Update plan: truncate over dd, direct mount, chown+sync before fa2json, drop losetup

test/Manual experiment.md

@@ -0,0 +1,82 @@
+## Manual experiment
+
+> [!NOTE]
+> This manual experiment shows how we can do the testing (teardown not included). Note that we don't need the `losetup`-stuff, we know where everything is.
+
+### Compile
+```sh
+gcc fs-watcher.c json-writer.c -o fa2json
+```
+### Create image file
+```sh
+mktemp /tmp/fa2json-test-XXXXXX.img
+```
+
+```text
+/tmp/fa2json-test-UrwpOb.img
+```
+
+```sh
+truncate -s 10M /tmp/fa2json-test-UrwpOb.img
+```
+
+```sh
+mkfs.ext4 /tmp/fa2json-test-UrwpOb.img
+```
+
+```text
+mke2fs 1.47.3 (8-Jul-2025)
+Discarding device blocks: done
+Creating filesystem with 10240 1k blocks and 2560 inodes
+Filesystem UUID: 035c508e-dec0-4a21-a4d1-1efb6fa72415
+Superblock backups stored on blocks:
+        8193
+
+Allocating group tables: done
+Writing inode tables: done
+Creating journal (1024 blocks): done
+Writing superblocks and filesystem accounting information: done
+```
+
+
+### Create mount point
+
+```sh
+mktemp -d /tmp/fa2json-mnt-XXXXXX
+```
+
+```text
+/tmp/fa2json-mnt-ts2Dik
+```
+
+### Mount loop device
+```sh
+sudo mount /tmp/fa2json-test-UrwpOb.img /tmp/fa2json-mnt-ts2Dik/
+```
+
+> [!NOTE]
+> In a different terminal I now ran - but we could do this after `chown` or possibly `chown` + `sync`?
+> ```sh
+> fa2json /tmp/fa2json-mnt-ts2Dik
+> ```
+
+### Let current user own file system
+```sh
+sudo chown $(id -u) /tmp/fa2json-mnt-ts2Dik/
+```
+
+#### `fa2json` output
+```json
+{"ts": [1772658052, 704412412, 386988, 865842867], "name": "/tmp/fa2json-mnt-ts2Dik/.", "mask": 1073741828}
+```
+
+### Touch marker
+```sh
+touch /tmp/fa2json-mnt-ts2Dik/MARKER
+```
+
+#### `fa2json` output
+```json
+{"ts": [1772658064, 151070715, 387000, 312501190], "name": "/tmp/fa2json-mnt-ts2Dik/MARKER", "mask": 256}
+{"ts": [1772658064, 151099105, 387000, 312529600], "name": "/tmp/fa2json-mnt-ts2Dik/MARKER", "mask": 12}
+```

test/PLAN.md

@@ -21,15 +21,20 @@ Requires root (`fanotify` FID reporting and `mount` both need `CAP_SYS_ADMIN`).
 
 ## Setup
 
-1. Create a temporary image file (`mktemp`)
-2. `dd` 10M of zeros into it
+1. Create a temporary image file (`mktemp /tmp/fa2json-test-XXXXXX.img`)
+2. `truncate -s 10M` the image (sparse file, no need for `dd`)
 3. `mkfs.ext4` the image
-4. `losetup --find --show` to attach it as a loop device
-5. `mount` the loop device to a temporary directory (`mktemp -d`)
-6. Spawn `fa2json <mountpoint>` as a child process
-7. Attach a `readline` interface to its stdout; parse each line as JSON and
+4. Create a temporary mount directory (`mktemp -d /tmp/fa2json-mnt-XXXXXX`)
+5. `sudo mount <img> <mntdir>` (no `losetup` needed — `mount` accepts image files directly)
+6. `sudo chown $(id -u) <mntdir>` to hand ownership to the current user
+7. `sync` to flush before fa2json starts listening
+8. `sudo` spawn `fa2json <mountpoint>` as a child process (needs `CAP_SYS_ADMIN`)
+9. Attach a `readline` interface to its stdout; parse each line as JSON and
    push into an event buffer
 
+Steps 6 and 7 ensure the `chown` event never enters the fa2json stream, and
+all subsequent FS operations run unprivileged.
+
 ---
 
 ## Teardown
@@ -37,10 +42,9 @@ Requires root (`fanotify` FID reporting and `mount` both need `CAP_SYS_ADMIN`).
 Runs unconditionally in a `finally` block:
 
 1. Kill the `fa2json` child process
-2. `umount <mountpoint>`
-3. `losetup -d <loopdev>`
-4. `rm` the image file
-5. `rmdir` the mount directory
+2. `sudo umount <mountpoint>`
+3. `rm` the image file
+4. `rmdir` the mount directory
 
 ---