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Re: [Qemu-devel] [PATCH v3 0/5] file descriptor passing using pass-fd

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH v3 0/5] file descriptor passing using pass-fd
Date: Tue, 19 Jun 2012 17:57:19 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1

Am 19.06.2012 17:46, schrieb Eric Blake:
> On 06/14/2012 09:55 AM, Corey Bryant wrote:
>> This patch series adds the pass-fd QMP monitor command, which allows
>> an fd to be passed via SCM_RIGHTS, and returns the received file
>> descriptor.  Support is also added to the block layer to allow QEMU
>> to dup the fd when the filename is of the /dev/fd/X format.  This
>> is useful if MAC policy prevents QEMU from opening specific types
>> of files.
> Something to think about on how this will interact with Jeff's work on
> block-commit.  That is an interface where qemu must reopen backing files
> which were previously O_RDONLY to now be O_RDWR.  By default (when
> open() is supported in qemu, and qemu is using the actual file name),
> this means 'block-commit' works without needing any fd names.  But with
> this new fd-passing approach, a file originally opened as O_RDONLY
> /dev/fd/21 will need to be reopened, but the reopened fd will (likely)
> not be 21.  In other words, we need to make sure 'block-commit' supports
> the ability to pass in optional arguments that specify the file name of
> the backing file to be reopened, so that libvirt can pass in O_RDWR fds
> to replace the existing O_RDONLY fd, and be aware that the /dev/fd/nn
> naming of the reopen will be different.

Adding an extra argument to each command that reopens (as in
bdrv_reopen(), i.e. changes flags) internally is one option. In my
opinion not a particularly nice one, though.

Maybe it's better to have a monitor command that just prepares a reopen
and means "for the next reopen of /dev/fd/42, the passed FD will have
the right flags (if it hasn't, the reopen will fail)". We can use dup2()
to keep the "name" stable.


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