diff --git a/src/glsl/blob.c b/src/glsl/blob.c
index a4003cf7178..dd4341be961 100644
--- a/src/glsl/blob.c
+++ b/src/glsl/blob.c
@@ -100,6 +100,21 @@ blob_create(void *mem_ctx)
    return blob;
 }
 
+bool
+blob_overwrite_bytes(struct blob *blob,
+                     size_t offset,
+                     const void *bytes,
+                     size_t to_write)
+{
+   /* Detect an attempt to overwrite data out of bounds. */
+   if (offset < 0 || blob->size - offset < to_write)
+      return false;
+
+   memcpy(blob->data + offset, bytes, to_write);
+
+   return true;
+}
+
 bool
 blob_write_bytes(struct blob *blob, const void *bytes, size_t to_write)
 {
@@ -134,6 +149,14 @@ blob_write_uint32(struct blob *blob, uint32_t value)
    return blob_write_bytes(blob, &value, sizeof(value));
 }
 
+bool
+blob_overwrite_uint32 (struct blob *blob,
+                       size_t offset,
+                       uint32_t value)
+{
+   return blob_overwrite_bytes(blob, offset, &value, sizeof(value));
+}
+
 bool
 blob_write_uint64(struct blob *blob, uint64_t value)
 {
diff --git a/src/glsl/blob.h b/src/glsl/blob.h
index af32d22e498..ec903ec140f 100644
--- a/src/glsl/blob.h
+++ b/src/glsl/blob.h
@@ -107,6 +107,24 @@ blob_write_bytes (struct blob *blob, const void *bytes, size_t to_write);
 uint8_t *
 blob_reserve_bytes (struct blob *blob, size_t to_write);
 
+/**
+ * Overwrite some data previously written to the blob.
+ *
+ * Writes data to an existing portion of the blob at an offset of \offset.
+ * This data range must have previously been written to the blob by one of the
+ * blob_write_* calls.
+ *
+ * For example usage, see blob_overwrite_uint32
+ *
+ * \return True unless the requested offset or offset+to_write lie outside
+ * the current blob's size.
+ */
+bool
+blob_overwrite_bytes (struct blob *blob,
+                      size_t offset,
+                      const void *bytes,
+                      size_t to_write);
+
 /**
  * Add a uint32_t to a blob.
  *
@@ -120,6 +138,31 @@ blob_reserve_bytes (struct blob *blob, size_t to_write);
 bool
 blob_write_uint32 (struct blob *blob, uint32_t value);
 
+/**
+ * Overwrite a uint32_t previously written to the blob.
+ *
+ * Writes a uint32_t value to an existing portion of the blob at an offset of
+ * \offset.  This data range must have previously been written to the blob by
+ * one of the blob_write_* calls.
+ *
+ *
+ * The expected usage is something like the following pattern:
+ *
+ *	size_t offset;
+ *
+ *	offset = blob->size;
+ *	blob_write_uint32 (blob, 0); // placeholder
+ *	... various blob write calls, writing N items ...
+ *	blob_overwrite_uint32 (blob, offset, N);
+ *
+ * \return True unless the requested position or position+to_write lie outside
+ * the current blob's size.
+ */
+bool
+blob_overwrite_uint32 (struct blob *blob,
+                       size_t offset,
+                       uint32_t value);
+
 /**
  * Add a uint64_t to a blob.
  *