The arguments to entry probes are the same as the arguments to the corresponding operating system kernel function. These arguments can be accessed as int64_t values by using the arg0, arg1, arg2, ... variables.

If the function has a return value, the return value is stored in arg1 of the return probe. If a function does not have a return value, arg1 is not defined.

While a given function only has a single point of entry, it might have many different points where it returns to its caller. FBT collects a function's multiple return sites into a single return probe. If you want to know the exact return path, you can examine the return probe arg0 value, which indicates the offset in bytes of the returning instruction in the function text.