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Re: [Qemu-devel] [PATCH 0/4] virtio: handle zoned backing devices

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH 0/4] virtio: handle zoned backing devices
Date: Wed, 10 Jul 2019 13:33:08 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2

On 10/07/19 13:02, Kevin Wolf wrote:
> Hm... Actually, file-posix implements .bdrv_check_perm and could just
> refuse attaching a parent there if it doesn't request a specific
> permission like BLK_PERM_SUPPORT_ZONED. That should give us the
> whitelist semantics through existing infrastructure.

I'd like Dmitry to have something more precise to base his work on.  The
permissions system is really complicated and I never really wrapped my
head around it, so I need your help.

IIUC, blkconf_apply_backend_options would grow a new argument (like
"resizable") and that argument would add BLK_PERM_SUPPORT_ZONED to the
perm that blkconf_apply_backend_options passes to blk_set_perm.  On the
other side raw_check_perm would say something like

    if (is_zoned(s) && !(perm & BLK_PERM_SUPPORT_ZONED)) {
        return -ENOTSUP;

Is this correct?

In addition, BLK_PERM_SUPPORT_ZONED would have to be a shared
permission, since it's possible to assign the same block device to
multiple scsi-block devices.  So BLK_PERM_SUPPORT_ZONED would be added
unconditionally to shared_perm.


ps: I have always thought that shared_perm is expressed the wrong way
and should have been "denied_perm".  How hard would it be to change that

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