[PATCH 4/4] fs: devfs: make the default automount reachable under cdev aliases

[Sascha Hauer <s.hauer@pengutronix.de>]

A cdev with a default automount is mounted by walking into
/mnt/<cdevname>, so /mnt only ever knows a cdev under its real name. With
a USB disk registered as "disk0" and aliased to "usbdisk0" you can read
/dev/usbdisk0.data, but mounting it still requires knowing that it really
is disk0.0.

Link the aliases next to the automount directory, so that /mnt/usbdisk0.data
points to disk0.0 and walking into it triggers the same automount as
/mnt/disk0.0 does. This also covers the aliases a partition has always
had: /mnt/disk0.data did not exist so far even though /dev/disk0.data
did.

The links are created and removed along with the alias itself, hence
devfs_remove() has to remove the aliases before dropping the automount -
cdev_mnt_unlink() needs DEVFS_HAS_AUTOMOUNT to still be set to know that
there is anything to unlink.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Assisted-by: Claude:claude-opus-5
---
 fs/devfs-core.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/fs/devfs-core.c b/fs/devfs-core.c
index d647be9b9d..35a4bdfdc4 100644
--- a/fs/devfs-core.c
+++ b/fs/devfs-core.c
@@ -520,6 +520,35 @@ static void devfs_unlink(const char *name)
 	free(path);
 }
 
+/*
+ * A cdev with a default automount is mounted by walking into
+ * /mnt/<cdevname>. Link the aliases next to it so that an alias can be used
+ * for mounting just as well.
+ */
+static void cdev_mnt_symlink(struct cdev *cdev, const char *linkname)
+{
+	char *path;
+
+	if (!(cdev->flags & DEVFS_HAS_AUTOMOUNT))
+		return;
+
+	path = xasprintf("/mnt/%s", linkname);
+	symlink(cdev->name, path);
+	free(path);
+}
+
+static void cdev_mnt_unlink(struct cdev *cdev, const char *linkname)
+{
+	char *path;
+
+	if (!(cdev->flags & DEVFS_HAS_AUTOMOUNT))
+		return;
+
+	path = xasprintf("/mnt/%s", linkname);
+	unlink(path);
+	free(path);
+}
+
 void devfs_init(void)
 {
 	struct cdev *cdev;
@@ -588,6 +617,7 @@ static int __devfs_add_alias(struct cdev *cdev, const char *name,
 	list_add_tail(&alias->list, &cdev->aliases);
 
 	cdev_symlink(cdev, name);
+	cdev_mnt_symlink(cdev, name);
 
 	return 0;
 }
@@ -686,6 +716,7 @@ static void devfs_remove_aliases(struct cdev *cdev)
 
 	list_for_each_entry_safe(alias, tmp, &cdev->aliases, list) {
 		devfs_unlink(alias->name);
+		cdev_mnt_unlink(cdev, alias->name);
 		list_del(&alias->list);
 		free(alias->name);
 		free(alias);
@@ -706,6 +737,7 @@ int devfs_remove(struct cdev *cdev)
 
 	devfs_unlink(cdev->name);
 
+	/* Must happen before the automount is dropped below */
 	devfs_remove_aliases(cdev);
 
 	cdev_remove_default_automount(cdev);

-- 
2.47.3