Splicing to/from a tty

[Oliver Giles]

Commit 36e2c7421f02 (fs: don't allow splice read/write without explicit ops) broke my userspace application which talks to an SSL VPN by splice()ing between "openssl s_client" and "pppd". The latter operates over a pty, and since that commit there is no fallback for splice()ing between a pipe and a pty, or any tty for that matter.

The above commit mentions switching them to the iter ops and using generic_file_splice_read. IIUC, this would require implementing iter ops also on the line disciplines, which sounds pretty disruptive.

For my case, I attempted to instead implement splice_write and splice_read in tty_fops; I managed to get splice_write working calling ld->ops->write, but splice_read is not so simple because the tty_ldisc_ops read method expects a userspace buffer. So I cannot see how to implement this without either (a) using set_fs, or (b) implementing iter ops on all line disciplines.

Is splice()ing between a tty and a pipe worth supporting at all? Not a big deal for my use case at least, but it used to work.