suricata: bound stack trace formatting in signal handler

The crash handler built its stack trace by advancing a pointer with
the snprintf return value. That value is the length the output would
have had, not what was written, so once the trace filled msg the
pointer moved past the end of the buffer. Every size argument after
that, SC_LOG_MAX_LOG_MSG_LEN - (temp - msg), was negative and
converted to a huge size_t, and snprintf went on writing into the
stack past msg. With a deep enough stack the handler faults before it
can log the crash it exists to report.

The frame name lookup also compared unw_get_proc_name() against
UNW_ENOMEM, but libunwind returns the negated code, so the test
never fired. A lookup that failed with UNW_ENOINFO fell through
to the success branch and printed the name buffer, which
libunwind had not written.

Track space used with offset and use that on each snprintf call. Advance
by MIN(cw, remaining - 1) to prevent overruns.  The loop stops when
the buffer is full and closes the trace with "..." so a clipped message
is not read as a whole stack.

Failed symbol lookups now print "[unknown]:". -UNW_ENOMEM is not one of
them, since libunwind returns a truncated but usable name with it.
name starts as "?" so nothing is printed from it either way.

Ticket: 8846

src/suricata.c

@@ -1,4 +1,4 @@
-/* Copyright (C) 2007-2025 Open Information Security Foundation
+/* Copyright (C) 2007-2026 Open Information Security Foundation
  *
  * You can copy, redistribute or modify this Program under the terms of
  * the GNU General Public License version 2 as published by the Free
@@ -346,29 +346,38 @@ static void SignalHandlerUnexpected(int sig_num, siginfo_t *info, void *context)
         goto terminate;
     }
 
-    char *temp = msg;
-    int cw = snprintf(temp, SC_LOG_MAX_LOG_MSG_LEN - (temp - msg), "stacktrace:sig %d:", sig_num);
-    temp += cw;
+    int cw = snprintf(msg, sizeof(msg), "stacktrace:sig %d:", sig_num);
+    if (cw < 0)
+        goto terminate;
+    size_t offset = MIN((size_t)cw, sizeof(msg) - 1);
+
     r = 1;
-    while (r > 0) {
+    while (r > 0 && offset < sizeof(msg) - 1) {
         if (unw_is_signal_frame(&cursor) == 0) {
             unw_word_t off;
-            char name[256];
-            if (unw_get_proc_name(&cursor, name, sizeof(name), &off) == UNW_ENOMEM) {
-                cw = snprintf(temp, SC_LOG_MAX_LOG_MSG_LEN - (temp - msg), "[unknown]:");
+            char name[256] = "?";
+            int ret = unw_get_proc_name(&cursor, name, sizeof(name), &off);
+            /* -UNW_ENOMEM means the name was truncated to fit; it is still usable. */
+            if (ret != 0 && ret != -UNW_ENOMEM) {
+                cw = snprintf(msg + offset, sizeof(msg) - offset, "[unknown]:");
             } else {
-                cw = snprintf(
-                        temp, SC_LOG_MAX_LOG_MSG_LEN - (temp - msg), "%s+0x%08" PRIx64, name, off);
+                cw = snprintf(msg + offset, sizeof(msg) - offset, "%s+0x%08" PRIx64, name, off);
             }
-            temp += cw;
+            if (cw < 0)
+                break;
+            offset += MIN((size_t)cw, sizeof(msg) - offset - 1);
         }
 
         r = unw_step(&cursor);
-        if (r > 0) {
-            cw = snprintf(temp, SC_LOG_MAX_LOG_MSG_LEN - (temp - msg), ";");
-            temp += cw;
+        if (r > 0 && offset < sizeof(msg) - 1) {
+            msg[offset++] = ';';
+            msg[offset] = '\0';
         }
     }
+    /* Mark a trace that ran out of room so a clipped tail is not read as a
+     * complete stack. */
+    if (offset >= sizeof(msg) - 1)
+        memcpy(msg + sizeof(msg) - 4, "...", 4);
     SCLogError("%s", msg);
 
 terminate: