Candidates should be able to properly patch a kernel to add support for new hardware. This objective also includes being able to properly remove kernel patches from already patched kernels.
Key files, terms and utilities include:
| patch |
| gzip |
| bzip2 |
Makefile |
A kernel patchfile contains a difference listing produced by the diff command. The patch command is used to apply the contents of the patchfile to the kernel sources.
Patching a kernel is very straightforward:
/usr/src.
/usr/src.
patchfile
If patch is unable to apply a part of a patch, it
puts that part in a reject file. The name of a reject file is the name
of the output file plus a .rej suffix, or a # if
the addition of “.rej” would generate a
filename that is too long. In case even the addition of a mere
# would result in a filename that is too long, the
last character of the filename is replaced with a #.
The patch command and it's most common options:
Strip the smallest prefix containing number
leading slashes from each file name found in the patch file. A
sequence of one or more adjacent slashes is counted as a single
slash. This controls how file names found in the patch file are
treated, in case you keep your files in a different directory
than the person who sent out the patch. For example, supposing
the file name in the patch file was
/u/howard/src/blurfl/blurfl.c, then setting
-p0 gives the entire file name unmodified, and setting -p1 gives
u/howard/src/blurfl/blurfl.c.
Work silently, unless an error occurs.
Remove output files that are empty after the patches have been applied. Normally this option is unnecessary, since patch can examine the time stamps on the header to determine whether a file should exist after patching. However, if the input is not a context diff or if patch is conforming to POSIX, patch does not remove empty patched files unless this option is given. When patch removes a file, it also attempts to remove any empty ancestor directories.
Assume that this patch was created with the old and new files swapped. patch attempts to swap each hunk around before applying it and rejects will come out in the swapped format. The -R option does not work with ed diff scripts because there is too little information to reconstruct the reverse operation. If the first hunk of a patch fails, patch reverses the hunk to see if it can be applied that way. If it can, you are asked if you want to have the -R option set. If it can't, the patch continues to be applied normally. (Note: this method cannot detect a reversed patch if it is a normal diff and if the first command is an append (i.e. it should have been a delete) since appends always succeed, due to the fact that a null context matches anywhere. Luckily, most patches add or change lines rather than delete them, so most reversed normal diffs begin with a delete, which fails, triggering the heuristic.)
For more information consult the man-pages of the diff command and the patch command.
A kernel patch can be removed from a production kernel by removing it from the production kernel source tree and compiling a new kernel.
To remove the patch from the production kernel simply try to apply the
patch a second time, patch will ask you if it should assume
“-R” as the following example
illustrates:
# patch -p1 < patch-2.4.13-pre3
patching file linux/Documentation/Configure.help
Reversed (or previously applied) patch detected! Assume -R? [n] y