selftests: livepatch: Avoid running the tests if kernel-devel is missing

By checking if KDIR is a valid directory we can safely skip the tests if
kernel-devel isn't installed (default value of KDIR), or if KDIR
variable passed doesn't exists.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202402191417.XULH88Ct-lkp@intel.com/
Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

selftests: livepatch: Test livepatching a heavily called syscall

The test proves that a syscall can be livepatched. It is interesting
because syscalls are called a tricky way. Also the process gets
livepatched either when sleeping in the userspace or when entering
or leaving the kernel space.

The livepatch is a bit tricky:
1. The syscall function name is architecture specific. Also
ARCH_HAS_SYSCALL_WRAPPER must be taken in account.

2. The syscall must stay working the same way for other processes
on the system. It is solved by decrementing a counter only
for PIDs of the test processes. It means that the test processes
has to call the livepatched syscall at least once.

The test creates one userspace process per online cpu. The processes
are calling getpid in a busy loop. The intention is to create random
locations when the livepatch gets enabled. Nothing is guarantted.
The magic is in the randomness.

Reviewed-by: Joe Lawrence <joe.lawrence@redhat.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

livepatch: Move tests from lib/livepatch to selftests/livepatch

The modules are being moved from lib/livepatch to
tools/testing/selftests/livepatch/test_modules.

This code moving will allow writing more complex tests, like for example an
userspace C code that will call a livepatched kernel function.

The modules are now built as out-of-tree
modules, but being part of the kernel source means they will be maintained.

Another advantage of the code moving is to be able to easily change,
debug and rebuild the tests by running make on the selftests/livepatch
directory, which is not currently possible since the modules on
lib/livepatch are build and installed using the "modules" target.

The current approach also keeps the ability to execute the tests manually
by executing the scripts inside selftests/livepatch directory, as it's
currently supported. If the modules are modified, they needed to be
rebuilt before running the scripts though.

The modules are built before running the selftests when using the
kselftest invocations:

make kselftest TARGETS=livepatch
or
make -C tools/testing/selftests/livepatch run_tests

Having the modules being built as out-of-modules requires changing the
currently used 'modprobe' by 'insmod' and adapt the test scripts that
check for the kernel message buffer.

Now it is possible to only compile the modules by running:

make -C tools/testing/selftests/livepatch/

This way the test modules and other test program can be built in order
to be packaged if so desired.

As there aren't any modules being built on lib/livepatch, remove the
TEST_LIVEPATCH Kconfig and it's references.

Note: "make gen_tar" packages the pre-built binaries into the tarball.
It means that it will store the test modules pre-built for
the kernel running on the build host.

Note that these modules need not binary compatible with
the kernel built from the same sources. But the same
is true for other packaged selftest binaries.

The entire kernel sources are needed for rebuilding
the selftests on another system.

Reviewed-by: Joe Lawrence <joe.lawrence@redhat.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>