diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index a90dba0662f..bc04d9dfc13 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -3745,6 +3745,16 @@ my_shmem_init(void *arg)
       areas are left in an abandoned state and any attempt to attach or
       re-initialize them will fail until the server is restarted.
      </para>
+     <para>
+      A backend attaching to an already allocated shared memory area must
+      normally pass the same <literal>size</literal> that was used to allocate
+      it to <function>ShmemRequestStruct()</function>; a mismatch results in
+      an error.  If the attaching backend does not know the size in advance,
+      it can pass <literal>SHMEM_ATTACH_UNKNOWN_SIZE</literal> instead, to
+      skip this cross-check.  <literal>SHMEM_ATTACH_UNKNOWN_SIZE</literal> can
+      only be used to attach to an area that already exists; using it before
+      allocating the area results in an error.
+     </para>
     </sect3>
 
     <sect3 id="xfunc-shared-addin-dynamic">
diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c
index f971ee24192..3345735dff0 100644
--- a/src/backend/storage/ipc/shmem.c
+++ b/src/backend/storage/ipc/shmem.c
@@ -131,6 +131,7 @@
 #include <unistd.h>
 
 #include "access/slru.h"
+#include "common/int.h"
 #include "fmgr.h"
 #include "funcapi.h"
 #include "miscadmin.h"
@@ -535,6 +536,9 @@ InitShmemIndexEntry(ShmemRequest *request)
 	size_t		allocated_size;
 	void	   *structPtr;
 
+	/* Size must be known at this point. */
+	Assert(request->options->size != SHMEM_ATTACH_UNKNOWN_SIZE);
+
 	/* look it up in the shmem index */
 	index_entry = (ShmemIndexEnt *)
 		hash_search(ShmemIndex, name, HASH_ENTER_NULL, &found);
@@ -836,6 +840,8 @@ ShmemAllocNoError(Size size)
  *
  * Also sets *allocated_size to the number of bytes allocated, which will
  * be equal to the number requested plus any padding we choose to add.
+ *
+ * Returns NULL in case space can not be allocated.
  */
 static void *
 ShmemAllocRaw(Size size, Size alignment, Size *allocated_size)
@@ -866,8 +872,13 @@ ShmemAllocRaw(Size size, Size alignment, Size *allocated_size)
 	rawStart = ShmemAllocator->free_offset;
 	newStart = TYPEALIGN(alignment, rawStart);
 
-	newFree = newStart + size;
-	if (newFree <= ShmemSegHdr->totalsize)
+	/*
+	 * newFree = newStart + size, which is the start of the remaining space
+	 * after the allocation.  If it exceeds the shmem segment size, we don't
+	 * have enough space available.
+	 */
+	if (!pg_add_size_overflow(newStart, size, &newFree) &&
+		newFree <= ShmemSegHdr->totalsize)
 	{
 		newSpace = (char *) ShmemBase + newStart;
 		ShmemAllocator->free_offset = newFree;
@@ -1024,7 +1035,14 @@ ProcessShmemRequestsAfterStartup(const ShmemCallbacks *callbacks)
 			found_any = true;
 		}
 		else
+		{
+			if (request->options->size == SHMEM_ATTACH_UNKNOWN_SIZE)
+				ereport(ERROR,
+						(errmsg("cannot attach to shared memory struct \"%s\" because it does not exist",
+								request->options->name),
+						 errdetail("SHMEM_ATTACH_UNKNOWN_SIZE can only be used to attach to an existing shared memory structure.")));
 			notfound_any = true;
+		}
 	}
 	if (found_any && notfound_any)
 		elog(ERROR, "some of the requested shmem areas have already been initialized");
diff --git a/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl b/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl
index 9432b42181b..7d961fe2790 100644
--- a/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl
+++ b/src/test/modules/test_shmem/t/001_late_shmem_alloc.pl
@@ -31,6 +31,23 @@ cmp_ok($attach_count2, '>', $attach_count1,
 
 $node->stop;
 
+###
+# Test that trying to allocate a new shmem area with size =
+# SHMEM_ATTACH_UNKNOWN_SIZE (-1) fails.
+###
+$node->append_conf('postgresql.conf', "test_shmem.area_size = -1");
+$node->start;
+
+my (undef, undef, $stderr) =
+  $node->psql("postgres", "SELECT get_test_shmem_attach_count();");
+like(
+	$stderr,
+	qr/cannot attach to shared memory struct "test_shmem area" because it does not exist/,
+	"unknown size request for a nonexistent area fails");
+
+$node->stop;
+$node->adjust_conf('postgresql.conf', 'test_shmem.area_size', undef);
+
 ###
 # Test allocating memory after startup in single-user mode
 ###
diff --git a/src/test/modules/test_shmem/test_shmem.c b/src/test/modules/test_shmem/test_shmem.c
index 6cf47dc8968..2a7e13512bf 100644
--- a/src/test/modules/test_shmem/test_shmem.c
+++ b/src/test/modules/test_shmem/test_shmem.c
@@ -35,12 +35,11 @@ typedef struct TestShmemData
 
 static TestShmemData *TestShmem;
 
-#define MIN_TEST_AREA_BYTES sizeof(TestShmemData)
-#define DEFAULT_TEST_AREA_BYTES MIN_TEST_AREA_BYTES
+#define DEFAULT_TEST_AREA_BYTES sizeof(TestShmemData)
 #define MAX_TEST_AREA_BYTES 1000000
 
 static bool attached_or_initialized = false;
-static int	test_shmem_area_size = MIN_TEST_AREA_BYTES;
+static int	test_shmem_area_size = DEFAULT_TEST_AREA_BYTES;
 static bool test_shmem_guc_defined = false;
 
 static void test_shmem_request(void *arg);
@@ -100,12 +99,16 @@ _PG_init(void)
 
 	if (!test_shmem_guc_defined)
 	{
+		/*
+		 * The minimum size that makes sense is sizeof(TestShmemData), but we
+		 * allow -1 so that we can test passing SHMEM_ATTACH_UNKNOWN_SIZE.
+		 */
 		DefineCustomIntVariable("test_shmem.area_size",
 								"Size of the shmem area to request.",
 								NULL,
 								&test_shmem_area_size,
 								DEFAULT_TEST_AREA_BYTES,
-								MIN_TEST_AREA_BYTES,
+								-1,
 								MAX_TEST_AREA_BYTES,
 								PGC_USERSET,
 								GUC_UNIT_BYTE,