf to specify an entry for your
319   old kernel image (say, /vmlinux.old) in case the new one does not
320   work.  See the LILO docs for more information.
321
322   After reinstalling LILO, you should be all set.  Shutdown the system,
323   reboot, and enjoy!
324
325   If you ever need to change the default root device, video mode,
326   etc. in the kernel image, use your bootloader's boot options
327   where appropriate.  No need to recompile the kernel to change
328   these parameters.
329
330 - Reboot with the new kernel and enjoy.
331
332If something goes wrong
333-----------------------
334
335 - If you have problems that seem to be due to kernel bugs, please check
336   the file MAINTAINERS to see if there is a particular person associated
337   with the part of the kernel that you are having trouble with. If there
338   isn't anyone listed there, then the second best thing is to mail
339   them to me (torvalds@linux-foundation.org), and possibly to any other
340   relevant mailing-list or to the newsgroup.
341
342 - In all bug-reports, *please* tell what kernel you are talking about,
343   how to duplicate the problem, and what your setup is (use your common
344   sense).  If the problem is new, tell me so, and if the problem is
345   old, please try to tell me when you first noticed it.
346
347 - If the bug results in a message like::
348
349     unable to handle kernel paging request at address C0000010
350     Oops: 0002
351     EIP:   0010:XXXXXXXX
352     eax: xxxxxxxx   ebx: xxxxxxxx   ecx: xxxxxxxx   edx: xxxxxxxx
353     esi: xxxxxxxx   edi: xxxxxxxx   ebp: xxxxxxxx
354     ds: xxxx  es: xxxx  fs: xxxx  gs: xxxx
355     Pid: xx, process nr: xx
356     xx xx xx xx xx xx xx xx xx xx
357
358   or similar kernel debugging information on your screen or in your
359   system log, please duplicate it *exactly*.  The dump may look
360   incomprehensible to you, but it does contain information that may
361   help debugging the problem.  The text above the dump is also
362   important: it tells something about why the kernel dumped code (in
363   the above example, it's due to a bad kernel pointer). More information
364   on making sense of the dump is in Documentation/admin-guide/bug-hunting.rst
365
366 - If you compiled the kernel with CONFIG_KALLSYMS you can send the dump
367   as is, otherwise you will have to use the ``ksymoops`` program to make
368   sense of the dump (but compiling with CONFIG_KALLSYMS is usually preferred).
369   This utility can be downloaded from
370   https://www.kernel.org/pub/linux/utils/kernel/ksymoops/ .
371   Alternatively, you can do the dump lookup by hand:
372
373 - In debugging dumps like the above, it helps enormously if you can
374   look up what the EIP value means.  The hex value as such doesn't help
375   me or anybody else very much: it will depend on your particular
376   kernel setup.  What you should do is take the hex value from the EIP
377   line (ignore the ``0010:``), and look it up in the kernel namelist to
378   see which kernel function contains the offending address.
379
380   To find out the kernel function name, you'll need to find the system
381   binary associated with the kernel that exhibited the symptom.  This is
382   the file 'linux/vmlinux'.  To extract the namelist and match it against
383   the EIP from the kernel crash, do::
384
385     nm vmlinux | sort | less
386
387   This will give you a list of kernel addresses sorted in ascending
388   order, from which it is simple to find the function that contains the
389   offending address.  Note that the address given by the kernel
390   debugging messages will not necessarily match exactly with the
391   function addresses (in fact, that is very unlikely), so you can't
392   just 'grep' the list: the list will, however, give you the starting
393   point of each kernel function, so by looking for the function that
394   has a starting address lower than the one you are searching for but
395   is followed by a function with a higher address you will find the one
396   you want.  In fact, it may be a good idea to include a bit of
397   "context" in your problem report, giving a few lines around the
398   interesting one.
399
400   If you for some reason cannot do the above (you have a pre-compiled
401   kernel image or similar), telling me as much about your setup as
402   possible will help.  Please read
403   'Documentation/admin-guide/reporting-issues.rst' for details.
404
405 - Alternatively, you can use gdb on a running kernel. (read-only; i.e. you
406   cannot change values or set break points.) To do this, first compile the
407   kernel with -g; edit arch/x86/Makefile appropriately, then do a ``make
408   clean``. You'll also need to enable CONFIG_PROC_FS (via ``make config``).
409
410   After you've rebooted with the new kernel, do ``gdb vmlinux /proc/kcore``.
411   You can now use all the usual gdb commands. The command to look up the
412   point where your system crashed is ``l *0xXXXXXXXX``. (Replace the XXXes
413   with the EIP value.)
414
415   gdb'ing a non-running kernel currently fails because ``gdb`` (wrongly)
416   disregards the starting offset for which the kernel is compiled.
417