Squashfs: Ensure all readahead pages have been used

In the recent work to remove page->index, a sanity check
that ensured all the readhead pages were covered by the
Squashfs data block was removed [1].

To avoid any regression, this commit adds the sanity check
back in an equivalent way.  Namely the page actor will now
return error if any pages are unused after completion.

[1] https://lore.kernel.org/all/20240818235847.170468-3-phillip@squashfs.org.uk/

--

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Link: https://lore.kernel.org/r/20240822233106.121522-1-phillip@squashfs.org.uk
V3: last_page should be actor->last_page
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/squashfs/file.c

@@ -535,7 +535,7 @@ static int squashfs_readahead_fragment(struct page **page,
 
 	last_page = squashfs_page_actor_free(actor);
 
-	if (copied == expected) {
+	if (copied == expected && !IS_ERR(last_page)) {
 		/* Last page (if present) may have trailing bytes not filled */
 		bytes = copied % PAGE_SIZE;
 		if (bytes && last_page)
@@ -625,7 +625,7 @@ static void squashfs_readahead(struct readahead_control *ractl)
 
 		last_page = squashfs_page_actor_free(actor);
 
-		if (res == expected) {
+		if (res == expected && !IS_ERR(last_page)) {
 			int bytes;
 
 			/* Last page (if present) may have trailing bytes not filled */

fs/squashfs/file_direct.c

@@ -80,7 +80,7 @@ int squashfs_readpage_block(struct page *target_page, u64 block, int bsize,
 	if (res < 0)
 		goto mark_errored;
 
-	if (res != expected) {
+	if (res != expected || IS_ERR(last_page)) {
 		res = -EIO;
 		goto mark_errored;
 	}

fs/squashfs/page_actor.h

@@ -33,10 +33,11 @@ extern struct squashfs_page_actor *squashfs_page_actor_init_special(
 				loff_t start_index);
 static inline struct page *squashfs_page_actor_free(struct squashfs_page_actor *actor)
 {
-	struct page *last_page = actor->last_page;
+	struct page *last_page = actor->next_page == actor->pages ? actor->last_page : ERR_PTR(-EIO);
 
 	kfree(actor->tmp_buffer);
 	kfree(actor);
+
 	return last_page;
 }
 static inline void *squashfs_first_page(struct squashfs_page_actor *actor)