livepatch's consistency model requires that no live patched function must be found on any task's stack during a transition process after a live patch is applied. It is achieved by walking through stacks of all blocked tasks.
The user might also want to define more functions to search for without them being patched at all. It may either help with preparing a live patch, which would otherwise require adding more functions just to achieve the consistency, or it can be used to overcome deficiencies the stack checking inherently has.
Consider the following example, in which GCC may optimize function parent() so that a part of it is moved to a different section (child.cold()) and parent() jumps to it. If both parent() and child2() are to patching targets, things can break easily if a task sleeps in child.cold() and new patched child2() changes ABI. parent() is not found on the stack, child.cold() jumps back to parent() eventually and new child2() is called.
parent(): /* to-be-patched */ ... jmp child.cold() /* cannot be patched */ ... schedule() ... jmp <back> ... call child2() /* to-be-patched */ ...
The patch set adds a new API which allows the user to specify such functions.
v1: https://lore.kernel.org/all/20211119090327.12811-1-mbenes@suse.cz/
Changes: -------- v2: - no separate klp_funcs, stack_only attribute is defined - tests rewritten
Miroslav Benes (2): livepatch: Allow user to specify functions to search for on a stack selftests/livepatch: Test of the API for specifying functions to search for on a stack
include/linux/livepatch.h | 3 + kernel/livepatch/core.c | 28 ++- kernel/livepatch/patch.c | 6 + kernel/livepatch/transition.c | 5 +- lib/livepatch/Makefile | 5 +- lib/livepatch/test_klp_func_stack_only_demo.c | 66 ++++++++ .../test_klp_func_stack_only_demo2.c | 61 +++++++ lib/livepatch/test_klp_func_stack_only_mod.c | 70 ++++++++ tools/testing/selftests/livepatch/Makefile | 3 +- .../livepatch/test-func-stack-only.sh | 159 ++++++++++++++++++ 10 files changed, 402 insertions(+), 4 deletions(-) create mode 100644 lib/livepatch/test_klp_func_stack_only_demo.c create mode 100644 lib/livepatch/test_klp_func_stack_only_demo2.c create mode 100644 lib/livepatch/test_klp_func_stack_only_mod.c create mode 100755 tools/testing/selftests/livepatch/test-func-stack-only.sh