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Re: [Qemu-devel] [RFC PATCH 0/4] block: file descriptor passing using -f

From: Kevin Wolf
Subject: Re: [Qemu-devel] [RFC PATCH 0/4] block: file descriptor passing using -filefd and getfd_file
Date: Tue, 22 May 2012 16:45:44 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1

Am 22.05.2012 16:30, schrieb Corey Bryant:
> On 05/22/2012 04:18 AM, Kevin Wolf wrote:
>> Am 21.05.2012 22:19, schrieb Corey Bryant:
>>> libvirt's sVirt security driver provides SELinux MAC isolation for
>>> Qemu guest processes and their corresponding image files.  In other
>>> words, sVirt uses SELinux to prevent a QEMU process from opening
>>> files that do not belong to it.
>>> sVirt provides this support by labeling guests and resources with
>>> security labels that are stored in file system extended attributes.
>>> Some file systems, such as NFS, do not support the extended
>>> attribute security namespace, and therefore cannot support sVirt
>>> isolation.
>>> A solution to this problem is to provide fd passing support, where
>>> libvirt opens files and passes file descriptors to QEMU.  This,
>>> along with SELinux policy to prevent QEMU from opening files, can
>>> provide image file isolation for NFS files.
>>> This patch series adds the -filefd command-line option and the
>>> getfd_file monitor command.  This will enable libvirt to open a
>>> file and push the corresponding filename and file descriptor to
>>> QEMU.  When QEMU needs to "open" a file, it will first check if the
>>> file descriptor was passed by either of these methods before
>>> attempting to actually open the file.
>> I thought we decided to avoid making some file names magic, and instead
>> go for the obvious /dev/fd/42?
> I understand that open("/dev/fd/42") would be the same as dup(42), but 
> I'm not sure that I'm entirely clear on how this would work.  Could you 
> give an example?

With your approach you open the file outside qemu, pass the fd to qemu
along with a file name that it's supposed to replace and then you use
that fake file name:

(qemu) getfd_file abc
(qemu) drive_add 0 file=abc,...

Instead you could use the existing getfd command and avoid the translation:

(qemu) getfd
(qemu) drive_add 0 file=/dev/fd/42,...

Er, well. Just that getfd doesn't return the assigned fd today, so the
management tool doesn't know it. We would have to add that.


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