[PATCH 2/2] netfs: Fix missing zero-length check in unbuffered write

From: David Howells
Date: Mon Jan 29 2024 - 04:51:05 EST


Fix netfs_unbuffered_write_iter() to return immediately if
generic_write_checks() returns 0, indicating there's nothing to write.
Note that netfs_file_write_iter() already does this.

Also, whilst we're at it, put in checks for the size being zero before we
even take the locks. Note that generic_write_checks() can still reduce the
size to zero, so we still need that check.

Without this, a warning similar to the following is logged to dmesg:

netfs: Zero-sized write [R=1b6da]

and the syscall fails with EIO, e.g.:

/sbin/ldconfig.real: Writing of cache extension data failed: Input/output error

This can be reproduced on 9p by:

xfs_io -f -c 'pwrite 0 0' /xfstest.test/foo

Fixes: 153a9961b551 ("netfs: Implement unbuffered/DIO write support")
Reported-by: Eric Van Hensbergen <ericvh@xxxxxxxxxx>
Link: https://lore.kernel.org/r/ZbQUU6QKmIftKsmo@FV7GG9FTHL/
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
cc: Jeff Layton <jlayton@xxxxxxxxxx>
cc: v9fs@xxxxxxxxxxxxxxx
cc: linux_oss@xxxxxxxxxxxxx
cc: netfs@xxxxxxxxxxxxxxx
cc: linux-fsdevel@xxxxxxxxxxxxxxx
---
fs/netfs/buffered_write.c | 3 +++
fs/netfs/direct_write.c | 5 ++++-
2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/netfs/buffered_write.c b/fs/netfs/buffered_write.c
index a3059b3168fd..9a0d32e4b422 100644
--- a/fs/netfs/buffered_write.c
+++ b/fs/netfs/buffered_write.c
@@ -477,6 +477,9 @@ ssize_t netfs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)

_enter("%llx,%zx,%llx", iocb->ki_pos, iov_iter_count(from), i_size_read(inode));

+ if (!iov_iter_count(from))
+ return 0;
+
if ((iocb->ki_flags & IOCB_DIRECT) ||
test_bit(NETFS_ICTX_UNBUFFERED, &ictx->flags))
return netfs_unbuffered_write_iter(iocb, from);
diff --git a/fs/netfs/direct_write.c b/fs/netfs/direct_write.c
index 60a40d293c87..bee047e20f5d 100644
--- a/fs/netfs/direct_write.c
+++ b/fs/netfs/direct_write.c
@@ -139,6 +139,9 @@ ssize_t netfs_unbuffered_write_iter(struct kiocb *iocb, struct iov_iter *from)

_enter("%llx,%zx,%llx", iocb->ki_pos, iov_iter_count(from), i_size_read(inode));

+ if (!iov_iter_count(from))
+ return 0;
+
trace_netfs_write_iter(iocb, from);
netfs_stat(&netfs_n_rh_dio_write);

@@ -146,7 +149,7 @@ ssize_t netfs_unbuffered_write_iter(struct kiocb *iocb, struct iov_iter *from)
if (ret < 0)
return ret;
ret = generic_write_checks(iocb, from);
- if (ret < 0)
+ if (ret <= 0)
goto out;
ret = file_remove_privs(file);
if (ret < 0)