Kernel+Userland: Add mount MS_SRCHIDDEN option

Either we mount from a loop device or other source, the user might want
to obfuscate the given source for security reasons, so this option will
ensure this will happen.
If passed during a mount, the source will be hidden when reading from
the /sys/kernel/df node.

Kernel/API/POSIX/unistd.h

@@ -30,6 +30,7 @@ extern "C" {
 #define MS_WXALLOWED (1 << 6)
 #define MS_AXALLOWED (1 << 7)
 #define MS_NOREGULAR (1 << 8)
+#define MS_SRCHIDDEN (1 << 9)
 
 enum {
     _SC_MONOTONIC_CLOCK,

Kernel/FileSystem/SysFS/Subsystems/Kernel/DiskUsage.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <AK/JsonObjectSerializer.h>
+#include <Kernel/API/POSIX/unistd.h>
 #include <Kernel/Devices/Loop/LoopDevice.h>
 #include <Kernel/FileSystem/FileBackedFileSystem.h>
 #include <Kernel/FileSystem/SysFS/Subsystems/Kernel/DiskUsage.h>
@@ -40,18 +41,22 @@ ErrorOr<void> SysFSDiskUsage::try_generate(KBufferBuilder& builder)
         TRY(fs_object.add("readonly"sv, fs.is_readonly()));
         TRY(fs_object.add("mount_flags"sv, mount.flags()));
 
-        if (fs.is_file_backed()) {
-            auto& file = static_cast<const FileBackedFileSystem&>(fs).file();
-            if (file.is_loop_device()) {
-                auto& device = static_cast<LoopDevice const&>(file);
-                auto path = TRY(device.custody().try_serialize_absolute_path());
-                TRY(fs_object.add("source"sv, path->view()));
+        if (mount.flags() & MS_SRCHIDDEN) {
+            TRY(fs_object.add("source"sv, "unknown"));
+        } else {
+            if (fs.is_file_backed()) {
+                auto& file = static_cast<const FileBackedFileSystem&>(fs).file();
+                if (file.is_loop_device()) {
+                    auto& device = static_cast<LoopDevice const&>(file);
+                    auto path = TRY(device.custody().try_serialize_absolute_path());
+                    TRY(fs_object.add("source"sv, path->view()));
+                } else {
+                    auto pseudo_path = TRY(static_cast<const FileBackedFileSystem&>(fs).file_description().pseudo_path());
+                    TRY(fs_object.add("source"sv, pseudo_path->view()));
+                }
             } else {
-                auto pseudo_path = TRY(static_cast<const FileBackedFileSystem&>(fs).file_description().pseudo_path());
-                TRY(fs_object.add("source"sv, pseudo_path->view()));
+                TRY(fs_object.add("source"sv, "none"));
             }
-        } else {
-            TRY(fs_object.add("source"sv, "none"));
         }
 
         TRY(fs_object.finish());

Userland/Utilities/mount.cpp

@@ -43,6 +43,8 @@ static int parse_options(StringView options)
             flags |= MS_AXALLOWED;
         else if (part == "noregular")
             flags |= MS_NOREGULAR;
+        else if (part == "srchidden")
+            flags |= MS_SRCHIDDEN;
         else
             warnln("Ignoring invalid option: {}", part);
     }
@@ -181,6 +183,8 @@ static ErrorOr<void> print_mounts()
             out(",nodev");
         if (mount_flags & MS_NOREGULAR)
             out(",noregular");
+        if (mount_flags & MS_SRCHIDDEN)
+            out(",srcobfuscate");
         if (mount_flags & MS_NOEXEC)
             out(",noexec");
         if (mount_flags & MS_NOSUID)