[PATCH] io_uring: gate REQ_F_ISREG on !S_ANON_INODE as well

[Jens Axboe <axboe@kernel.dk>]

io_uring marks a request as dealing with a regular file on S_ISREG. This
drives things like retries on short reads or writes, which is generally
not expected on a regular file (or bdev). Applications tend to not
expect that, so io_uring tries hard to ensure it doesn't deliver short
IO on regular files.

However, a recent commit added S_IFREG to anonymous inodes. When
io_uring is used to read from various things that are backed by anon
inodes, like eventfd, timerfd, etc, then it'll now all of a sudden wait
for more data when rather than deliver what was read or written in a
single operation. This breaks applications that issue reads on anon
inodes, if they ask for more data than a single read delivers.

Add a check for !S_ANON_INODE as well before setting REQ_F_ISREG to
prevent that.

Cc: Christian Brauner <brauner@kernel.org>
Cc: stable@vger.kernel.org
Link: https://github.com/ghostty-org/ghostty/discussions/7720
Fixes: cfd86ef7e8e7 ("anon_inode: use a proper mode internally")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 5111ec040c53..73648d26a622 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -1666,11 +1666,12 @@ static void io_iopoll_req_issued(struct io_kiocb *req, unsigned int issue_flags)
 
 io_req_flags_t io_file_get_flags(struct file *file)
 {
+	struct inode *inode = file_inode(file);
 	io_req_flags_t res = 0;
 
 	BUILD_BUG_ON(REQ_F_ISREG_BIT != REQ_F_SUPPORT_NOWAIT_BIT + 1);
 
-	if (S_ISREG(file_inode(file)->i_mode))
+	if (S_ISREG(inode->i_mode) && !(inode->i_flags & S_ANON_INODE))
 		res |= REQ_F_ISREG;
 	if ((file->f_flags & O_NONBLOCK) || (file->f_mode & FMODE_NOWAIT))
 		res |= REQ_F_SUPPORT_NOWAIT;

-- 
Jens Axboe