diff --git a/libavcodec/cbs.c b/libavcodec/cbs.c
index 61cf8e3466..7c1aa005c2 100644
--- a/libavcodec/cbs.c
+++ b/libavcodec/cbs.c
@@ -871,3 +871,166 @@ int ff_cbs_alloc_unit_content2(CodedBitstreamContext *ctx,
 
     return 0;
 }
+
+static int cbs_clone_unit_content(AVBufferRef **clone_ref,
+                                  CodedBitstreamUnit *unit,
+                                  const CodedBitstreamUnitTypeDescriptor *desc)
+{
+    uint8_t *src, *copy;
+    uint8_t **src_ptr, **copy_ptr;
+    AVBufferRef **src_buf, **copy_buf;
+    int err, i;
+
+    av_assert0(unit->content);
+    src = unit->content;
+
+    copy = av_memdup(src, desc->content_size);
+    if (!copy)
+        return AVERROR(ENOMEM);
+
+    for (i = 0; i < desc->nb_ref_offsets; i++) {
+        src_ptr  = (uint8_t**)(src + desc->ref_offsets[i]);
+        src_buf  = (AVBufferRef**)(src_ptr + 1);
+        copy_ptr = (uint8_t**)(copy + desc->ref_offsets[i]);
+        copy_buf = (AVBufferRef**)(copy_ptr + 1);
+
+        if (!*src_ptr) {
+            av_assert0(!*src_buf);
+            continue;
+        }
+        if (!*src_buf) {
+            // We can't handle a non-refcounted pointer here - we don't
+            // have enough information to handle whatever structure lies
+            // at the other end of it.
+            err = AVERROR(EINVAL);
+            goto fail;
+        }
+
+        // src_ptr is required to point somewhere inside src_buf.  If it
+        // doesn't, there is a bug somewhere.
+        av_assert0(*src_ptr >= (*src_buf)->data &&
+                   *src_ptr <  (*src_buf)->data + (*src_buf)->size);
+
+        *copy_buf = av_buffer_ref(*src_buf);
+        if (!*copy_buf) {
+            err = AVERROR(ENOMEM);
+            goto fail;
+        }
+        *copy_ptr = (*copy_buf)->data + (*src_ptr - (*src_buf)->data);
+    }
+
+    *clone_ref = av_buffer_create(copy, desc->content_size,
+                                  desc->content_free ? desc->content_free :
+                                  cbs_default_free_unit_content,
+                                  (void*)desc, 0);
+    if (!*clone_ref) {
+        err = AVERROR(ENOMEM);
+        goto fail;
+    }
+
+    return 0;
+
+fail:
+    for (--i; i >= 0; i--)
+        av_buffer_unref((AVBufferRef**)(copy + desc->ref_offsets[i]));
+    av_freep(&copy);
+    *clone_ref = NULL;
+    return err;
+}
+
+int ff_cbs_make_unit_refcounted(CodedBitstreamContext *ctx,
+                                CodedBitstreamUnit *unit)
+{
+    const CodedBitstreamUnitTypeDescriptor *desc;
+    AVBufferRef *ref;
+    int err;
+
+    av_assert0(unit->content);
+    if (unit->content_ref) {
+        // Already refcounted, nothing to do.
+        return 0;
+    }
+
+    desc = cbs_find_unit_type_desc(ctx, unit);
+    if (!desc)
+        return AVERROR(ENOSYS);
+
+    switch (desc->content_type) {
+    case CBS_CONTENT_TYPE_POD:
+        ref = av_buffer_alloc(desc->content_size);
+        if (!ref)
+            return AVERROR(ENOMEM);
+        memcpy(ref->data, unit->content, desc->content_size);
+        err = 0;
+        break;
+
+    case CBS_CONTENT_TYPE_INTERNAL_REFS:
+        err = cbs_clone_unit_content(&ref, unit, desc);
+        break;
+
+    case CBS_CONTENT_TYPE_COMPLEX:
+        if (!desc->content_clone)
+            return AVERROR_PATCHWELCOME;
+        err = desc->content_clone(&ref, unit);
+        break;
+
+    default:
+        av_assert0(0 && "Invalid content type.");
+    }
+
+    if (err < 0)
+        return err;
+
+    unit->content_ref = ref;
+    unit->content     = ref->data;
+    return 0;
+}
+
+int ff_cbs_make_unit_writable(CodedBitstreamContext *ctx,
+                              CodedBitstreamUnit *unit)
+{
+    const CodedBitstreamUnitTypeDescriptor *desc;
+    AVBufferRef *ref;
+    int err;
+
+    // This can only be applied to refcounted units.
+    err = ff_cbs_make_unit_refcounted(ctx, unit);
+    if (err < 0)
+        return err;
+    av_assert0(unit->content && unit->content_ref);
+
+    if (av_buffer_is_writable(unit->content_ref))
+        return 0;
+
+    desc = cbs_find_unit_type_desc(ctx, unit);
+    if (!desc)
+        return AVERROR(ENOSYS);
+
+    switch (desc->content_type) {
+    case CBS_CONTENT_TYPE_POD:
+        err = av_buffer_make_writable(&unit->content_ref);
+        break;
+
+    case CBS_CONTENT_TYPE_INTERNAL_REFS:
+        err = cbs_clone_unit_content(&ref, unit, desc);
+        break;
+
+    case CBS_CONTENT_TYPE_COMPLEX:
+        if (!desc->content_clone)
+            return AVERROR_PATCHWELCOME;
+        err = desc->content_clone(&ref, unit);
+        break;
+
+    default:
+        av_assert0(0 && "Invalid content type.");
+    }
+    if (err < 0)
+        return err;
+
+    if (desc->content_type != CBS_CONTENT_TYPE_POD) {
+        av_buffer_unref(&unit->content_ref);
+        unit->content_ref = ref;
+    }
+    unit->content = unit->content_ref->data;
+    return 0;
+}
diff --git a/libavcodec/cbs.h b/libavcodec/cbs.h
index ea8d942894..3a054aa8f3 100644
--- a/libavcodec/cbs.h
+++ b/libavcodec/cbs.h
@@ -400,4 +400,33 @@ void ff_cbs_delete_unit(CodedBitstreamFragment *frag,
                         int position);
 
 
+/**
+ * Make the content of a unit refcounted.
+ *
+ * If the unit is not refcounted, this will do a deep copy of the unit
+ * content to new refcounted buffers.
+ *
+ * It is not valid to call this function on a unit which does not have
+ * decomposed content.
+ */
+int ff_cbs_make_unit_refcounted(CodedBitstreamContext *ctx,
+                                CodedBitstreamUnit *unit);
+
+/**
+ * Make the content of a unit writable so that internal fields can be
+ * modified.
+ *
+ * If it is known that there are no other references to the content of
+ * the unit, does nothing and returns success.  Otherwise (including the
+ * case where the unit content is not refcounted), it does a full clone
+ * of the content (including any internal buffers) to make a new copy,
+ * and replaces the existing references inside the unit with that.
+ *
+ * It is not valid to call this function on a unit which does not have
+ * decomposed content.
+ */
+int ff_cbs_make_unit_writable(CodedBitstreamContext *ctx,
+                              CodedBitstreamUnit *unit);
+
+
 #endif /* AVCODEC_CBS_H */
diff --git a/libavcodec/cbs_internal.h b/libavcodec/cbs_internal.h
index 35159f9c5f..d991e1eedf 100644
--- a/libavcodec/cbs_internal.h
+++ b/libavcodec/cbs_internal.h
@@ -80,6 +80,7 @@ typedef const struct CodedBitstreamUnitTypeDescriptor {
     size_t ref_offsets[CBS_MAX_REF_OFFSETS];
 
     void (*content_free)(void *opaque, uint8_t *data);
+    int  (*content_clone)(AVBufferRef **ref, CodedBitstreamUnit *unit);
 } CodedBitstreamUnitTypeDescriptor;
 
 typedef struct CodedBitstreamType {