[PATCH v2] erofs: fix metadata reads at the end of file-backed images

[Hengyu Liang <hengyul@cs.unc.edu>]

Commit e587a984d332 ("erofs: use dedicated meta inodes for file-backed
mounts") made erofs_read_meta_folio() read the metadata of a
file-backed mount one whole folio at a time.

However, an image only has to be a multiple of its block size. With a
block size below the page size, it can end in the middle of a page. The
read of that last page is then short, and erofs_fileio_ki_complete()
fails it with -EIO. As of now, inodes in the last page of such an image
cannot be read, and an image smaller than a page cannot be mounted.

The issue can be reproduced with util-linux 2.41, whose mount uses the
image file directly:

    mkdir src
    for i in $(seq 50); do echo $i > src/$i; done
    mkfs.erofs -b512 test.erofs src
    mount -t erofs test.erofs /mnt
    ls -l /mnt

Before that commit, ls -l lists all 50 files. After it, ls -l reports
"Input/output error" for 24 of them.

This patch reads the folio only up to the end of the backing file and
zeroes the rest, as the page cache of the backing file did before. A
folio that lies wholly past the end is zeroed without a read.

Fixes: e587a984d332 ("erofs: use dedicated meta inodes for file-backed mounts")
Cc: stable@vger.kernel.org
Signed-off-by: Hengyu Liang <hengyul@cs.unc.edu>
---
Changes in v2:
- Also zero a folio that lies wholly past the end of the backing file,
  without a read, as suggested by Gao Xiang.
- v1: https://lore.kernel.org/r/20261023174232.3282900-1-hengyul@cs.unc.edu

Tested in KVM with KASAN on 734bef548a96: the images used for v1, plus
three images cut so that whole folios lie past the end. With this
patch every complete image reads back correctly, and the cut images
give the same results as v7.2 (EFSCORRUPTED instead of EIO for the
inodes past the end).

 fs/erofs/fileio.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/fs/erofs/fileio.c b/fs/erofs/fileio.c
index c36a90a1a4cb..a77af604479f 100644
--- a/fs/erofs/fileio.c
+++ b/fs/erofs/fileio.c
@@ -142,17 +142,30 @@ int erofs_read_meta_folio(struct file *file, struct folio *folio)
 		.dev = { .m_pa = folio_pos(folio), },
 	};
 	struct inode *inode = folio_inode(folio);
+	size_t len = folio_size(folio);
+	loff_t pos, isize;
 	int err;
 
 	err = erofs_map_dev(inode->i_sb, &io.dev);
 	if (err)
 		return err;
 
+	pos = io.dev.m_dif->fsoff + io.dev.m_pa;
+	isize = i_size_read(file_inode(io.dev.m_dif->file));
+	/* the image may end before the end of the folio, zero the rest */
+	if (pos + folio_size(folio) > isize) {
+		len = isize > pos ? isize - pos : 0;
+		folio_zero_segment(folio, len, folio_size(folio));
+		if (!len) {
+			folio_end_read(folio, true);
+			return 0;
+		}
+	}
+
 	io.rq = erofs_fileio_rq_alloc(inode->i_sb, io.dev.m_dif->file);
 	io.rq->bio.bi_end_io = erofs_fileio_meta_endio;
-	io.rq->bio.bi_iter.bi_sector =
-		(io.dev.m_dif->fsoff + io.dev.m_pa) >> 9;
-	bio_add_folio_nofail(&io.rq->bio, folio, folio_size(folio), 0);
+	io.rq->bio.bi_iter.bi_sector = pos >> 9;
+	bio_add_folio_nofail(&io.rq->bio, folio, len, 0);
 	erofs_fileio_rq_submit(io.rq);
 	return 0;
 }

base-commit: 734bef548a96e8774d6eef5dfa2026c6edc4e80b
-- 
2.53.0