ngx_sprintf() has implicit limit of 65536 bytes so on Linux/ppc64
in 32-bit mode the errstr is somewhere at 0xffffd680, the "last" overflows
and ngx_vsnprintf() does not print at all

src/core/ngx_log.c

diff --git a/src/core/ngx_log.c b/src/core/ngx_log.c
index ea4e7f2..175190c 100644
--- a/src/core/ngx_log.c
+++ b/src/core/ngx_log.c
@@ -92,13 +92,14 @@
 
     p = errstr + ngx_cached_err_log_time.len;
 
-    p = ngx_sprintf(p, " [%s] ", err_levels[level]);
+    p = ngx_snprintf(p, last - p, " [%s] ", err_levels[level]);
 
     /* pid#tid */
-    p = ngx_sprintf(p, "%P#" NGX_TID_T_FMT ": ", ngx_log_pid, ngx_log_tid);
+    p = ngx_snprintf(p, last - p, "%P#" NGX_TID_T_FMT ": ",
+                    ngx_log_pid, ngx_log_tid);
 
     if (log->connection) {
-        p = ngx_sprintf(p, "*%uA ", log->connection);
+        p = ngx_snprintf(p, last - p, "*%uA ", log->connection);
     }
 
 #if (NGX_HAVE_VARIADIC_MACROS)