kcov: split ioctl handling into locked and unlocked parts

kcov: split ioctl handling into locked and unlocked parts

Patch series "kcov: improve mmap processing", v3.

Subsequent mmaps of the same kcov descriptor currently do not update the
virtual memory of the task and yet return 0 (success).  This is
counter-intuitive and may lead to unexpected memory access errors.

Also, this unnecessarily limits the functionality of kcov to only the
simplest usage scenarios.  Kcov instances are effectively forever attached
to their first address spaces and it becomes impossible to e.g.  reuse the
same kcov handle in forked child processes without mmapping the memory
first.  This is exactly what we tried to do in syzkaller and inadvertently
came upon this behavior.

This patch series addresses the problem described above.

This patch (of 3):

Currently all ioctls are de facto processed under a spinlock in order to
serialise them.  This, however, prohibits the use of vmalloc and other
memory management functions in the implementations of those ioctls,
unnecessary complicating any further changes to the code.

Let all ioctls first be processed inside the kcov_ioctl() function which
should execute the ones that are not compatible with spinlock and then
pass control to kcov_ioctl_locked() for all other ones.
KCOV_REMOTE_ENABLE is processed both in kcov_ioctl() and
kcov_ioctl_locked() as the steps are easily separable.

Although it is still compatible with a spinlock, move KCOV_INIT_TRACE
handling to kcov_ioctl(), so that the changes from the next commit are
easier to follow.

Link: https://lkml.kernel.org/r/20220117153634.150357-1-nogikh@google.com
Link: https://lkml.kernel.org/r/20220117153634.150357-2-nogikh@google.com
Signed-off-by: Aleksandr Nogikh <nogikh@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Taras Madan <tarasmadan@google.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>