From 0739b5df11f2125f2f4b592e19c0d240620c5df6 Mon Sep 17 00:00:00 2001
From: Liav A <liavalb@gmail.com>
Date: Fri, 9 Feb 2024 17:17:14 +0200
Subject: [PATCH] Utilities/mount: Automatically mount regular files with loop
 devices

Check if the source fd is not a block device file using the fstat
syscall and if that's the case, try to mount the file using a temporary
loop device.
---
 Base/usr/share/man/man8/mount.md |  3 +++
 Userland/Utilities/mount.cpp     | 22 +++++++++++++++++++++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/Base/usr/share/man/man8/mount.md b/Base/usr/share/man/man8/mount.md
index 44827728d2..b69e1fb385 100644
--- a/Base/usr/share/man/man8/mount.md
+++ b/Base/usr/share/man/man8/mount.md
@@ -44,6 +44,9 @@ Additionally, the name `defaults` is accepted and ignored.
 ```sh
 # mount devpts /dev/pts -t devpts -o noexec,nosuid
 # mount /home/anon/myfile.txt /tmp/foo -o bind
+
+# mount a regular file using a temporary loop device
+$ mount /home/anon/myfilesystem.bin /mnt
 ```
 
 ## See also
diff --git a/Userland/Utilities/mount.cpp b/Userland/Utilities/mount.cpp
index dca6e69255..573fbececb 100644
--- a/Userland/Utilities/mount.cpp
+++ b/Userland/Utilities/mount.cpp
@@ -7,6 +7,7 @@
 #include <AK/JsonArray.h>
 #include <AK/JsonObject.h>
 #include <AK/JsonValue.h>
+#include <AK/String.h>
 #include <LibCore/ArgsParser.h>
 #include <LibCore/DirIterator.h>
 #include <LibCore/File.h>
@@ -197,6 +198,21 @@ static ErrorOr<void> print_mounts()
     return {};
 }
 
+static ErrorOr<void> mount_using_loop_device(int inode_fd, StringView mountpoint, StringView fs_type, int flags)
+{
+    int devctl_fd = TRY(Core::System::open("/dev/devctl"sv, O_RDONLY));
+    int value = inode_fd;
+    TRY(Core::System::ioctl(devctl_fd, DEVCTL_CREATE_LOOP_DEVICE, &value));
+    int loop_device_index = value;
+
+    auto loop_device_path = TRY(String::formatted("/dev/loop/{}", loop_device_index));
+    int loop_device_fd = TRY(Core::System::open(loop_device_path.bytes_as_string_view(), O_RDONLY));
+
+    auto result = Core::System::mount(loop_device_fd, mountpoint, fs_type, flags);
+    TRY(Core::System::ioctl(devctl_fd, DEVCTL_DESTROY_LOOP_DEVICE, &loop_device_index));
+    return result;
+}
+
 ErrorOr<int> serenity_main(Main::Arguments arguments)
 {
     StringView source;
@@ -242,7 +258,11 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
         } else {
             if (fs_type.is_empty())
                 fs_type = "ext2"sv;
-            TRY(Core::System::mount(fd, mountpoint, fs_type, flags));
+            auto stat = TRY(Core::System::fstat(fd));
+            if (!S_ISBLK(stat.st_mode))
+                TRY(mount_using_loop_device(fd, mountpoint, fs_type, flags));
+            else
+                TRY(Core::System::mount(fd, mountpoint, fs_type, flags));
         }
         return 0;
     }