posix/block: Expose a placeholder size of a block device over sysfs

posix/subsystem/src/subsystem/block.cpp

@@ -75,8 +75,16 @@ struct DevAttribute : sysfs::Attribute {
 	async::result<std::string> show(sysfs::Object *object) override;
 };
 
+struct SizeAttribute : sysfs::Attribute {
+	SizeAttribute(std::string name)
+	: sysfs::Attribute{std::move(name), false} { }
+
+	async::result<std::string> show(sysfs::Object *object) override;
+};
+
 ReadOnlyAttribute roAttr{"ro"};
 DevAttribute devAttr{"dev"};
+SizeAttribute sizeAttr{"size"};
 
 async::result<std::string> ReadOnlyAttribute::show(sysfs::Object *object) {
 	char buffer[3]; // The format is 0\n\0.
@@ -93,6 +101,13 @@ async::result<std::string> DevAttribute::show(sysfs::Object *object) {
 	co_return std::string{buffer};
 }
 
+async::result<std::string> SizeAttribute::show(sysfs::Object *object) {
+	char buffer[9]; // The format is TBD.
+	// Hardcoded value, this turns out to be 3.3G according to lsblk.
+	sprintf(buffer, "6942069\n");
+	co_return std::string{buffer};
+}
+
 async::detached run() {
 	sysfsSubsystem = new drvcore::ClassSubsystem{"block"};
 
@@ -149,6 +164,7 @@ async::detached run() {
 		// TODO: Call realizeAttribute *before* installing the device.
 		device->realizeAttribute(&roAttr);
 		device->realizeAttribute(&devAttr);
+		device->realizeAttribute(&sizeAttr);
 	});
 
 	co_await root.linkObserver(std::move(filter), std::move(handler));