[PATCH] net/9p: show warning on Tread/Twrite if wrong file mode

[Christian Schoenebeck <linux_oss@crudebyte.com>]

The netfs changes (eb497943fa21) introduced cases where 'Tread' was sent
to 9p server on a fid that was opened in write-only file mode. It took
some time to find the cause of the symptoms observed (EBADF errors in
user space apps). Add warnings to detect such issues easier in future.

Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Link: https://lore.kernel.org/netdev/3645230.Tf70N6zClz@silver/
---
As requested by Dominique, here a clean version of my previous
EBADF trap code to be merged. Dominique, if you already have an
equivalent patch queued, then just go ahead. I don't mind.

I'm currently testing your EBADF fix patch and the discussed,
slightly adjusted versions. Looking good so far, but I'll report
back later on.


 net/9p/client.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/9p/client.c b/net/9p/client.c
index 8bba0d9cf975..05dead12702d 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -1555,6 +1555,8 @@ p9_client_read(struct p9_fid *fid, u64 offset, struct iov_iter *to, int *err)
 	int total = 0;
 	*err = 0;
 
+	WARN_ON((fid->mode & O_ACCMODE) == O_WRONLY);
+
 	while (iov_iter_count(to)) {
 		int count;
 
@@ -1648,6 +1650,8 @@ p9_client_write(struct p9_fid *fid, u64 offset, struct iov_iter *from, int *err)
 	p9_debug(P9_DEBUG_9P, ">>> TWRITE fid %d offset %llu count %zd\n",
 		 fid->fid, offset, iov_iter_count(from));
 
+	WARN_ON((fid->mode & O_ACCMODE) == O_RDONLY);
+
 	while (iov_iter_count(from)) {
 		int count = iov_iter_count(from);
 		int rsize = fid->iounit;
-- 
2.30.2