linux-kernel - Re: Splicing to/from a tty

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Date:   Wed, 20 Jan 2021 23:14:39 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christoph Hellwig <hch@....de>,
        Johannes Berg <johannes@...solutions.net>,
        Oliver Giles <ohw.giles@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: Splicing to/from a tty

On Wed, Jan 20, 2021 at 11:27:26AM -0800, Linus Torvalds wrote:
> On Wed, Jan 20, 2021 at 11:11 AM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > Why do we care about O_APPEND on anything without FMODE_PWRITE (including
> > pipes), anyway?  All writes there ignore position, after all...
> 
> We shouldn't care.
> 
> Also, I think we should try to move away from FMODE_PWRITE/PREAD
> entirely, and use FMODE_STREAM as the primary "this thing doesn't have
> a position at all".
> 
> That's what gets rid of all the f_pos locking etc after all. The
> FMODE_PWRITE/PREAD flags are I think legacy (although we do seem to
> have the seq_file case that normally allows position on reads, but not
> on writes, so we may need to keep all three bits).

Umm...  Why do we clear FMODE_PWRITE there, anyway?  It came in
commit 915a29ec1c5e34283a6231af1036114e4d612cb0
Author: Linus Torvalds <torvalds@...970.osdl.org>
Date:   Sat Aug 7 02:08:23 2004 -0700

    Add pread/pwrite support bits to match the lseek bit.
    
    This also removes the ESPIPE logic from pipes and seq_files,
    since the VFS layer now supports it.

with seq_read() losing the special-cased pread prevention and
seq_open() getting a ban on both pread and pwrite.  With
pread() support added in 2009, and (pointless) pwrite prohibition
left in place.