From: Theodore Ts'o <tytso@mit.edu>
Subject: Re: [PATCH 5/7] mke2fs: proceed if the user doesn't type anything
 after 5 seconds
Date: Mon, 28 Apr 2014 19:26:34 -0400
Message-ID: <20140428232634.GD7857@thunk.org>
References: <1398556834-31913-1-git-send-email-tytso@mit.edu>
 <1398556834-31913-5-git-send-email-tytso@mit.edu>
 <535E74D4.4050805@redhat.com>
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Cc: Ext4 Developers List <linux-ext4@vger.kernel.org>
To: Eric Sandeen <sandeen@redhat.com>
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On Mon, Apr 28, 2014 at 10:33:40AM -0500, Eric Sandeen wrote:
> On 4/26/14, 7:00 PM, Theodore Ts'o wrote:
> > If mke2fs needs to ask the user for permission, and the user doesn't
> > type anything for five seconds, proceed as if the user had said yes.
> > 
> > This will allow us to add more stringent checks without breaking
> > existing scripts (much).
> 
> Hm, this sounds a little dangerous - "-F" overrides a lot.

Actually, if you take a look at what we use proceed_question() for, it
doesn't actually override anything (up until now) that might lead to
data loss.  It's for things like trying to create an file system with
a block size greater than 4k on an x86 platform, creating a file
system larger than the apparent block size, etc.  The main goal was to
make sure the user actually *sees* the darned message.

Perhaps the only case where proceed_question() can prevent data loss
is the one where the user typo's /dev/sda3 as /dev/sda.  Everything
else is in the category of "we want to make sure the user sees the
warning".

The motivation behind this is adding this safety check:

% ./misc/mke2fs -t ext4 -L test-filesystem /dev/sdc3 8M
mke2fs 1.42.9 (4-Feb-2014)
/dev/sdc3 contains a ext4 file system labelled 'test-filesystem'
Proceed anyway (or wait 5 seconds) ? (y,n) 

Previously, we would blithely blow away /dev/sdc3 without even giving
a warning.  So if stdin (fd 0) is not a tty, we skip this test
entirely --- otherwise existing scripts would fail.  However, if a
script is attached to a tty, we would end up stalling the script
waiting for the user to answer yes/no where previously no question
would be asked at all.  This is the case where it's important that
proceed_question() will now pause five seconds, and then continue.

		   		   	    	       - Ted