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

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Message-ID: <5ce6fe95ee9c4e0f9699a8dc48ec7eab@AcuMS.aculab.com>
Date:   Thu, 21 Jan 2021 10:08:30 +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: 21 January 2021 01:04
> On Wed, Jan 20, 2021 at 4:38 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > OK...  I wonder how many debugfs writable files allow pwrite() with
> > BS results...
> 
> I hope some of them check for "pos == 0" when they start parsing integers.
> 
> But honestly, I don't think it's a big deal. We've had these things
> that just basically assume that whenever you write, the offset just
> doesn't matter at all, and as long as some number comes in one single
> write call, we accept it.
> 
> Because even if you end up doing something like just
> 
>    echo $SOMETHING > /sys/xyz/abc
> 
> and that "$SOMETHING" could be done multiple writes, in practice it
> all works out just fine and it never really is. You almost have to try
> to screw up with something like
> 
>   (echo -n 3; echo -n 4) > /sys/xyz/abc
> 
> to actually see two writes of "3" and "4" instead of one write with
> "34". And honestly, if somebody does something like that, do we really
> care? They might get 3, they might get 4, and they might get 34. They
> get what they deserve.

Or worse:
	echo abc >/sys/xyz/abc
	echo x | dd bs=1 count=2 oseek=1 conv=notrunc of=/sys/xyz/abc
which (if I got the dd command right) would generate "axc" on a real file.

OTOH multiple short reads are quite likely.
Best not done on a counter...

	David

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