symbol lookup in ddb - bad heuristic

[Valery Ushakov <uwe%stderr.spb.ru@localhost>]

db_printsym has the following heuristic:

  revision 1.68
  date: 2021-12-13 04:25:29 +0300;  author: chs;  state: Exp;  lines: +16 -2;  commitid: MT9cIBmUIZU1AqkD;
  ddb: fix function names of "noreturn" functions in stack traces.

  when looking up function names for stack traces (where the addresses
  are the return addresses of function calls), if the address is the
  first instruction in the function, assume that the function being
  called is marked "noreturn" and that the function containing the
  call is actually the function immediately before the address that we
  looked up.  to find the correct function name, do the lookup again
  with (address - 1) and then add one to the offset within the
  function that we find.


That causes breakpoints on a function entry to be misreported:

  Breakpoint in pid 0.0 (system) at netbsd:sysbeepdetach+0x21: pushl %ebp
  ...
  db{0}> show break
   Map      Count    Address
  *0x0     1    netbsd:sysbeepdetach+0x21
  db{0}> x/i sysbeepdetach+0x21   
  netbsd:clockintr:       pushl   %ebp

May be the hack need to be applied only with a new special flag, say,
KSYMS_RET?  Then we can define separate DB_STGY_PROC (no heuristic)
and DB_STGY_RET (with the heuristic).

The downside is that all MD db_stack_trace_print functions need to be
adjusted, but it actually makes sense to use both strategies there,
b/c when we are traversing an interrupt/exception frame, the
DB_STACK_PROC (without the heuristic) is the right thing to use, but
unwinding a call needs DB_STACK_RET (with the new flag).

Thoughts?

-uwe