support: Add support_blob_repeat_allocate_shared

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

3 files changed, 68 insertions, 20 deletions

diff --git a/support/blob_repeat.c b/support/blob_repeat.c
index a7aa9bf4c7..cd6297e026 100644
--- a/support/blob_repeat.c
+++ b/support/blob_repeat.c
@@ -125,10 +125,11 @@ minimum_stride_size (size_t page_size, size_t element_size)
 }
 
 /* Allocations larger than maximum_small_size potentially use mmap
-   with alias mappings.  */
+   with alias mappings.  If SHARED, the alias mappings are created
+   using MAP_SHARED instead of MAP_PRIVATE.  */
 static struct support_blob_repeat
 allocate_big (size_t total_size, const void *element, size_t element_size,
-              size_t count)
+              size_t count, bool shared)
 {
   unsigned long page_size = xsysconf (_SC_PAGESIZE);
   size_t stride_size = minimum_stride_size (page_size, element_size);
@@ -213,7 +214,11 @@ allocate_big (size_t total_size, const void *element, size_t element_size,
   {
     size_t remaining_size = total_size;
     char *current = target;
-    int flags = MAP_FIXED | MAP_FILE | MAP_PRIVATE;
+    int flags = MAP_FIXED | MAP_FILE;
+    if (shared)
+      flags |= MAP_SHARED;
+    else
+      flags |= MAP_PRIVATE;
 #ifdef MAP_NORESERVE
     flags |= MAP_NORESERVE;
 #endif
@@ -251,8 +256,8 @@ allocate_big (size_t total_size, const void *element, size_t element_size,
 }
 
 struct support_blob_repeat
-support_blob_repeat_allocate (const void *element, size_t element_size,
-                              size_t count)
+repeat_allocate (const void *element, size_t element_size,
+		 size_t count, bool shared)
 {
   size_t total_size;
   if (__builtin_mul_overflow (element_size, count, &total_size))
@@ -263,7 +268,21 @@ support_blob_repeat_allocate (const void *element, size_t element_size,
   if (total_size <= maximum_small_size)
     return allocate_malloc (total_size, element, element_size, count);
   else
-    return allocate_big (total_size, element, element_size, count);
+    return allocate_big (total_size, element, element_size, count, shared);
+}
+
+struct support_blob_repeat
+support_blob_repeat_allocate (const void *element, size_t element_size,
+                              size_t count)
+{
+  return repeat_allocate (element, element_size, count, false);
+}
+
+struct support_blob_repeat
+support_blob_repeat_allocate_shared (const void *element, size_t element_size,
+				     size_t count)
+{
+  return repeat_allocate (element, element_size, count, true);
 }
 
 void
diff --git a/support/blob_repeat.h b/support/blob_repeat.h
index 12f33bcd02..519458cf50 100644
--- a/support/blob_repeat.h
+++ b/support/blob_repeat.h
@@ -38,7 +38,17 @@ struct support_blob_repeat support_blob_repeat_allocate (const void *element,
                                                          size_t element_size,
                                                          size_t count);
 
-/* Deallocate the blob created by support_blob_repeat_allocate.  */
+/* Like support_blob_repeat_allocate, except that copy-on-write
+   semantics are disabled.  This means writing to one part of the blob
+   can affect other parts.  It is possible to map non-shared memory
+   over parts of the resulting blob using MAP_ANONYMOUS | MAP_FIXED
+   | MAP_PRIVATE, so that writes to these parts do not affect
+   others.  */
+struct support_blob_repeat support_blob_repeat_allocate_shared
+  (const void *element, size_t element_size, size_t count);
+
+/* Deallocate the blob created by support_blob_repeat_allocate or
+   support_blob_repeat_allocate_shared.  */
 void support_blob_repeat_free (struct support_blob_repeat *);
 
 #endif /* SUPPORT_BLOB_REPEAT_H */
diff --git a/support/tst-support_blob_repeat.c b/support/tst-support_blob_repeat.c
index a0eb9d2b89..b61d6b249a 100644
--- a/support/tst-support_blob_repeat.c
+++ b/support/tst-support_blob_repeat.c
@@ -17,6 +17,7 @@
    <https://www.gnu.org/licenses/>.  */
 
 #include <stdio.h>
+#include <string.h>
 #include <support/blob_repeat.h>
 #include <support/check.h>
 
@@ -63,21 +64,39 @@ do_test (void)
     }
   support_blob_repeat_free (&repeat);
 
-  repeat = support_blob_repeat_allocate ("012345678", 9, 10 * 1000 * 1000);
-  if (repeat.start == NULL)
-    puts ("warning: not enough memory for large mapping");
-  else
+  for (int do_shared = 0; do_shared < 2; ++do_shared)
     {
-      unsigned char *p = repeat.start;
-      for (int i = 0; i < 10 * 1000 * 1000; ++i)
-        for (int j = 0; j <= 8; ++j)
-          if (p[i * 9 + j] != '0' + j)
-            {
-              printf ("error: element %d index %d\n", i, j);
-              TEST_COMPARE (p[i * 9 + j], '0' + j);
-            }
+      if (do_shared)
+        repeat = support_blob_repeat_allocate_shared ("012345678", 9,
+                                                      10 * 1000 * 1000);
+      else
+        repeat = support_blob_repeat_allocate ("012345678", 9,
+                                               10 * 1000 * 1000);
+      if (repeat.start == NULL)
+        puts ("warning: not enough memory for large mapping");
+      else
+        {
+          unsigned char *p = repeat.start;
+          for (int i = 0; i < 10 * 1000 * 1000; ++i)
+            for (int j = 0; j <= 8; ++j)
+              if (p[i * 9 + j] != '0' + j)
+                {
+                  printf ("error: element %d index %d\n", i, j);
+                  TEST_COMPARE (p[i * 9 + j], '0' + j);
+                }
+
+          enum { total_size = 9 * 10 * 1000 * 1000 };
+          p[total_size - 1] = '\0';
+          asm ("" ::: "memory");
+          if (do_shared)
+            /* The write is repeated in multiple places earlier in the
+               string due to page sharing.  */
+            TEST_VERIFY (strlen (repeat.start) < total_size - 1);
+          else
+            TEST_COMPARE (strlen (repeat.start), total_size - 1);
+        }
+      support_blob_repeat_free (&repeat);
     }
-  support_blob_repeat_free (&repeat);
 
   return 0;
 }