[PATCH 5/4] Squashfs: Ensure all readahead pages have been used

[Phillip Lougher <phillip@squashfs.org.uk>]

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>
---
 fs/squashfs/file.c        | 4 ++--
 fs/squashfs/file_direct.c | 2 +-
 fs/squashfs/page_actor.h  | 6 +++++-
 3 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/fs/squashfs/file.c b/fs/squashfs/file.c
index 5a3745e52025..21aaa96856c1 100644
--- a/fs/squashfs/file.c
+++ b/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 */
diff --git a/fs/squashfs/file_direct.c b/fs/squashfs/file_direct.c
index 646d4d421f99..22251743fadf 100644
--- a/fs/squashfs/file_direct.c
+++ b/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;
 	}
diff --git a/fs/squashfs/page_actor.h b/fs/squashfs/page_actor.h
index c6d837f0e9ca..f770a906c2c6 100644
--- a/fs/squashfs/page_actor.h
+++ b/fs/squashfs/page_actor.h
@@ -37,7 +37,11 @@ static inline struct page *squashfs_page_actor_free(struct squashfs_page_actor *
 
 	kfree(actor->tmp_buffer);
 	kfree(actor);
-	return last_page;
+
+	if (actor->next_page == actor->pages)
+		return last_page;
+	else
+		return ERR_PTR(-EIO);
 }
 static inline void *squashfs_first_page(struct squashfs_page_actor *actor)
 {
-- 
2.39.2