On Wed, 18 Nov 2020 13:48:37 -0600
Segher Boessenkool [off-list ref] wrote:

quoted

With it_func being the func from the struct tracepoint_func, which is a
void pointer, it is typecast to the function that is defined by the
tracepoint. args is defined as the arguments that match the proto.  

If you have at most four or so args, what you wnat to do will work on
all systems the kernel currently supports, as far as I can tell.  It
is not valid C, and none of the compilers have an extension for this
either.  But it will likely work.

Well, unfortunately, there's tracepoints with many more than 4 arguments. I
think there's one with up to 13!

quoted

The problem we are solving is on the removal case, if the memory is tight,
it is possible that the new array can not be allocated. But we must still
remove the called function. The idea in this case is to replace the
function saved with a stub. The above loop will call the stub and not the
removed function until another update happens.

This thread is about how safe is it to call:

void tp_stub_func(void) { return ; }

instead of the function that was removed?  

Exactly as safe as calling a stub defined in asm.  The undefined
behaviour happens if your program has such a call, it doesn't matter
how the called function is defined, it doesn't have to be C.

quoted

Thus, we are indeed calling that stub function from a call site that is not
using the same parameters.

The question is, will this break?  

It is unlikely to break if you use just a few arguments, all of simple
scalar types.  Just hope you will never encounter a crazy ABI :-)

But in most cases, all the arguments are of scaler types, as anything else
is not recommended, because copying is always slower than just passing a
pointer, especially since it would need to be copied for every instance of
that loop. I could do an audit to see if there's any that exist, and perhaps
even add some static checker to make sure they don't.

-- Steve