Utilities: Add the lsdev utility

This utility lists all devices' major number allocations, for character
and block devices. It can help the user to figure out the DeviceMapper
service manages spawning of device nodes in /dev and other associated
files under the /tmp/system/devicemap directory.

Base/usr/share/man/man8/lsdev.md

@@ -0,0 +1,26 @@
+## Name
+
+lsdev - list devices' major number allocations
+
+## Synopsis
+
+```**sh
+$ lsdev
+```
+
+## Description
+
+lsdev is a utility for displaying devices' major number allocations, for
+character and block devices both. It shows the allocated number and its
+family name per allocation.
+
+## Files
+
+* `/sys/kernel/chardev_major_allocs` - list of the major number allocations for character devices.
+* `/sys/kernel/blockdev_major_allocs` - list of the major number allocations for block devices.
+
+## Examples
+
+```sh
+$ lsdev
+```

Userland/Utilities/CMakeLists.txt

@@ -7,7 +7,7 @@ list(APPEND REQUIRED_TARGETS
     touch tr true umount uname uniq uptime w wc which whoami xargs yes
 )
 list(APPEND RECOMMENDED_TARGETS
-    aconv adjtime aplay abench asctl bt checksum chres cksum copy fortune gzip install keymap lsirq lsof lspci lzcat man mkfs.fat mknod mktemp
+    aconv adjtime aplay abench asctl bt checksum chres cksum copy fortune gzip install keymap lsdev lsirq lsof lspci lzcat man mkfs.fat mknod mktemp
     nc netstat notify ntpquery open passwd pixelflut pls printf pro shot strings tar tt unzip wallpaper xzcat zip
 )
 

Userland/Utilities/lsdev.cpp

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+/*
+ * Copyright (c) 2022, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/JsonArray.h>
+#include <AK/JsonObject.h>
+#include <AK/NumberFormat.h>
+#include <AK/StringView.h>
+#include <LibCore/ArgsParser.h>
+#include <LibCore/File.h>
+#include <LibCore/System.h>
+#include <LibMain/Main.h>
+
+ErrorOr<int> serenity_main(Main::Arguments arguments)
+{
+    TRY(Core::System::pledge("stdio rpath"));
+    TRY(Core::System::unveil("/sys/kernel/chardev_major_allocs", "r"));
+    TRY(Core::System::unveil("/sys/kernel/blockdev_major_allocs", "r"));
+    TRY(Core::System::unveil(nullptr, nullptr));
+
+    Core::ArgsParser args_parser;
+    args_parser.set_general_help("List major device number allocations.");
+    args_parser.parse(arguments);
+
+    outln("Character devices:");
+    {
+        auto file = TRY(Core::File::open("/sys/kernel/chardev_major_allocs"sv, Core::File::OpenMode::Read));
+        auto file_contents = TRY(file->read_until_eof());
+        auto json_result = TRY(JsonValue::from_string(file_contents));
+        auto const& json = json_result.as_array();
+        json.for_each([&](auto& value) {
+            auto& major_number_allocation_object = value.as_object();
+            auto family_name = major_number_allocation_object.get_byte_string("family_name"sv).value_or({});
+            auto allocated_number = major_number_allocation_object.get_u64("allocated_number"sv).value_or(0);
+            outln("{:3d} {}", allocated_number, family_name);
+        });
+    }
+
+    outln();
+    outln("Block devices:");
+    {
+        auto file = TRY(Core::File::open("/sys/kernel/blockdev_major_allocs"sv, Core::File::OpenMode::Read));
+        auto file_contents = TRY(file->read_until_eof());
+        auto json_result = TRY(JsonValue::from_string(file_contents));
+        auto const& json = json_result.as_array();
+        json.for_each([&](auto& value) {
+            auto& major_number_allocation_object = value.as_object();
+            auto family_name = major_number_allocation_object.get_byte_string("family_name"sv).value_or({});
+            auto allocated_number = major_number_allocation_object.get_u64("allocated_number"sv).value_or(0);
+            outln("{:3d} {}", allocated_number, family_name);
+        });
+    }
+
+    return 0;
+}