> You could try putting this into gdb.exp somewhere:
> 
>     proc gdb_trace_error {name ignore op} {
>       error "Attempt to write to $name."
>     }
> 
>     trace variable prms_id w gdb_trace_error
>     trace variable bug_id w gdb_trace_error

Nice trick! Attached is a patch that implements something along those
lines...  Ideally, we would really like to check any use of these
variables (including reads and unsets), but these variable do get
accessed during the run (computing some pass strings). In practice,
all the uses I've found were setting them, so it is probably going
to be good enough.

Tested on x86_64-linux.

-- 
Joel