linux-kernel - Re: [question] What happens when dd writes data to a missing device?

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Message-ID: <20201011203151.GD3576660@ZenIV.linux.org.uk>
Date:   Sun, 11 Oct 2020 21:31:51 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>
Cc:     Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [question] What happens when dd writes data to a missing device?

On Mon, Oct 12, 2020 at 12:46:03AM +0500, Mikhail Gavrilov wrote:
> Hi folks!
> I have a question.
> What happens when dd writes data to a missing device?
> 
> For example:
> # dd if=/home/mikhail/Downloads/Fedora-Workstation-Live-x86_64-Rawhide-20201010.n.0.iso
> of=/dev/adb
> 
> Today I and wrongly entered /dev/adb instead of /dev/sdb,
> and what my surprise was when the data began to be written to the
> /dev/adb device without errors.
> 
> But my surprise was even greater when cat /dev/adb started to display
> the written data.
> 
> I have a question:
> Where the data was written

Into a file called "/dev/adb", of course.

> and could it damage the stored data in
> memory or on disk?

Why would it?  There's nothing magical about /dev - the same thing happened
as if you said
dd if=/home/mikhail/Downloads/whatever.iso of=/tmp/adb
or, for that matter, of=/home/mikhail/copy-of-that-damn-iso - it had been
asked to write into file with that name if it already existed or to create it
and write into it if it didn't exist...  So it had created a file in /dev
with name adb and stored a copy into it.  You might run out of space if the
file had been large enough, but that's about it...

Try ls -l /dev/adb /dev/sdb and compare these two - sdb will be something
like
brw-rw---- 1 root disk 8, 16 ....
and adb -
-rw-rw---- 1 root <some group> <size of that sucker> ...

Block device and regular file respectively...  man mknod if you are
curious about device nodes and creating them manually - usually that's
done by scripts called by udev when it discovers devices, but that's
what they boil down to in the end.

Again, there's nothing magical about /dev or the names of specific
device nodes created in it - it's just the usual place to put that
stuff into, but that's it; you could call mknod(2) to create such
device nodes in any directory, using any names.