fdtoverlay: Allow adding labels to __overlay__ nodes in overlays

When applying overlays, we merge symbols from the overlay into the target
tree.  At the moment the logic for this assumes all symbols in the overlay
are attached to a node of the form:
	/fragment@XXX/__overlay__/relative/path

And will end up applied to the relative/path node under the fragment's
target.

However, this disallows the case of a symbol in the form just:
	/fragment@XXX/__overlay__

This does have a pretty obvious sensible meaning: attach the new symbol
directly to the fragment's target, but we don't currently do that.

It's pretty easy to workaround this limitation in one's overlays, but it's
also easy to handle in the overlay applying code, so we might as well
extend it to cover this case.

Reported-by: Christophe Braillon
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

libfdt/fdt_overlay.c

@@ -733,8 +733,6 @@ static int overlay_symbol_update(void *fdt, void *fdto)
 		/* keep end marker to avoid strlen() */
 		e = path + path_len;
 
-		/* format: /<fragment-name>/__overlay__/<relative-subnode-path> */
-
 		if (*path != '/')
 			return -FDT_ERR_BADVALUE;
 
@@ -748,11 +746,18 @@ static int overlay_symbol_update(void *fdt, void *fdto)
 
 		/* verify format; safe since "s" lies in \0 terminated prop */
 		len = sizeof("/__overlay__/") - 1;
-		if ((e - s) < len || memcmp(s, "/__overlay__/", len))
+		if ((e - s) > len && (memcmp(s, "/__overlay__/", len) == 0)) {
+			/* /<fragment-name>/__overlay__/<relative-subnode-path> */
+			rel_path = s + len;
+			rel_path_len = e - rel_path;
+		} else if ((e - s) == len
+			   && (memcmp(s, "/__overlay__", len - 1) == 0)) {
+			/* /<fragment-name>/__overlay__ */
+			rel_path = "";
+			rel_path_len = 0;
+		} else {
 			return -FDT_ERR_BADOVERLAY;
-
-		rel_path = s + len;
-		rel_path_len = e - rel_path;
+		}
 
 		/* find the fragment index in which the symbol lies */
 		ret = fdt_subnode_offset_namelen(fdto, 0, frag_name,

tests/run_tests.sh

@@ -949,6 +949,18 @@ fdtoverlay_tests() {
 
     # test that fdtoverlay manages to apply overlays with long target path
     run_fdtoverlay_test lpath "/test-node/sub-test-node/sub-test-node-with-very-long-target-path/test-0" "prop" "-ts" ${basedtb} ${target_long_pathdtb} ${overlay_long_pathdtb}
+
+    # test adding a label to the root of a fragment
+    stacked_base_nolabel=stacked_overlay_base_nolabel.dts
+    stacked_base_nolabeldtb=stacked_overlay_base_nolabel.test.dtb
+    stacked_addlabel=stacked_overlay_addlabel.dts
+    stacked_addlabeldtb=stacked_overlay_addlabel.test.dtb
+    stacked_addlabel_targetdtb=stacked_overlay_target_nolabel.fdtoverlay.test.dtb
+
+    run_dtc_test -@ -I dts -O dtb -o $stacked_base_nolabeldtb $stacked_base_nolabel
+    run_dtc_test -@ -I dts -O dtb -o $stacked_addlabeldtb $stacked_addlabel
+
+    run_fdtoverlay_test baz "/foonode/barnode/baznode" "baz-property" "-ts" ${stacked_base_nolabeldtb} ${stacked_addlabel_targetdtb} ${stacked_addlabeldtb} ${stacked_bardtb} ${stacked_bazdtb}
 }
 
 pylibfdt_tests () {

tests/stacked_overlay_addlabel.dts

@@ -0,0 +1,13 @@
+/dts-v1/;
+/plugin/;
+/ {
+	fragment@1 {
+		target-path = "/foonode";
+		foo: __overlay__ {
+			overlay-1-property;
+			bar: barnode {
+				bar-property = "bar";
+			};
+		};
+	};
+};

tests/stacked_overlay_base_nolabel.dts

@@ -0,0 +1,6 @@
+/dts-v1/;
+/ {
+	foonode {
+		foo-property = "foo";
+	};
+};