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, 0 insertions, 15 deletions

diff --git a/samples/livepatch/livepatch-shadow-fix2.c b/samples/livepatch/livepatch-shadow-fix2.c
index 53c1794bdc5f..ff9948f0ec00 100644
--- a/samples/livepatch/livepatch-shadow-fix2.c
+++ b/samples/livepatch/livepatch-shadow-fix2.c
@@ -128,21 +128,6 @@ static int livepatch_shadow_fix2_init(void)
 {
         int ret;
 
-        if (!klp_have_reliable_stack() && !patch.immediate) {
-                /*
-                 * WARNING: Be very careful when using 'patch.immediate' in
-                 * your patches.  It's ok to use it for simple patches like
-                 * this, but for more complex patches which change function
-                 * semantics, locking semantics, or data structures, it may not
-                 * be safe.  Use of this option will also prevent removal of
-                 * the patch.
-                 *
-                 * See Documentation/livepatch/livepatch.txt for more details.
-                 */
-                patch.immediate = true;
-                pr_notice("The consistency model isn't supported for your architecture.  Bypassing safety mechanisms and applying the patch immediately.\n");
-        }
-
         ret = klp_register_patch(&patch);
         if (ret)
                 return ret;