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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 12 Jul 2018 09:07:36 -0700
Message-ID: <CA+55aFwkiFdhGmT8gaSohAQGMWpGUZBQ0dhGNGWEN4cpjK7XHA@mail.gmail.com>
Subject: Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for
 superblock creation [ver #9]
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     David Howells <dhowells@redhat.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jann Horn <jannh@google.com>
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On Thu, Jul 12, 2018 at 9:00 AM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> Wait a sec - that's only a problem if your command contains pointer-chasing
> et.al.

No.

It's a problem if anybody ever does something like "let's have a
helper splice thread that uses splice to move data automatically from
one buffer to another".

And yes, it's something people have wanted.

Seriously. I'm putting my foot down. NO COMMANDS IN WRITE DATA!

We have made that mistake in the past. Having done stupid things in
the past is not an excuse for doing so again. Quite the reverse.
Making the same mistake and not learning from your mistakes is the
sign of stupidity.

So I repeat: write is for data. If you want an action, you do it with
ioctl, or you do it with a system call.

              Linus