On Wed, 9 Jan 2019, Petr Mladek wrote: > From: Jason Baron <jbaron@xxxxxxxxxx> > > Sometimes we would like to revert a particular fix. Currently, this > is not easy because we want to keep all other fixes active and we > could revert only the last applied patch. > > One solution would be to apply new patch that implemented all > the reverted functions like in the original code. It would work > as expected but there will be unnecessary redirections. In addition, > it would also require knowing which functions need to be reverted at > build time. > > Another problem is when there are many patches that touch the same > functions. There might be dependencies between patches that are > not enforced on the kernel side. Also it might be pretty hard to > actually prepare the patch and ensure compatibility with the other > patches. > > Atomic replace && cumulative patches: > > A better solution would be to create cumulative patch and say that > it replaces all older ones. > > This patch adds a new "replace" flag to struct klp_patch. When it is > enabled, a set of 'nop' klp_func will be dynamically created for all > functions that are already being patched but that will no longer be > modified by the new patch. They are used as a new target during > the patch transition. > > The idea is to handle Nops' structures like the static ones. When > the dynamic structures are allocated, we initialize all values that > are normally statically defined. > > The only exception is "new_func" in struct klp_func. It has to point > to the original function and the address is known only when the object > (module) is loaded. Note that we really need to set it. The address is > used, for example, in klp_check_stack_func(). > > Nevertheless we still need to distinguish the dynamically allocated > structures in some operations. For this, we add "nop" flag into > struct klp_func and "dynamic" flag into struct klp_object. They > need special handling in the following situations: > > + The structures are added into the lists of objects and functions > immediately. In fact, the lists were created for this purpose. > > + The address of the original function is known only when the patched > object (module) is loaded. Therefore it is copied later in > klp_init_object_loaded(). > > + The ftrace handler must not set PC to func->new_func. It would cause > infinite loop because the address points back to the beginning of > the original function. > > + The various free() functions must free the structure itself. > > Note that other ways to detect the dynamic structures are not considered > safe. For example, even the statically defined struct klp_object might > include empty funcs array. It might be there just to run some callbacks. > > Also note that the safe iterator must be used in the free() functions. > Otherwise already freed structures might get accessed. > > Special callbacks handling: > > The callbacks from the replaced patches are _not_ called by intention. > It would be pretty hard to define a reasonable semantic and implement it. > > It might even be counter-productive. The new patch is cumulative. It is > supposed to include most of the changes from older patches. In most cases, > it will not want to call pre_unpatch() post_unpatch() callbacks from > the replaced patches. It would disable/break things for no good reasons. > Also it should be easier to handle various scenarios in a single script > in the new patch than think about interactions caused by running many > scripts from older patches. Not to say that the old scripts even would > not expect to be called in this situation. > > Removing replaced patches: > > One nice effect of the cumulative patches is that the code from the > older patches is no longer used. Therefore the replaced patches can > be removed. It has several advantages: > > + Nops' structs will no longer be necessary and might be removed. > This would save memory, restore performance (no ftrace handler), > allow clear view on what is really patched. > > + Disabling the patch will cause using the original code everywhere. > Therefore the livepatch callbacks could handle only one scenario. > Note that the complication is already complex enough when the patch > gets enabled. It is currently solved by calling callbacks only from > the new cumulative patch. > > + The state is clean in both the sysfs interface and lsmod. The modules > with the replaced livepatches might even get removed from the system. > > Some people actually expected this behavior from the beginning. After all > a cumulative patch is supposed to "completely" replace an existing one. > It is like when a new version of an application replaces an older one. > > This patch does the first step. It removes the replaced patches from > the list of patches. It is safe. The consistency model ensures that > they are no longer used. By other words, each process works only with > the structures from klp_transition_patch. > > The removal is done by a special function. It combines actions done by > __disable_patch() and klp_complete_transition(). But it is a fast > track without all the transaction-related stuff. > > Signed-off-by: Jason Baron <jbaron@xxxxxxxxxx> > [pmladek@xxxxxxxx: Split, reuse existing code, simplified] > Signed-off-by: Petr Mladek <pmladek@xxxxxxxx> > Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> > Cc: Jessica Yu <jeyu@xxxxxxxxxx> > Cc: Jiri Kosina <jikos@xxxxxxxxxx> > Cc: Miroslav Benes <mbenes@xxxxxxx> Acked-by: Miroslav Benes <mbenes@xxxxxxx> Miroslav
