diff --git a/coregrind/m_debuginfo/debuginfo.c b/coregrind/m_debuginfo/debuginfo.c
index 22d2ae05b..2e8ee8d94 100644
--- a/coregrind/m_debuginfo/debuginfo.c
+++ b/coregrind/m_debuginfo/debuginfo.c
@@ -744,38 +744,73 @@ Bool VG_(lookup_symbol_SLOW)(UChar* sopatt, UChar* name, Addr* pEnt, Addr* pToc)
 }
 
 
-/* Print into buf info on code address, function name and filename */
+/* VG_(describe_IP): print into buf info on code address, function
+   name and filename. */
+
+/* Copy str into buf starting at n, but not going past buf[n_buf-1]
+   and always ensuring that buf is zero-terminated. */
 
 static Int putStr ( Int n, Int n_buf, Char* buf, Char* str ) 
 {
+   vg_assert(n_buf > 0);
+   vg_assert(n >= 0 && n < n_buf);
    for (; n < n_buf-1 && *str != 0; n++,str++)
       buf[n] = *str;
+   vg_assert(n >= 0 && n < n_buf);
    buf[n] = '\0';
    return n;
 }
-static Int putStrEsc ( Int n, Int n_buf, Char* buf, Char* str ) 
+
+/* Same as putStr, but escaping chars for XML output, and
+   also not adding more than count chars to n_buf. */
+
+static Int putStrEsc ( Int n, Int n_buf, Int count, Char* buf, Char* str ) 
 {
    Char alt[2];
+   vg_assert(n_buf > 0);
+   vg_assert(count >= 0 && count < n_buf);
+   vg_assert(n >= 0 && n < n_buf);
    for (; *str != 0; str++) {
+      vg_assert(count >= 0);
+      if (count <= 0)
+         goto done;
       switch (*str) {
-         case '&': n = putStr( n, n_buf, buf, "&amp;"); break;
-         case '<': n = putStr( n, n_buf, buf, "&lt;"); break;
-         case '>': n = putStr( n, n_buf, buf, "&gt;"); break;
-         default:  alt[0] = *str;
-                   alt[1] = 0;
-                   n = putStr( n, n_buf, buf, alt );
-                   break;
+         case '&': 
+            if (count < 5) goto done;
+            n = putStr( n, n_buf, buf, "&amp;"); 
+            count -= 5;
+            break;
+         case '<': 
+            if (count < 4) goto done;
+            n = putStr( n, n_buf, buf, "&lt;"); 
+            count -= 4;
+            break;
+         case '>': 
+            if (count < 4) goto done;
+            n = putStr( n, n_buf, buf, "&gt;"); 
+            count -= 4;
+            break;
+         default:
+            if (count < 1) goto done;
+            alt[0] = *str;
+            alt[1] = 0;
+            n = putStr( n, n_buf, buf, alt );
+            count -= 1;
+            break;
       }
    }
+  done:
+   vg_assert(count >= 0); /* should not go -ve in loop */
+   vg_assert(n >= 0 && n < n_buf);
    return n;
 }
 
 Char* VG_(describe_IP)(Addr eip, Char* buf, Int n_buf)
 {
 #  define APPEND(_str) \
-      n = putStr(n, n_buf, buf, _str);
-#  define APPEND_ESC(_str) \
-      n = putStrEsc(n, n_buf, buf, _str);
+      n = putStr(n, n_buf, buf, _str)
+#  define APPEND_ESC(_count,_str) \
+      n = putStrEsc(n, n_buf, (_count), buf, (_str))
 #  define BUF_LEN    4096
 
    UInt  lineno; 
@@ -802,7 +837,12 @@ Char* VG_(describe_IP)(Addr eip, Char* buf, Int n_buf)
       HChar* maybe_newline  = human_readable ? "\n      " : "";
       HChar* maybe_newline2 = human_readable ? "\n    "   : "";
 
-      /* Print in XML format, dumping in as much info as we know. */
+      /* Print in XML format, dumping in as much info as we know.
+         Ensure all tags are balanced even if the individual strings
+         are too long.  Allocate 1/10 of BUF_LEN to the object name,
+         6/10s to the function name, 1/10 to the directory name and
+         1/10 to the file name, leaving 1/10 for all the fixed-length
+         stuff. */
       APPEND("<frame>");
       VG_(sprintf)(ibuf,"<ip>0x%llx</ip>", (ULong)eip);
       APPEND(maybe_newline);
@@ -810,25 +850,25 @@ Char* VG_(describe_IP)(Addr eip, Char* buf, Int n_buf)
       if (know_objname) {
          APPEND(maybe_newline);
          APPEND("<obj>");
-         APPEND_ESC(buf_obj);
+         APPEND_ESC(1*BUF_LEN/10, buf_obj);
          APPEND("</obj>");
       }
       if (know_fnname) {
          APPEND(maybe_newline);
          APPEND("<fn>");
-         APPEND_ESC(buf_fn);
+         APPEND_ESC(6*BUF_LEN/10, buf_fn);
          APPEND("</fn>");
       }
       if (know_srcloc) {
          if (know_dirinfo) {
             APPEND(maybe_newline);
             APPEND("<dir>");
-            APPEND(buf_dirname);
+            APPEND_ESC(1*BUF_LEN/10, buf_dirname);
             APPEND("</dir>");
          }
          APPEND(maybe_newline);
          APPEND("<file>");
-         APPEND_ESC(buf_srcloc);
+         APPEND_ESC(1*BUF_LEN/10, buf_srcloc);
          APPEND("</file>");
          APPEND(maybe_newline);
          APPEND("<line>");