A request from IRC suggests a useful construct: where watchpoint probes'
addresses are computed on the fly.  Syntax could look like this:

global addr
probe kernel.data(addr).length(300) {  }
probe kernel.function("something") { addr = $myaddr }

so each time "something" hits, the address would be recomputed and the
hardware breakpoint probe reset.

This would require the same sorts of constraints on the globals as already
enforced for probe point conditionals.  This would also require the sorts
of on-the-fly registration/unregistration logic as pending for PR10995.

Some probe point types may permit such parametrization in several fields.
The dwarf ones would probably not support it in any slot, but
kprobe.function(SYM), timer.s(NN) could.