5554 kmdb can't trace stacks that begin within itself
Reviewed by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>

*** 206,216 ****
          long fr_argv[32];
          int start_index; /* index to save_instr where to start comparison */
          int err;
          int i;
  
!         struct {
                  uintptr_t fr_savfp;
                  uintptr_t fr_savpc;
          } fr;
  
          uintptr_t fp = gsp->kregs[KREG_RBP];
--- 206,216 ----
          long fr_argv[32];
          int start_index; /* index to save_instr where to start comparison */
          int err;
          int i;
  
!         struct fr {
                  uintptr_t fr_savfp;
                  uintptr_t fr_savpc;
          } fr;
  
          uintptr_t fp = gsp->kregs[KREG_RBP];
*** 223,232 ****
--- 223,234 ----
  
          GElf_Sym s;
          mdb_syminfo_t sip;
          mdb_ctf_funcinfo_t mfp;
          int xpv_panic = 0;
+         int advance_tortoise = 1;
+         uintptr_t tortoise_fp = 0;
  #ifndef _KMDB
          int xp;
  
          if ((mdb_readsym(&xp, sizeof (xp), "xpv_panicking") != -1) && (xp > 0))
                  xpv_panic = 1;
*** 235,257 ****
          bcopy(gsp, &gregs, sizeof (gregs));
  
          while (fp != 0) {
                  int args_style = 0;
  
!                 /*
!                  * Ensure progress (increasing fp), and prevent
!                  * endless loop with the same FP.
!                  */
!                 if (fp <= lastfp) {
!                         err = EMDB_STKFRAME;
                          goto badfp;
                  }
!                 if (mdb_tgt_vread(t, &fr, sizeof (fr), fp) != sizeof (fr)) {
                          err = EMDB_NOMAP;
                          goto badfp;
                  }
  
                  if ((mdb_tgt_lookup_by_addr(t, pc, MDB_TGT_SYM_FUZZY,
                      NULL, 0, &s, &sip) == 0) &&
                      (mdb_ctf_func_info(&s, &sip, &mfp) == 0)) {
                          int return_type = mdb_ctf_type_kind(mfp.mtf_return);
                          mdb_ctf_id_t args_types[5];
--- 237,274 ----
          bcopy(gsp, &gregs, sizeof (gregs));
  
          while (fp != 0) {
                  int args_style = 0;
  
!                 if (mdb_tgt_vread(t, &fr, sizeof (fr), fp) != sizeof (fr)) {
!                         err = EMDB_NOMAP;
                          goto badfp;
                  }
! 
!                 if (tortoise_fp == 0) {
!                         tortoise_fp = fp;
!                 } else {
!                         if (advance_tortoise != 0) {
!                                 struct fr tfr;
! 
!                                 if (mdb_tgt_vread(t, &tfr, sizeof (tfr),
!                                     tortoise_fp) != sizeof (tfr)) {
                                          err = EMDB_NOMAP;
                                          goto badfp;
                                  }
  
+                                 tortoise_fp = tfr.fr_savfp;
+                         }
+ 
+                         if (fp == tortoise_fp) {
+                                 err = EMDB_STKFRAME;
+                                 goto badfp;
+                         }
+                 }
+ 
+                 advance_tortoise = !advance_tortoise;
+ 
                  if ((mdb_tgt_lookup_by_addr(t, pc, MDB_TGT_SYM_FUZZY,
                      NULL, 0, &s, &sip) == 0) &&
                      (mdb_ctf_func_info(&s, &sip, &mfp) == 0)) {
                          int return_type = mdb_ctf_type_kind(mfp.mtf_return);
                          mdb_ctf_id_t args_types[5];