# HG changeset patch
# User Ruslan Ermilov <ru@nginx.com>
# Date 1480748499 -10800
# Node ID 87d7e640b45dc03e47d405dbf872b6f1f4ea7782
# Parent  2af776c22aec42b8d16714ab3d008225362dd5c0
Slab: improved double free detection.

Previously, an attempt to double free the starting page of the
free range was not detected.

diff --git a/src/core/ngx_slab.c b/src/core/ngx_slab.c
--- a/src/core/ngx_slab.c
+++ b/src/core/ngx_slab.c
@@ -597,7 +597,7 @@ ngx_slab_free_locked(ngx_slab_pool_t *po
             goto wrong_chunk;
         }
 
-        if (slab == NGX_SLAB_PAGE_FREE) {
+        if (!(slab & NGX_SLAB_PAGE_START)) {
             ngx_slab_error(pool, NGX_LOG_ALERT,
                            "ngx_slab_free(): page is already free");
             goto fail;