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Re: [libvirt] [PATCH v4 4/4] qemu-config: Add new -add-fd command line option





On 10/19/2012 07:05 AM, Kevin Wolf wrote:
Am 18.10.2012 23:37, schrieb Corey Bryant:


On 10/18/2012 04:43 PM, Eric Blake wrote:
On 10/18/2012 01:19 PM, Corey Bryant wrote:
This option can be used for passing file descriptors on the
command line.  It mirrors the existing add-fd QMP command which
allows an fd to be passed to QEMU via SCM_RIGHTS and added to an
fd set.

This can be combined with commands such as -drive to link file
descriptors in an fd set to a drive:

      qemu-kvm -add-fd fd=3,set=2,opaque="rdwr:/path/to/file"
               -add-fd fd=4,set=2,opaque="rdonly:/path/to/file"
               -drive file=/dev/fdset/2,index=0,media=disk

This example adds dups of fds 3 and 4, and the accompanying opaque
strings to the fd set with ID=2.  qemu_open() already knows how
to handle a filename of this format.  qemu_open() searches the
corresponding fd set for an fd and when it finds a match, QEMU
goes on to use a dup of that fd just like it would have used an
fd that it opened itself.

Signed-off-by: Corey Bryant <coreyb linux vnet ibm com>

+
+    if (fcntl(fd, F_GETFD) & FD_CLOEXEC) {
+        qerror_report(ERROR_CLASS_GENERIC_ERROR,
+                      "fd is not valid or already in use");
+        return -1;
+    }

Hmm, I was about to call you on the fact that you didn't check whether
fcntl() succeeded; but then realized that in the failure case it is
required by POSIX to return -1 which happens to include the FD_CLOEXEC
bit, so you actually ended up with a sneaky optimization that does the
right thing for both open and closed fds.

Yep it works for both cases.  I have to admit I stumbled into this at
first and then decided to leave it this way since it worked. :)

I wouldn't be surprised to find such subtleties in Fabrice's code, but
I'm not sure if adding new instances is the best idea ever. :-)

Perhaps a comment in the code is warranted (after all, it is not
immediately apparent from reading just this if statement why it works);
maybe "/* All fds inherited across exec() necessarily have FD_CLOEXEC
clear, while qemu sets FD_CLOEXEC on all other fds opened from command
line arguments */".  But I'm not going to require a v5 just for a
comment addition.

I agree, a comment would be useful.  Maybe Kevin can add if this series
gets pushed?

I'll amend the following to this patch, hope you both agree with the change:

diff --git a/vl.c b/vl.c
index 47095a2..5fb40da 100644
--- a/vl.c
+++ b/vl.c
@@ -792,7 +792,7 @@ static int parse_sandbox(QemuOpts *opts, void *opaque)

  static int parse_add_fd(QemuOpts *opts, void *opaque)
  {
-    int fd, dupfd;
+    int fd, dupfd, flags;
      int64_t fdset_id;
      const char *fd_opaque = NULL;

@@ -812,7 +812,12 @@ static int parse_add_fd(QemuOpts *opts, void *opaque)
          return -1;
      }

-    if (fcntl(fd, F_GETFD) & FD_CLOEXEC) {
+    /*
+     * All fds inherited across exec() necessarily have FD_CLOEXEC
+     * clear, while qemu sets FD_CLOEXEC on all other fds used internally.
+     */
+    flags = fcntl(fd, F_GETFD);
+    if (flags == -1 || (flags & FD_CLOEXEC)) {
          qerror_report(ERROR_CLASS_GENERIC_ERROR,
                        "fd is not valid or already in use");
          return -1;


That works for me.  Thanks Kevin!

--
Regards,
Corey Bryant


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