[libvirt] [PATCH 1/3] utils: Implement virCommandPassFDGetFDIndex

[Martin Kletzander]

On Tue, Feb 24, 2015 at 09:24:51AM -0500, Stefan Berger wrote:
>On 02/24/2015 08:34 AM, Martin Kletzander wrote:
>>On Mon, Feb 23, 2015 at 06:50:46AM -0500, Stefan Berger wrote:
>>>Implement virCommandPassFDGetFDIndex to determine the index a given
>>>file descriptor will have when passed to the child process. When this
>>>function is called, a flag is set to prevent the reordering of the
>>>file descriptors.
>>>
>>>Signed-off-by: Stefan Berger <stefanb at linux.vnet.ibm.com>
>>>---
>>>src/libvirt_private.syms |  1 +
>>>src/util/vircommand.c    | 33 +++++++++++++++++++++++++++++++++
>>>src/util/vircommand.h    |  3 +++
>>>3 files changed, 37 insertions(+)
>>>
>>>diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
>>>index c156b40..aadb833 100644
>>>--- a/src/libvirt_private.syms
>>>+++ b/src/libvirt_private.syms
>>>@@ -1213,6 +1213,7 @@ virCommandNewArgList;
>>>virCommandNewArgs;
>>>virCommandNonblockingFDs;
>>>virCommandPassFD;
>>>+virCommandPassFDGetFDIndex;
>>>virCommandPassListenFDs;
>>>virCommandRawStatus;
>>>virCommandRequireHandshake;
>>>diff --git a/src/util/vircommand.c b/src/util/vircommand.c
>>>index 6527d85..2616446 100644
>>>--- a/src/util/vircommand.c
>>>+++ b/src/util/vircommand.c
>>>@@ -67,6 +67,7 @@ enum {
>>>    VIR_EXEC_RUN_SYNC   = (1 << 3),
>>>    VIR_EXEC_ASYNC_IO   = (1 << 4),
>>>    VIR_EXEC_LISTEN_FDS = (1 << 5),
>>>+    VIR_EXEC_FIXED_FDS  = (1 << 6),
>>
>>The descriptors shouldn't get reordered *unless* there is
>>VIR_EXEC_LISTEN_FDS flag added, so FIXED_FDS shouldn't be necessary.
>>Or is there a bug in that approach that needs fixing?
>
>Hm, it depends on the order in which APIs are called where we either
>need to protect against re-ordering (if we use indices) or we cannot
>allow indices to be used if we need to reorder them. So, actually the
>way the patch is, it isn't correct, either.
>
>The problem of having to prevent the reordering of file descriptors
>when we are building the QEMU command line with file descriptors
>already exists today. So maybe I could just remove this flag and the
>related check knowing that reordering of file descriptors and building
>the QEMU command line that contains file descriptor (from before the
>order) would not be allowed today without any of the patches I am
>trying to add.
>

You surely and safely can.  The point behind reordering FDs was that
when you're using FD passing as done by systemd, those FDs passed must
start from number 3.  However, because we are already passing a bunch
of file descriptors to QEMU and other commands, we *must not* reorder
any file descriptors because it wouldn't work, of course.

Martin
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