On arm64, kprobes always take an exception and so create a struct
pt_regs through the usual exception entry logic. Similarly kretprobes
taskes and exception for function entry, but for function returns it
uses a trampoline which attempts to create a struct pt_regs without
taking an exception.

This is problematic for a few reasons, including:

1) The kretprobes trampoline neither saves nor restores all of the
   portions of PSTATE. Before invoking the handler it saves a number of
   portions of PSTATE, and after returning from the handler it restores
   NZCV before returning to the original return address provided by the
   handler.

2) The kretprobe trampoline constructs the PSTATE value piecemeal from
   special purpose registers as it cannot read all of PSTATE atomically
   without taking an exception. This is somewhat fragile, and it's not
   possible to reliably recover PSTATE information which only exists on
   some physical CPUs (e.g. when SSBS support is mismatched).

   Today the kretprobes trampoline does not record:

   - BTYPE
   - SSBS
   - ALLINT
   - SS
   - PAN
   - UAO
   - DIT
   - TCO

   ... and this will only get worse with future architecture extensions
   which add more PSTATE bits.

3) The kretprobes trampoline doesn't store portions of struct pt_regs
   (e.g. the PMR value when using pseudo-NMIs). Due to this, helpers
   which operate on a struct pt_regs, such as interrupts_enabled(), may
   not work correctly.

4) The function entry and function exit handlers run in different
   contexts. The entry handler will always be run in a debug exception
   context (which is currently treated as an NMI), but the return will
   be treated as whatever context the instrumented function was executed
   in. The differences between these contexts are liable to cause
   problems (e.g. as the two can be differently interruptible or
   preemptible, adversely affecting synchronization between the
   handlers).

5) As the kretprobes trampoline runs in the same context as the code
   being probed, it is subject to the same single-stepping context,
   which may not be desirable if this is being driven by the kprobes
   handlers.

Overall, this is fragile, painful to maintain, and gets in the way of
supporting other things (e.g. RELIABLE_STACKTRACE, FEAT_NMI).

This patch addresses these issues by replacing the kretprobes trampoline
with a `BRK` instruction, and using an exception boundary to acquire and
restore the regs, in the same way as the regular kprobes trampoline.

Ive tested this atop v6.8-rc3:

| KTAP version 1
| 1..1
|     KTAP version 1
|     # Subtest: kprobes_test
|     # module: test_kprobes
|     1..7
|     ok 1 test_kprobe
|     ok 2 test_kprobes
|     ok 3 test_kprobe_missed
|     ok 4 test_kretprobe
|     ok 5 test_kretprobes
|     ok 6 test_stacktrace_on_kretprobe
|     ok 7 test_stacktrace_on_nested_kretprobe
| # kprobes_test: pass:7 fail:0 skip:0 total:7
| # Totals: pass:7 fail:0 skip:0 total:7
| ok 1 kprobes_test
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Florent Revest <revest@chromium.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/20240208145916.2004154-1-mark.rutland@arm.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>