Re: [PATCH v2 00/20] block/export: Allow exporting BDSs via FUSE

[Daniel P . Berrangé]

On Tue, Sep 22, 2020 at 12:49:12PM +0200, Max Reitz wrote:
> Based-on: <20200907182011.521007-1-kwolf@redhat.com>
>           (“block/export: Add infrastructure and QAPI for block exports”)
> 
> (Though its patch 16 needs a s/= \&blk_exp_nbd/= drv/ on top.)
> 
> v1: https://lists.nongnu.org/archive/html/qemu-block/2019-12/msg00451.html
> 
> Branch: https://github.com/XanClic/qemu.git fuse-exports-v2
> Branch: https://git.xanclic.moe/XanClic/qemu.git fuse-exports-v2
> 
> 
> Hi,
> 
> Ever since I found out that you can mount FUSE filesystems on regular
> files (not just directories), I had the idea of adding FUSE block
> exports to qemu where you can export block nodes as raw images.  The
> best thing is that you’d be able to mount an image on itself, so
> whatever format it may be in, qemu lets it appear as a raw image (and
> you can then use regular tools like dd on it).
> 
> The performance is quite bad so far, but we can always try to improve it
> if the need arises.  For now I consider it mostly a cute feature to get
> easy access to the raw contents of image files in any format (without
> requiring root rights).

Aside from the iotests, so you forsee any particular use cases
where this feature is desirable / important ?

Looking at it from a security POV, I'm not thrilled about the
idea of granting QEMU permission to use the mount syscall for
seccomp or SELinux. IOW, I expect this feature won't be something
we want to expose in QEMU guests managed by libvirt, which would
limit how widely it can be used.

QEMU can export NBD. Would it make sense to do this as an NBD
client ? There's already https://libguestfs.org/nbdfuse.1.html
but IIUC that exposes it as a file within a dir. Presumably
it is not too hard to make it support exposing it directly as
a file too.

I wonder how performance compares between your native FUSE
impl in QEMU vs NBD FUSE ?


Regards,
Daniel
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