Re: Splicing to/from a tty

[Oliver Giles]

On Wed Jan 20, 2021 at 5:56 AM NZDT, Robert Karszniewicz wrote:
>
> I have bisected this issue down to this commit:
> 4d03e3cc5982 ("fs: don't allow kernel reads and writes without iter
> ops")
>
> Another case I've also noticed is writing to a serial connection:
> kernel write not supported for file /ttymxc0 (pid: 252 comm: cat)
>

Tangentially, I hit the same thing when hacking on this. Here's a diff
making the implementation match the comment:

--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -541,7 +541,7 @@ ssize_t __kernel_write(struct file *file, const void *buf, size_t count, loff_t
         * Also fail if ->write_iter and ->write are both wired up as that
         * implies very convoluted semantics.
         */
-       if (unlikely(!file->f_op->write_iter || file->f_op->write))
+       if (unlikely(file->f_op->write_iter && file->f_op->write))
                return warn_unsupported(file, "write");
 
        init_sync_kiocb(&kiocb, file);