ftrace_hash_ipmodify_enable() checks IPMODIFY and DIRECT ftrace_ops on
the same kernel function. When needed, ftrace_hash_ipmodify_enable()
calls ops->ops_func() to prepare the direct ftrace (BPF trampoline) to
share the same function as the IPMODIFY ftrace (livepatch).

ftrace_hash_ipmodify_enable() is called in register_ftrace_direct() path,
but not called in modify_ftrace_direct() path. As a result, the following
operations will break livepatch:

1. Load livepatch to a kernel function;
2. Attach fentry program to the kernel function;
3. Attach fexit program to the kernel function.

After 3, the kernel function being used will not be the livepatched
version, but the original version.

Fix this by adding __ftrace_hash_update_ipmodify() to
__modify_ftrace_direct() and adjust some logic around the call.

Signed-off-by: Song Liu <[email protected]>
Reviewed-by: Jiri Olsa <[email protected]>

Both livepatch and BPF trampoline use ftrace. Special attention is needed
when livepatch and fexit program touch the same function at the same
time, because livepatch updates a kernel function and the BPF trampoline
need to call into the right version of the kernel function.

Use samples/livepatch/livepatch-sample.ko for the test.

The test covers two cases:
  1) When a fentry program is loaded first. This exercises the
     modify_ftrace_direct code path.
  2) When a fentry program is loaded first. This exercises the
     register_ftrace_direct code path.

Signed-off-by: Song Liu <[email protected]>
Reviewed-by: Jiri Olsa <[email protected]>