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

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Date:   Wed, 20 Jan 2021 22:25:56 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>
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

From: Linus Torvalds
> Sent: 20 January 2021 19:27
> 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).
> 
> Anyway, I think that with FMODE_STREAM, O_APPEND definitely should be a no-op.

I also wonder if pread/pwrite with offset == 0 should be valid
on things where the offset makes no sense.

I'm rather surprised the offset isn't just silently ignored
for devices where seeking is non-sensical.
You might want to error it for mag tapes, but not pipes,
ttys, sockets etc.

I really can't remember what SYSV, Solaris or NetBSD do.

	David

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