linux-kernel - Re: [PATCH] Syscalls: reboot: Add options to the reboot syscall to remount filesystems ro

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Date:	Thu, 3 Mar 2011 15:36:27 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mark Lord <kernel@...savvy.com>
Cc:	Ken Sumrall <ksumrall@...roid.com>, linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Jens Axboe <axboe@...nel.dk>,
	Matthew Wilcox <matthew@....cx>,
	Eric Paris <eparis@...hat.com>,
	Dave Young <hidave.darkstar@...il.com>,
	Jiri Slaby <jslaby@...e.cz>, James Morris <jmorris@...ei.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] Syscalls: reboot: Add options to the reboot syscall to
 remount filesystems ro

On Thu, Mar 3, 2011 at 10:28 AM, Mark Lord <kernel@...savvy.com> wrote:
> On 11-03-03 12:45 PM, Linus Torvalds wrote:
>>
>> If you can change whatever user-land process that does the reboot
>> system call (and clearly you can, since you're adding new commands and
>> using those), then why the heck don't you just do the remount-ro from
>> that same user land?
>
> Is there a system call for emergency_remount_*() ?
> I've been writing to /proc/proc/sysrq-trigger to accomplish this.

So I don't know what this has to do with "emergency_remount()" - we're
talking about a regular controlled shutdown/reset. The fact that the
patch used the emergency_remount() code seems to be purely an
implementation issue, nothing more.

And while sysrq-trigger certainly works (when it's enabled, but that's
true of /proc too, of course), I do think it's a rather odd way of
solving the problem, when the simple "just remount read-only" is what
the code actually _wants_ to do.

But you're certainly right that it takes less code to open
/proc/sysrq-trigger and writing a single byte to it than it does to do
the straightforward "let's just do the normal mount thing".

                             Linus
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