linux-kernel - Re: [PATCH] um: Accept /dev/fd/* uml block devices

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Date:	Sun, 28 Jul 2013 12:25:33 +0200
From:	Gabriel de Perthuis <g2p.code@...il.com>
To:	Richard Weinberger <richard@....at>
CC:	Jeff Dike <jdike@...toit.com>,
	user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] um: Accept /dev/fd/* uml block devices

Le dim. 28 juil. 2013 10:12:37 CEST, Richard Weinberger a écrit :
> Am 27.07.2013 17:23, schrieb Gabriel de Perthuis:
>> Useful for
>> * limiting privileges
>> * opening block devices O_EXCL
>
> So, the goal of this patch is to allow passing a file descriptor
> number as block device instead of a file?

Yes.  It turns out it already works, but not after dropping privileges.

> I assume you have already a wrapper around UML which exec()'s it such that
> it can reuse a fd?

Yes, vido: https://github.com/g2p/vido

Here's the relevant commit:
https://github.com/g2p/vido/commit/42d4b86eab13d90ee63138b73146485dc4e47ec6

>> Use dup to work around the fact /proc/self/fd
>> can't be opened after dropping privileges.
>> This proc behaviour doesn't match TLPI and might be a bug.
>>
>> Qemu has a slightly more complex fdset approach
>> that provides fds with different access permissions.
>
> I really don't like that you patch os_open_file(), this is a
> generic function.

The justification was that it unbreaks open("/dev/fd") to be more like
standards suggest, but I can see how that makes it a special case.

> What about this one?
> Allow ubda= (and all other UML block device kernel parameters) to
> accept arguments like file:/foo/bar and fd:N.
> Where N is a number and file: is default such that we do not break
> old kernels.

Okay, I'll add a prefix.  Maybe file:// + /abs/path | rel/path
since that's already standard.
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