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

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Message-ID: <b814a441-1f5c-682a-4621-abc3259b7409@interlog.com>
Date:   Sun, 11 Oct 2020 21:57:04 -0400
From:   Douglas Gilbert <dgilbert@...erlog.com>
To:     Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [question] What happens when dd writes data to a missing device?

On 2020-10-11 3:46 p.m., 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 and could it damage the stored data in
> memory or on disk?

Others have answered your direct question.

You may find 'oflag=nocreat' helpful if you (or others) do _not_ want
a regular file created in /dev ; for example: if you have misspelt a
device name.
That flag may also be helpful in unstable systems (e.g. where device
nodes are disappearing and re-appearing) as it can be a real pain
if you manage to create a regular file with a name like /dev/sdc when
the disk usually occupying that node is temporarily offline. When
that disk comes back online then regular file '/dev/sdc' will stop
device node '/dev/sdc' from being created.

The solution is to remove the regular file /dev/sdc and you probably
need to power cycle that disk. If this becomes a regular event then
'oflag=nocreat' is your friend [see 'man dd' for a little more
information, it really should be expanded].

Doug Gilbert