Re: [PATCH 03/10] ftrace: Add register_ftrace_direct()

[Steven Rostedt]

On Fri, 8 Nov 2019 18:29:09 -0800
Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:

> Is there a way to do a replacement of direct call?

I'm curious to what the use case would be. The direct call added would
still need to be a trampoline, as the parameters need to be saved
before calling any other code, and then restored before going back to
the traced function. Couldn't you do a text poke on that trampoline to
change what gets called?

> If I use unregister(old)+register(new) some events will be missed.

> If I use register(new)+unregister(old) for short period of time both new and

Actually, as we only allow a single direct call to be added at any time,
the register(new) would fail if there was already an old at the
location.

> old will be triggering on all cpus which will likely confuse bpf tracing.
> Something like modify_ftrace_direct() should solve it. It's still racy. In a
> sense that some cpus will be executing old while other cpus will be executing
> new, but per-cpu there will be no double accounting. How difficult would be
> to add such feature?

All this said, it would actually be pretty trivial to implement this,
as when another ftrace_ops is attached to the direct call location, it
falls back to the direct helper. To implement a modify_ftrace_direct(),
all that would be needed to do is to:

1) Attached a ftrace_ops stub to the same function that has the direct
caller, that will cause ftrace to got to the loop routine, and the
direct helper would then define what gets called by what what
registered in the direct_functions array.

2) Change what gets called in the direct_functions array, and at that
moment, the helper function would be using that. May require syncing
CPUs to get all CPUs seeing the same thing.

3) Remove the ftrace_ops stub, which would put back the direct caller
in the fentry location, but this time with the new function.

-- Steve