Re: [libvirt] [PATCH v3 0/5] file descriptor passing using pass-fd
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.
Kevin