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.. _applying_patches: |
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Applying Patches To The Linux Kernel |
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++++++++++++++++++++++++++++++++++++ |
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Original by: |
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Jesper Juhl, August 2005 |
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.. note:: |
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This document is obsolete. In most cases, rather than using ``patch`` |
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manually, you'll almost certainly want to look at using Git instead. |
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A frequently asked question on the Linux Kernel Mailing List is how to apply |
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a patch to the kernel or, more specifically, what base kernel a patch for |
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one of the many trees/branches should be applied to. Hopefully this document |
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will explain this to you. |
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In addition to explaining how to apply and revert patches, a brief |
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description of the different kernel trees (and examples of how to apply |
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their specific patches) is also provided. |
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What is a patch? |
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================ |
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A patch is a small text document containing a delta of changes between two |
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different versions of a source tree. Patches are created with the ``diff`` |
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program. |
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To correctly apply a patch you need to know what base it was generated from |
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and what new version the patch will change the source tree into. These |
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should both be present in the patch file metadata or be possible to deduce |
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from the filename. |
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How do I apply or revert a patch? |
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================================= |
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You apply a patch with the ``patch`` program. The patch program reads a diff |
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(or patch) file and makes the changes to the source tree described in it. |
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Patches for the Linux kernel are generated relative to the parent directory |
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holding the kernel source dir. |
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This means that paths to files inside the patch file contain the name of the |
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kernel source directories it was generated against (or some other directory |
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names like "a/" and "b/"). |
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Since this is unlikely to match the name of the kernel source dir on your |
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local machine (but is often useful info to see what version an otherwise |
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unlabeled patch was generated against) you should change into your kernel |
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source directory and then strip the first element of the path from filenames |
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in the patch file when applying it (the ``-p1`` argument to ``patch`` does |
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this). |
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To revert a previously applied patch, use the -R argument to patch. |
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So, if you applied a patch like this:: |
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patch -p1 < ../patch-x.y.z |
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You can revert (undo) it like this:: |
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patch -R -p1 < ../patch-x.y.z |
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How do I feed a patch/diff file to ``patch``? |
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============================================= |
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This (as usual with Linux and other UNIX like operating systems) can be |
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done in several different ways. |
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In all the examples below I feed the file (in uncompressed form) to patch |
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via stdin using the following syntax:: |
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patch -p1 < path/to/patch-x.y.z |
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If you just want to be able to follow the examples below and don't want to |
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know of more than one way to use patch, then you can stop reading this |
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section here. |
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Patch can also get the name of the file to use via the -i argument, like |
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this:: |
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patch -p1 -i path/to/patch-x.y.z |
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If your patch file is compressed with gzip or xz and you don't want to |
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uncompress it before applying it, then you can feed it to patch like this |
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instead:: |
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xzcat path/to/patch-x.y.z.xz | patch -p1 |
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bzcat path/to/patch-x.y.z.gz | patch -p1 |
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If you wish to uncompress the patch file by hand first before applying it |
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(what I assume you've done in the examples below), then you simply run |
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gunzip or xz on the file -- like this:: |
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gunzip patch-x.y.z.gz |
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xz -d patch-x.y.z.xz |
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Which will leave you with a plain text patch-x.y.z file that you can feed to |
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patch via stdin or the ``-i`` argument, as you prefer. |
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A few other nice arguments for patch are ``-s`` which causes patch to be silent |
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except for errors which is nice to prevent errors from scrolling out of the |
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screen too fast, and ``--dry-run`` which causes patch to just print a listing of |
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what would happen, but doesn't actually make any changes. Finally ``--verbose`` |
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tells patch to print more information about the work being done. |
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Common errors when patching |
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=========================== |
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When patch applies a patch file it attempts to verify the sanity of the |
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file in different ways. |
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Checking that the file looks like a valid patch file and checking the code |
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around the bits being modified matches the context provided in the patch are |
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just two of the basic sanity checks patch does. |
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If patch encounters something that doesn't look quite right it has two |
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options. It can either refuse to apply the changes and abort or it can try |
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to find a way to make the patch apply with a few minor changes. |
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One example of something that's not 'quite right' that patch will attempt to |
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fix up is if all the context matches, the lines being changed match, but the |
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line numbers are different. This can happen, for example, if the patch makes |
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a change in the middle of the file but for some reasons a few lines have |
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been added or removed near the beginning of the file. In that case |
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everything looks good it has just moved up or down a bit, and patch will |
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usually adjust the line numbers and apply the patch. |
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Whenever patch applies a patch that it had to modify a bit to make it fit |
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it'll tell you about it by saying the patch applied with **fuzz**. |
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You should be wary of such changes since even though patch probably got it |
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right it doesn't /always/ get it right, and the result will sometimes be |
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wrong. |
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When patch encounters a change that it can't fix up with fuzz it rejects it |
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outright and leaves a file with a ``.rej`` extension (a reject file). You can |
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read this file to see exactly what change couldn't be applied, so you can |
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go fix it up by hand if you wish. |
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If you don't have any third-party patches applied to your kernel source, but |
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only patches from kernel.org and you apply the patches in the correct order, |
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and have made no modifications yourself to the source files, then you should |
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never see a fuzz or reject message from patch. If you do see such messages |
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anyway, then there's a high risk that either your local source tree or the |
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patch file is corrupted in some way. In that case you should probably try |
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re-downloading the patch and if things are still not OK then you'd be advised |
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to start with a fresh tree downloaded in full from kernel.org. |
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Let's look a bit more at some of the messages patch can produce. |
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If patch stops and presents a ``File to patch:`` prompt, then patch could not |
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find a file to be patched. Most likely you forgot to specify -p1 or you are |
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in the wrong directory. Less often, you'll find patches that need to be |
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applied with ``-p0`` instead of ``-p1`` (reading the patch file should reveal if |
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this is the case -- if so, then this is an error by the person who created |
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the patch but is not fatal). |
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If you get ``Hunk #2 succeeded at 1887 with fuzz 2 (offset 7 lines).`` or a |
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message similar to that, then it means that patch had to adjust the location |
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of the change (in this example it needed to move 7 lines from where it |
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expected to make the change to make it fit). |
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The resulting file may or may not be OK, depending on the reason the file |
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was different than expected. |
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This often happens if you try to apply a patch that was generated against a |
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different kernel version than the one you are trying to patch. |
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If you get a message like ``Hunk #3 FAILED at 2387.``, then it means that the |
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patch could not be applied correctly and the patch program was unable to |
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fuzz its way through. This will generate a ``.rej`` file with the change that |
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caused the patch to fail and also a ``.orig`` file showing you the original |
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content that couldn't be changed. |
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If you get ``Reversed (or previously applied) patch detected! Assume -R? [n]`` |
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then patch detected that the change contained in the patch seems to have |
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already been made. |
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If you actually did apply this patch previously and you just re-applied it |
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in error, then just say [n]o and abort this patch. If you applied this patch |
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previously and actually intended to revert it, but forgot to specify -R, |
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then you can say [**y**]es here to make patch revert it for you. |
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This can also happen if the creator of the patch reversed the source and |
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destination directories when creating the patch, and in that case reverting |
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the patch will in fact apply it. |
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A message similar to ``patch: **** unexpected end of file in patch`` or |
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``patch unexpectedly ends in middle of line`` means that patch could make no |
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sense of the file you fed to it. Either your download is broken, you tried to |
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feed patch a compressed patch file without uncompressing it first, or the patch |
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file that you are using has been mangled by a mail client or mail transfer |
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agent along the way somewhere, e.g., by splitting a long line into two lines. |
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Often these warnings can easily be fixed by joining (concatenating) the |
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two lines that had been split. |
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As I already mentioned above, these errors should never happen if you apply |
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a patch from kernel.org to the correct version of an unmodified source tree. |
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So if you get these errors with kernel.org patches then you should probably |
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assume that either your patch file or your tree is broken and I'd advise you |
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to start over with a fresh download of a full kernel tree and the patch you |
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wish to apply. |
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Are there any alternatives to ``patch``? |
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======================================== |
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Yes there are alternatives. |
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You can use the ``interdiff`` program (http://cyberelk.net/tim/patchutils/) to |
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generate a patch representing the differences between two patches and then |
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apply the result. |
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This will let you move from something like 5.7.2 to 5.7.3 in a single |
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step. The -z flag to interdiff will even let you feed it patches in gzip or |
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bzip2 compressed form directly without the use of zcat or bzcat or manual |
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decompression. |
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Here's how you'd go from 5.7.2 to 5.7.3 in a single step:: |
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interdiff -z ../patch-5.7.2.gz ../patch-5.7.3.gz | patch -p1 |
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Although interdiff may save you a step or two you are generally advised to |
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do the additional steps since interdiff can get things wrong in some cases. |
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Another alternative is ``ketchup``, which is a python script for automatic |
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downloading and applying of patches (https://www.selenic.com/ketchup/). |
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Other nice tools are diffstat, which shows a summary of changes made by a |
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patch; lsdiff, which displays a short listing of affected files in a patch |
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file, along with (optionally) the line numbers of the start of each patch; |
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and grepdiff, which displays a list of the files modified by a patch where |
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the patch contains a given regular expression. |
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Where can I download the patches? |
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================================= |
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The patches are available at https://kernel.org/ |
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Most recent patches are linked from the front page, but they also have |
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specific homes. |
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The 5.x.y (-stable) and 5.x patches live at |
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https://www.kernel.org/pub/linux/kernel/v5.x/ |
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The -rc patches are not stored on the webserver but are generated on |
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demand from git tags such as |
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https://git.kernel.org/torvalds/p/v5.1-rc1/v5.0 |
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The stable -rc patches live at |
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https://www.kernel.org/pub/linux/kernel/v5.x/stable-review/ |
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The 5.x kernels |
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=============== |
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These are the base stable releases released by Linus. The highest numbered |
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release is the most recent. |
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If regressions or other serious flaws are found, then a -stable fix patch |
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will be released (see below) on top of this base. Once a new 5.x base |
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kernel is released, a patch is made available that is a delta between the |
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previous 5.x kernel and the new one. |
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To apply a patch moving from 5.6 to 5.7, you'd do the following (note |
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that such patches do **NOT** apply on top of 5.x.y kernels but on top of the |
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base 5.x kernel -- if you need to move from 5.x.y to 5.x+1 you need to |
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first revert the 5.x.y patch). |
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Here are some examples:: |
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# moving from 5.6 to 5.7 |
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$ cd ~/linux-5.6 # change to kernel source dir |
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$ patch -p1 < ../patch-5.7 # apply the 5.7 patch |
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$ cd .. |
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$ mv linux-5.6 linux-5.7 # rename source dir |
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# moving from 5.6.1 to 5.7 |
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$ cd ~/linux-5.6.1 # change to kernel source dir |
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$ patch -p1 -R < ../patch-5.6.1 # revert the 5.6.1 patch |
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# source dir is now 5.6 |
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$ patch -p1 < ../patch-5.7 # apply new 5.7 patch |
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$ cd .. |
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$ mv linux-5.6.1 linux-5.7 # rename source dir |
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The 5.x.y kernels |
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================= |
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Kernels with 3-digit versions are -stable kernels. They contain small(ish) |
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critical fixes for security problems or significant regressions discovered |
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in a given 5.x kernel. |
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This is the recommended branch for users who want the most recent stable |
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kernel and are not interested in helping test development/experimental |
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versions. |
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If no 5.x.y kernel is available, then the highest numbered 5.x kernel is |
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the current stable kernel. |
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.. note:: |
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The -stable team usually do make incremental patches available as well |
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as patches against the latest mainline release, but I only cover the |
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non-incremental ones below. The incremental ones can be found at |
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https://www.kernel.org/pub/linux/kernel/v5.x/incr/ |
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These patches are not incremental, meaning that for example the 5.7.3 |
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patch does not apply on top of the 5.7.2 kernel source, but rather on top |
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of the base 5.7 kernel source. |
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So, in order to apply the 5.7.3 patch to your existing 5.7.2 kernel |
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source you have to first back out the 5.7.2 patch (so you are left with a |
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base 5.7 kernel source) and then apply the new 5.7.3 patch. |
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Here's a small example:: |
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$ cd ~/linux-5.7.2 # change to the kernel source dir |
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$ patch -p1 -R < ../patch-5.7.2 # revert the 5.7.2 patch |
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$ patch -p1 < ../patch-5.7.3 # apply the new 5.7.3 patch |
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$ cd .. |
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$ mv linux-5.7.2 linux-5.7.3 # rename the kernel source dir |
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The -rc kernels |
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=============== |
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These are release-candidate kernels. These are development kernels released |
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by Linus whenever he deems the current git (the kernel's source management |
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tool) tree to be in a reasonably sane state adequate for testing. |
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These kernels are not stable and you should expect occasional breakage if |
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you intend to run them. This is however the most stable of the main |
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development branches and is also what will eventually turn into the next |
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stable kernel, so it is important that it be tested by as many people as |
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possible. |
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This is a good branch to run for people who want to help out testing |
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development kernels but do not want to run some of the really experimental |
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stuff (such people should see the sections about -next and -mm kernels below). |
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The -rc patches are not incremental, they apply to a base 5.x kernel, just |
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like the 5.x.y patches described above. The kernel version before the -rcN |
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suffix denotes the version of the kernel that this -rc kernel will eventually |
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turn into. |
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So, 5.8-rc5 means that this is the fifth release candidate for the 5.8 |
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kernel and the patch should be applied on top of the 5.7 kernel source. |
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Here are 3 examples of how to apply these patches:: |
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# first an example of moving from 5.7 to 5.8-rc3 |
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$ cd ~/linux-5.7 # change to the 5.7 source dir |
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$ patch -p1 < ../patch-5.8-rc3 # apply the 5.8-rc3 patch |
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$ cd .. |
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$ mv linux-5.7 linux-5.8-rc3 # rename the source dir |
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# now let's move from 5.8-rc3 to 5.8-rc5 |
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$ cd ~/linux-5.8-rc3 # change to the 5.8-rc3 dir |
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$ patch -p1 -R < ../patch-5.8-rc3 # revert the 5.8-rc3 patch |
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$ patch -p1 < ../patch-5.8-rc5 # apply the new 5.8-rc5 patch |
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$ cd .. |
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$ mv linux-5.8-rc3 linux-5.8-rc5 # rename the source dir |
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# finally let's try and move from 5.7.3 to 5.8-rc5 |
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$ cd ~/linux-5.7.3 # change to the kernel source dir |
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$ patch -p1 -R < ../patch-5.7.3 # revert the 5.7.3 patch |
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$ patch -p1 < ../patch-5.8-rc5 # apply new 5.8-rc5 patch |
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$ cd .. |
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$ mv linux-5.7.3 linux-5.8-rc5 # rename the kernel source dir |
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The -mm patches and the linux-next tree |
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======================================= |
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The -mm patches are experimental patches released by Andrew Morton. |
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In the past, -mm tree were used to also test subsystem patches, but this |
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function is now done via the |
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`linux-next` (https://www.kernel.org/doc/man-pages/linux-next.html) |
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tree. The Subsystem maintainers push their patches first to linux-next, |
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and, during the merge window, sends them directly to Linus. |
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The -mm patches serve as a sort of proving ground for new features and other |
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experimental patches that aren't merged via a subsystem tree. |
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Once such patches has proved its worth in -mm for a while Andrew pushes |
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it on to Linus for inclusion in mainline. |
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The linux-next tree is daily updated, and includes the -mm patches. |
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Both are in constant flux and contains many experimental features, a |
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lot of debugging patches not appropriate for mainline etc., and is the most |
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experimental of the branches described in this document. |
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These patches are not appropriate for use on systems that are supposed to be |
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stable and they are more risky to run than any of the other branches (make |
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sure you have up-to-date backups -- that goes for any experimental kernel but |
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even more so for -mm patches or using a Kernel from the linux-next tree). |
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Testing of -mm patches and linux-next is greatly appreciated since the whole |
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point of those are to weed out regressions, crashes, data corruption bugs, |
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build breakage (and any other bug in general) before changes are merged into |
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the more stable mainline Linus tree. |
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But testers of -mm and linux-next should be aware that breakages are |
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more common than in any other tree. |
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This concludes this list of explanations of the various kernel trees. |
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I hope you are now clear on how to apply the various patches and help testing |
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the kernel. |
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Thank you's to Randy Dunlap, Rolf Eike Beer, Linus Torvalds, Bodo Eggert, |
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Johannes Stezenbach, Grant Coady, Pavel Machek and others that I may have |
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forgotten for their reviews and contributions to this document.
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