[RFC PATCH] fs/ntfs3: prevent positive E_NTFS_NONRESIDENT from reaching writeback error handling

[Vidhu Sarwal <vidhu.linux@gmail.com>]

attr_data_write_resident() returns the internal positive status code
E_NTFS_NONRESIDENT when the DATA attribute becomes non-resident.
ntfs_writepages() propagates this value into generic writeback error
handling, which expects a standard negative errno and triggers
WARN_ON_ONCE(err > 0).

Translate positive E_NTFS_* status codes to -EIO before recording
the writeback error. This prevents positive internal status codes from
reaching generic writeback code while preserving the existing behaviour
of treating this path as a writeback error.

Reported-by: syzbot+57f9327d593d301ce2a2@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=57f9327d593d301ce2a2
Signed-off-by: Vidhu Sarwal <vidhu.linux@gmail.com>
---
RFC:

I'd appreciate guidance on whether -EIO is the appropriate errno here.

E_NTFS_NONRESIDENT indicates that the DATA attribute became
non-resident during writeback. Translating it to -EIO prevents the
positive internal status code from reaching generic writeback code and
preserves the existing behaviour of treating this path as a writeback
error.

However, if the intended behaviour is to retry the write using the
non-resident path instead of failing it, then the correct solution would
be more than a simple errno translation. The proposed change assumes the
current terminal-error behaviour is intentional.
 fs/ntfs3/inode.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/ntfs3/inode.c b/fs/ntfs3/inode.c
index 0c9bd669117d..9458867c6b0b 100644
--- a/fs/ntfs3/inode.c
+++ b/fs/ntfs3/inode.c
@@ -1019,6 +1019,15 @@ static int ntfs_writepages(struct address_space *mapping,
 
 			folio_unlock(folio);
 			if (err2) {
+				/*
+				 * attr_data_write_resident() can return the
+				 * positive internal code E_NTFS_NONRESIDENT;
+				 * both mapping_set_error() and writeback_iter()
+				 * need a negative errno and warn on a positive
+				 * value.
+				 */
+				if (err2 > 0)
+					err2 = -EIO;
 				mapping_set_error(mapping, err2);
 				if (!err)
 					err = err2;

base-commit: a13c140cc289c0b7b3770bce5b3ad42ab35074aa
-- 
2.53.0