mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-12-15 23:14:31 +08:00
89e33ea732
Convert livepatch documentation to ReST format. The changes
are mostly trivial, as the documents are already on a good
shape. Just a few markup changes are needed for Sphinx to
properly parse the docs.
The conversion is actually:
- add blank lines and identation in order to identify paragraphs;
- fix tables markups;
- add some lists markups;
- mark literal blocks;
- The in-file TOC becomes a comment, in order to skip it from the
output, as Sphinx already generates an index there.
- adjust title markups.
At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Joe Lawrence <joe.lawrence@redhat.com>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
103 lines
3.8 KiB
ReStructuredText
103 lines
3.8 KiB
ReStructuredText
===================================
|
|
Atomic Replace & Cumulative Patches
|
|
===================================
|
|
|
|
There might be dependencies between livepatches. If multiple patches need
|
|
to do different changes to the same function(s) then we need to define
|
|
an order in which the patches will be installed. And function implementations
|
|
from any newer livepatch must be done on top of the older ones.
|
|
|
|
This might become a maintenance nightmare. Especially when more patches
|
|
modified the same function in different ways.
|
|
|
|
An elegant solution comes with the feature called "Atomic Replace". It allows
|
|
creation of so called "Cumulative Patches". They include all wanted changes
|
|
from all older livepatches and completely replace them in one transition.
|
|
|
|
Usage
|
|
-----
|
|
|
|
The atomic replace can be enabled by setting "replace" flag in struct klp_patch,
|
|
for example::
|
|
|
|
static struct klp_patch patch = {
|
|
.mod = THIS_MODULE,
|
|
.objs = objs,
|
|
.replace = true,
|
|
};
|
|
|
|
All processes are then migrated to use the code only from the new patch.
|
|
Once the transition is finished, all older patches are automatically
|
|
disabled.
|
|
|
|
Ftrace handlers are transparently removed from functions that are no
|
|
longer modified by the new cumulative patch.
|
|
|
|
As a result, the livepatch authors might maintain sources only for one
|
|
cumulative patch. It helps to keep the patch consistent while adding or
|
|
removing various fixes or features.
|
|
|
|
Users could keep only the last patch installed on the system after
|
|
the transition to has finished. It helps to clearly see what code is
|
|
actually in use. Also the livepatch might then be seen as a "normal"
|
|
module that modifies the kernel behavior. The only difference is that
|
|
it can be updated at runtime without breaking its functionality.
|
|
|
|
|
|
Features
|
|
--------
|
|
|
|
The atomic replace allows:
|
|
|
|
- Atomically revert some functions in a previous patch while
|
|
upgrading other functions.
|
|
|
|
- Remove eventual performance impact caused by core redirection
|
|
for functions that are no longer patched.
|
|
|
|
- Decrease user confusion about dependencies between livepatches.
|
|
|
|
|
|
Limitations:
|
|
------------
|
|
|
|
- Once the operation finishes, there is no straightforward way
|
|
to reverse it and restore the replaced patches atomically.
|
|
|
|
A good practice is to set .replace flag in any released livepatch.
|
|
Then re-adding an older livepatch is equivalent to downgrading
|
|
to that patch. This is safe as long as the livepatches do _not_ do
|
|
extra modifications in (un)patching callbacks or in the module_init()
|
|
or module_exit() functions, see below.
|
|
|
|
Also note that the replaced patch can be removed and loaded again
|
|
only when the transition was not forced.
|
|
|
|
|
|
- Only the (un)patching callbacks from the _new_ cumulative livepatch are
|
|
executed. Any callbacks from the replaced patches are ignored.
|
|
|
|
In other words, the cumulative patch is responsible for doing any actions
|
|
that are necessary to properly replace any older patch.
|
|
|
|
As a result, it might be dangerous to replace newer cumulative patches by
|
|
older ones. The old livepatches might not provide the necessary callbacks.
|
|
|
|
This might be seen as a limitation in some scenarios. But it makes life
|
|
easier in many others. Only the new cumulative livepatch knows what
|
|
fixes/features are added/removed and what special actions are necessary
|
|
for a smooth transition.
|
|
|
|
In any case, it would be a nightmare to think about the order of
|
|
the various callbacks and their interactions if the callbacks from all
|
|
enabled patches were called.
|
|
|
|
|
|
- There is no special handling of shadow variables. Livepatch authors
|
|
must create their own rules how to pass them from one cumulative
|
|
patch to the other. Especially that they should not blindly remove
|
|
them in module_exit() functions.
|
|
|
|
A good practice might be to remove shadow variables in the post-unpatch
|
|
callback. It is called only when the livepatch is properly disabled.
|