[PATCH v4] iomap: fix WARN_ON_ONCE() from unprivileged users

[Qian Cai <cai@lca.pw>]

It is trivial to trigger a WARN_ON_ONCE(1) in iomap_dio_actor() by
unprivileged users which would taint the kernel, or worse - panic if
panic_on_warn or panic_on_taint is set. Hence, just convert it to
pr_warn_ratelimited() to let users know their workloads are racing.
Thank Dave Chinner for the initial analysis of the racing reproducers.

Signed-off-by: Qian Cai <cai@lca.pw>
---
v4: use %pD4.
v3: Keep the default case and update the message.
v2: Record the path, pid and command as well.

 fs/iomap/direct-io.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index c1aafb2ab990..9519113ebc35 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -388,6 +388,16 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
 		return iomap_dio_bio_actor(inode, pos, length, dio, iomap);
 	case IOMAP_INLINE:
 		return iomap_dio_inline_actor(inode, pos, length, dio, iomap);
+	case IOMAP_DELALLOC:
+		/*
+		 * DIO is not serialised against mmap() access at all, and so
+		 * if the page_mkwrite occurs between the writeback and the
+		 * iomap_apply() call in the DIO path, then it will see the
+		 * DELALLOC block that the page-mkwrite allocated.
+		 */
+		pr_warn_ratelimited("Direct I/O collision with buffered writes! File: %pD4 Comm: %.20s\n",
+				    dio->iocb->ki_filp, current->comm);
+		return -EIO;
 	default:
 		WARN_ON_ONCE(1);
 		return -EIO;
-- 
2.18.4