[PATCH v2 3/7] exfat: validate cached iomaps during buffered writes

[Chi Zhiling <chizhiling@163.com>]

From: Chi Zhiling <chizhiling@kylinos.cn>

The next patch, "exfat: drop i_rwsem from exfat_page_mkwrite()", allows a
fault to advance valid_size under the folio lock without serializing
against an in-flight buffered write. As a result, valid_size may advance
into a folio covered by a cached iomap. Reusing that iomap would cause
__iomap_write_begin() to zero the wrong part of the block.

Add an ->iomap_valid callback that compares ->validity_cookie against the
current valid_size and forces a remap if valid_size has advanced into the
locked folio, whose pointer is stored in ->private by ->get_folio.

Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
---
 fs/exfat/iomap.c | 38 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/fs/exfat/iomap.c b/fs/exfat/iomap.c
index f3816509c6e5..26d216ef9c44 100644
--- a/fs/exfat/iomap.c
+++ b/fs/exfat/iomap.c
@@ -57,6 +57,7 @@ static int __exfat_iomap_begin(struct inode *inode, loff_t offset, loff_t length
 			iomap->addr = IOMAP_NULL_ADDR;
 			iomap->offset = offset;
 			iomap->length = length;
+			iomap->validity_cookie = valid_size;
 			return 0;
 		}
 
@@ -145,7 +146,7 @@ static int __exfat_iomap_begin(struct inode *inode, loff_t offset, loff_t length
 		}
 	}
 
-	iomap->flags |= IOMAP_F_MERGED;
+	iomap->validity_cookie = valid_size;
 out:
 	mutex_unlock(&sbi->s_lock);
 	return err;
@@ -252,6 +253,17 @@ const struct iomap_ops exfat_write_iomap_ops = {
 	.iomap_next	= exfat_write_iomap_next,
 };
 
+static struct folio *exfat_iomap_get_folio(struct iomap_iter *iter, loff_t pos,
+			unsigned int len)
+{
+	struct folio *folio = iomap_get_folio(iter, pos, len);
+
+	if (!IS_ERR(folio))
+		iter->iomap.private = folio;
+
+	return folio;
+}
+
 static void exfat_iomap_put_folio(struct inode *inode, loff_t pos,
 		unsigned int copied, struct folio *folio)
 {
@@ -264,8 +276,32 @@ static void exfat_iomap_put_folio(struct inode *inode, loff_t pos,
 	folio_put(folio);
 }
 
+static bool exfat_iomap_valid(struct inode *inode, const struct iomap *iomap)
+{
+	struct folio *folio = iomap->private;
+	loff_t old_valid_size = iomap->validity_cookie;
+	loff_t valid_size = exfat_get_valid_size(EXFAT_I(inode));
+
+	if (valid_size == old_valid_size)
+		return true;
+
+	if (!folio)
+		return false;
+
+	/* Still valid for this folio. */
+	if (old_valid_size >= folio_next_pos(folio))
+		return true;
+
+	if (valid_size <= folio_pos(folio))
+		return true;
+
+	return false;
+}
+
 const struct iomap_write_ops exfat_iomap_write_ops = {
+	.get_folio	= exfat_iomap_get_folio,
 	.put_folio	= exfat_iomap_put_folio,
+	.iomap_valid	= exfat_iomap_valid,
 };
 
 /*
-- 
2.53.0