[PATCH v3 2/5] qapi: Add pass-fd QMP command

[Corey Bryant <coreyb@xxxxxxxxxxxxxxxxxx>]

This patch adds the pass-fd QMP command using the QAPI framework.
Like the getfd command, it is used to pass a file descriptor via
SCM_RIGHTS.  However, the pass-fd command also returns the received
file descriptor, which is a difference in behavior from the getfd
command, which returns nothing.

The closefd command can be used to close a file descriptor that was
passed with the pass-fd command.

Note that when using getfd or pass-fd, there are some commands
(e.g. migrate with fd:name) that implicitly close the named fd.
When this is not the case, closefd must be used to avoid fd leaks.

Signed-off-by: Corey Bryant <coreyb@xxxxxxxxxxxxxxxxxx>
---
v2:
 -Introduce new QMP command to pass/return fd (lcapitulino@xxxxxxxxxx)
 -Use passfd as command name (berrange@xxxxxxxxxx)

v3:
 -Use pass-fd as command name (lcapitulino@xxxxxxxxxx)
 -Fail pass-fd if fdname already exists (lcapitulino@xxxxxxxxxx)
 -Add notes to QMP command describing behavior in more detail
  (lcapitulino@xxxxxxxxxx, eblake@xxxxxxxxxx)
 -Add note about fd leakage (eblake@xxxxxxxxxx)

 monitor.c        |   33 +++++++++++++++++++++++++++++++++
 qapi-schema.json |   19 +++++++++++++++++++
 qmp-commands.hx  |   34 ++++++++++++++++++++++++++++++++++
 3 files changed, 86 insertions(+)

diff --git a/monitor.c b/monitor.c
index 1a7f7e7..6d99368 100644
--- a/monitor.c
+++ b/monitor.c
@@ -2182,6 +2182,39 @@ static void do_inject_mce(Monitor *mon, const QDict *qdict)
 }
 #endif
 
+int64_t qmp_pass_fd(const char *fdname, Error **errp)
+{
+    mon_fd_t *monfd;
+    int fd;
+
+    fd = qemu_chr_fe_get_msgfd(cur_mon->chr);
+    if (fd == -1) {
+        error_set(errp, QERR_FD_NOT_SUPPLIED);
+        return -1;
+    }
+
+    if (qemu_isdigit(fdname[0])) {
+        error_set(errp, QERR_INVALID_PARAMETER_VALUE, "fdname",
+                  "a name not starting with a digit");
+        return -1;
+    }
+
+    QLIST_FOREACH(monfd, &cur_mon->fds, next) {
+        if (strcmp(monfd->name, fdname) == 0) {
+            error_set(errp, QERR_INVALID_PARAMETER_VALUE, "fdname",
+                      "a name that does not already exist");
+            return -1;
+        }
+    }
+
+    monfd = g_malloc0(sizeof(mon_fd_t));
+    monfd->name = g_strdup(fdname);
+    monfd->fd = fd;
+
+    QLIST_INSERT_HEAD(&cur_mon->fds, monfd, next);
+    return fd;
+}
+
 void qmp_getfd(const char *fdname, Error **errp)
 {
     mon_fd_t *monfd;
diff --git a/qapi-schema.json b/qapi-schema.json
index 26a6b84..ed99f23 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -1864,6 +1864,25 @@
 { 'command': 'netdev_del', 'data': {'id': 'str'} }
 
 ##
+# @pass-fd:
+#
+# Pass a file descriptor via SCM rights and assign it a name
+#
+# @fdname: file descriptor name
+#
+# Returns: The QEMU file descriptor that was received
+#          If file descriptor was not received, FdNotSupplied
+#          If @fdname is not valid, InvalidParameterType
+#
+# Since: 1.2.0
+#
+# Notes: If @fdname already exists, the command will fail.
+#        The 'closefd' command can be used to explicitly close the
+#        file descriptor when it is no longer needed.
+##
+{ 'command': 'pass-fd', 'data': {'fdname': 'str'}, 'returns': 'int' }
+
+##
 # @getfd:
 #
 # Receive a file descriptor via SCM rights and assign it a name
diff --git a/qmp-commands.hx b/qmp-commands.hx
index e3cf3c5..c039947 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -869,6 +869,40 @@ Example:
 EQMP
 
     {
+        .name       = "pass-fd",
+        .args_type  = "fdname:s",
+        .params     = "pass-fd name",
+        .help       = "pass a file descriptor via SCM rights and assign it a name",
+        .mhandler.cmd_new = qmp_marshal_input_pass_fd,
+    },
+
+SQMP
+pass-fd
+-------
+
+Pass a file descriptor via SCM rights and assign it a name.
+
+Arguments:
+
+- "fdname": file descriptor name (json-string)
+
+Return a json-int with the QEMU file descriptor that was received.
+
+Example:
+
+-> { "execute": "pass-fd", "arguments": { "fdname": "fd1" } }
+<- { "return": 42 }
+
+Notes:
+
+(1) If the name specified by the "fdname" argument already exists,
+    the command will fail.
+(2) The 'closefd' command can be used to explicitly close the file
+    descriptor when it is no longer needed.
+
+EQMP
+
+    {
         .name       = "getfd",
         .args_type  = "fdname:s",
         .params     = "getfd name",
-- 
1.7.10.2

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