[PATCH] 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.

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>
---
Tested in KVM with KASAN on a804c5742bff: file-backed mounts of images
with 512 to 4096 byte blocks, uncompressed and compressed, with and
without directio. With this patch every file reads back correctly;
without it, 24 of the 36 mounts give errors. Not tested on 16K or 64K
page kernels, where 4K-block images should be affected too.

 fs/erofs/fileio.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/fs/erofs/fileio.c b/fs/erofs/fileio.c
index c36a90a1a4cb..b4ed7fe7347c 100644
--- a/fs/erofs/fileio.c
+++ b/fs/erofs/fileio.c
@@ -142,17 +142,26 @@ 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 in the middle of the folio, zero the rest */
+	if (pos < isize && isize - pos < len) {
+		len = isize - pos;
+		folio_zero_segment(folio, len, folio_size(folio));
+	}
+
 	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: a804c5742bff224d4cb4d25dd383554d598be32b
-- 
2.53.0