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Re: [Qemu-devel] block-commit & dropping privs
From: |
Michael Tokarev |
Subject: |
Re: [Qemu-devel] block-commit & dropping privs |
Date: |
Fri, 27 Mar 2015 18:36:20 +0300 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Icedove/31.5.0 |
27.03.2015 17:49, Eric Blake wrote:
> On 03/27/2015 03:07 AM, Michael Tokarev wrote:
>> Hello.
>>
>> I tried to experiment with block-commit command, which propagates
>> changes accumulated in an overlay (qcow2) block image file back to
>> the base image file.
>>
>> And immediately faced a problem. All my VMs are run chrooted into
>> an empty dir and with low-priv user (using -runsa and -chroot options,
>> initially started as root). Ofcourse this low-priv qemu process
>> can't open the base image anymore, because it doesn't have the
>> necessary permissions and because the base file is inaccessible
>> within the chroot.
>>
>> So I wonder if we can avoid reopening the base img by always opening
>> it read-write (using a command-line option), does it make sense?
>
> It is already possible to open a file read-write on the command line, by
> using -add-fd twice to put both a read-only and a read-write fd handle
> to the same file in a single set N, then using -drive options to specify
> /dev/fdset/N rather than the file name. By that argument, I'm not sure
> if adding any other command line options is necessary.
How does fdSET work? How to use it? Will the BDS reopen work with an
fdset in an empty chroot?
Sorry I haven't seen this so far, and documentation is a bit vague.
I think I see how this should work, monitor_fdset_get_fd() will search
an FD with matching access mode flags... Ok, so two fds in an fdset,
one ro and one rw. And with that in mind, since qemu_open() checks if
the filename starts with /dev/fdset/, it should work inside a chroot.
Wonder how to specify cache mode, or should I open these with proper
O_DIRECT/O_SYNC/whatever? It looks like it's possible to change O_DIRECT
at runtime but not O_SYNC.
And the more interesting question is how to do that from shell.
Oh well.
Thanks,
/mjt
>> Or maybe there's some other possible solution to this, for example,
>> passing in a filedescriptor for the new base img over a unix socket?
>
> Hmm, we do allow QMP to pass in fds over Unix sockets already, but what
> we don't have yet is the ability to associate one of those just-passed
> fds to an existing open BDS (right now, you can only create a new fdset
> as tied to a new BDS, but block-commit can only target an open BDS;
> there is no way to pivot which BDS base is associated with another BDS).
> So maybe there is still room for improvement, especially since having a
> QMP solution is nicer than requiring foresight to pass in an fdset from
> the command line.
>