livepatch: Remove immediate feature

Immediate flag has been used to disable per-task consistency and patch
all tasks immediately. It could be useful if the patch doesn't change any
function or data semantics.

However, it causes problems on its own. The consistency problem is
currently broken with respect to immediate patches.

func            a
patches         1i
                2i
                3

When the patch 3 is applied, only 2i function is checked (by stack
checking facility). There might be a task sleeping in 1i though. Such
task is migrated to 3, because we do not check 1i in
klp_check_stack_func() at all.

Coming atomic replace feature would be easier to implement and more
reliable without immediate.

Thus, remove immediate feature completely and save us from the problems.

Note that force feature has the similar problem. However it is
considered as a last resort. If used, administrator should not apply any
new live patches and should plan for reboot into an updated kernel.

The architectures would now need to provide HAVE_RELIABLE_STACKTRACE to
fully support livepatch.

Signed-off-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

1 files changed, 1 insertions, 11 deletions

diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 1c3c9b27c916..41be6061b92f 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -366,11 +366,6 @@ static int __klp_enable_patch(struct klp_patch *patch)
         /*
          * A reference is taken on the patch module to prevent it from being
          * unloaded.
-         *
-         * Note: For immediate (no consistency model) patches we don't allow
-         * patch modules to unload since there is no safe/sane method to
-         * determine if a thread is still running in the patched code contained
-         * in the patch module once the ftrace registration is successful.
          */
         if (!try_module_get(patch->mod))
                 return -ENODEV;
@@ -890,12 +885,7 @@ int klp_register_patch(struct klp_patch *patch)
         if (!klp_initialized())
                 return -ENODEV;
 
-        /*
-         * Architectures without reliable stack traces have to set
-         * patch->immediate because there's currently no way to patch kthreads
-         * with the consistency model.
-         */
-        if (!klp_have_reliable_stack() && !patch->immediate) {
+        if (!klp_have_reliable_stack()) {
                 pr_err("This architecture doesn't have support for the livepatch consistency model.\n");
                 return -ENOSYS;
         }