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Re: [libvirt] [PATCH v3 4/4] qemu-config: Add new -add-fd command line option

On 10/17/2012 11:03 AM, Kevin Wolf wrote:
Am 17.10.2012 17:01, schrieb Eric Blake:
On 10/17/2012 08:02 AM, Kevin Wolf wrote:
Am 17.10.2012 06:16, schrieb Eric Blake:
I'm still seeing the corner case of:

qemu-kvm -add-fd fd=3,set=1 -add-fd fd=4,set=2 4<&-

where the dup(3) will populate fd 4 prior to the point where we get to
process the -add-fd fd=4 command to notice that the user started
qemu-kvm with fd 4 closed, and thus qemu will silently proceed to use
the wrong fd.

On the other hand, I'm not sure if that corner case is worth worrying
about, or if we just chalk it up to user stupidity (aka libvirt
programmer stupidity) if they did something like that (most likely,
because the management app forgot to clear FD_CLOEXEC before exec()ing

If you specify an FD number that isn't actually open when qemu is
stared, you can get any FD that qemu opens internally. I think the
correct answer to this problem is "then don't do that".

Overnight, I realized we do have one potential safety valve: we are
guaranteed that any fd inherited by the exec() of qemu-kvm has
FD_CLOEXEC clear, and we also strive to have qemu open/dup all of its
internal fds with FD_CLOEXEC set.  Therefore, it may be worth a sanity
check of fcntl(F_GETFD) to see if FD_CLOEXEC is set, and if so, the user
must have failed to pass in the fd, and we are now looking at a qemu
internal fd, and should therefore report failure.  But I'm not sure if
it's worth the extra code.

Hm, this sounds actually easy enough. I'll leave the decision to Corey,
but I like the idea.

Sure I can add this.

Corey Bryant

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