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

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Message-ID: <alpine.DEB.2.23.453.2010112150070.3140856@aurora.sdinet.de>
Date:   Sun, 11 Oct 2020 21:52:20 +0200 (CEST)
From:   Sven-Haegar Koch <haegar@...net.de>
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, 12 Oct 2020, Mikhail Gavrilov wrote:

> 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?

If the device node /dev/adb does not exist (most likely udev case when 
you don't have the device/no module loaded for it) then dd as root will 
just create a normal file inside the /dev ramdisk.

Only if the device node exists but is not handled then something else 
like an open error will happen.

c'ya
sven-haegar

-- 
Three may keep a secret, if two of them are dead.
- Ben F.