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

*** 195,205 ****
          mdb_tgt_gregset_t gregs;
          kreg_t *kregs = &gregs.kregs[0];
          int got_pc = (gsp->kregs[KREG_EIP] != 0);
          int err;
  
!         struct {
                  uintptr_t fr_savfp;
                  uintptr_t fr_savpc;
                  long fr_argv[32];
          } fr;
  
--- 195,205 ----
          mdb_tgt_gregset_t gregs;
          kreg_t *kregs = &gregs.kregs[0];
          int got_pc = (gsp->kregs[KREG_EIP] != 0);
          int err;
  
!         struct fr {
                  uintptr_t fr_savfp;
                  uintptr_t fr_savpc;
                  long fr_argv[32];
          } fr;
  
*** 208,236 ****
          uintptr_t lastfp = 0;
  
          ssize_t size;
          uint_t argc;
          int detect_exception_frames = 0;
  #ifndef _KMDB
          int xp;
  
          if ((mdb_readsym(&xp, sizeof (xp), "xpv_panicking") != -1) && (xp > 0))
                  detect_exception_frames = 1;
  #endif
  
          bcopy(gsp, &gregs, sizeof (gregs));
  
          while (fp != 0) {
- 
-                 /*
-                  * Ensure progress (increasing fp), and prevent
-                  * endless loop with the same FP.
-                  */
-                 if (fp <= lastfp) {
-                         err = EMDB_STKFRAME;
-                         goto badfp;
-                 }
                  if (fp & (STACK_ALIGN - 1)) {
                          err = EMDB_STKALIGN;
                          goto badfp;
                  }
                  if ((size = mdb_tgt_vread(t, &fr, sizeof (fr), fp)) >=
--- 208,229 ----
          uintptr_t lastfp = 0;
  
          ssize_t size;
          uint_t argc;
          int detect_exception_frames = 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))
                  detect_exception_frames = 1;
  #endif
  
          bcopy(gsp, &gregs, sizeof (gregs));
  
          while (fp != 0) {
                  if (fp & (STACK_ALIGN - 1)) {
                          err = EMDB_STKALIGN;
                          goto badfp;
                  }
                  if ((size = mdb_tgt_vread(t, &fr, sizeof (fr), fp)) >=
*** 240,249 ****
--- 233,265 ----
                  } else {
                          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 (got_pc && func(arg, pc, argc, fr.fr_argv, &gregs) != 0)
                          break;
  
                  kregs[KREG_ESP] = kregs[KREG_EBP];