Re: Splicing to/from a tty

From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christoph Hellwig <hch@lst.de>,
	Johannes Berg <johannes@sipsolutions.net>,
	Oliver Giles <ohw.giles@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: Splicing to/from a tty
Date: Thu, 21 Jan 2021 01:45:28 +0000	[thread overview]
Message-ID: <20210121014528.GG740243@zeniv-ca> (raw)
In-Reply-To: <CAHk-=whWGwcZXpqDFv-j2fcChtT1jE0ZMFCmQHp3BrSkp+XZ6A@mail.gmail.com>

On Wed, Jan 20, 2021 at 05:04:17PM -0800, Linus Torvalds wrote:

> The whole point of O_APPEND is that the position shouldn't matter.
> 
> And the whole point of "pwrite()" is that you specify a position.
> 
> So the two just do not go together - although we may have legacy
> issues, of course.

Our pwrite(2):
BUGS
       POSIX  requires  that  opening  a  file  with the O_APPEND flag
       should have no effect on the location at which pwrite() writes
       data.  However, on Linux, if a file is opened with O_APPEND,
       pwrite() appends data to the  end of the file, regardless of
       the value of offset.
POSIX pwrite(2):
	The pwrite() function shall be equivalent to write(), except that
	it writes into a given position and does not change the file offset
	(regardless of whether O_APPEND is set).  The first three arguments
	to pwrite() are the same as write() with the addition of a fourth
	argument offset for the desired position inside the file.  An attempt
	to perform a pwrite() on a file that is incapable of seeking shall
	result in an error.

I don't believe that we could change behaviour of our pwrite(2) without
breaking userland, even if we wanted to.  It's been that way since
2.1.60 when pwrite() had been first introduced - 23 years ago is more
than enough to have it cast in stone.  We do allow pwrite(2) with O_APPEND
and on such descriptors it acts like write(2) on the same.

> Now, splice() is able to do *both* write() and pwrite(), because
> unlike pwrite() it doesn't take a "pos" argument, it takes a _pointer_
> to pos. So with a NULL pointer, it's like read/write, and with a
> non-NULL pointer it is like pread/pwrite.
> 
> So I do think that "splice with non-NULL off_out and O_APPEND" should
> cause an error in general.

splice() triggers an error for seekable destination with O_APPEND and
with NULL off_out.  Same for splice() to socket with
	fcntl(sock_fd, F_SETFL, O_APPEND);
done first.

> Honestly, I don't think it's a huge deal. O_APPEND isn't that
> interesting, but I do hope that if we allow O_APPEND and a file
> position, then O_APPEND always overrides it.

It does, when it is allowed.