[Qemu-devel] [PATCH v7 2/6] qapi: Introduce add-fd, remove-fd, query-fdsets

[Corey Bryant]

This patch adds support that enables passing of file descriptors
to the QEMU monitor where they will be stored in specified file
descriptor sets.

A file descriptor set can be used by a client like libvirt to
store file descriptors for the same file.  This allows the
client to open a file with different access modes (O_RDWR,
O_WRONLY, O_RDONLY) and add/remove the passed fds to/from an fd
set as needed.  This will allow QEMU to (in a later patch in this
series) "open" and "reopen" the same file by dup()ing the fd in
the fd set that corresponds to the file, where the fd has the
matching access mode flag that QEMU requests.

The new QMP commands are:
  add-fd: Add a file descriptor to an fd set
  remove-fd: Remove a file descriptor from an fd set
  query-fdsets: Return information describing all fd sets

Note: These commands are not compatible with the existing getfd
and closefd QMP commands.

Signed-off-by: Corey Bryant <address@hidden>
---
v5:
 -This patch is new in v5 and replaces the pass-fd QMP command
  from v4.
 -By grouping fds in fd sets, we ease managability with an fd
  set per file, addressing concerns raised in v4 about handling
  "reopens" and preventing fd leakage. (address@hidden,
  address@hidden, address@hidden)

v6
 -Make @fd optional for remove-fd (address@hidden)
 -Make @fdset-id optional for add-fd (address@hidden)

v7:
 -Share fd sets among all monitor connections (address@hidden)
 -Added mon_refcount to keep track of monitor connection count.

 monitor.c        |  167 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 qapi-schema.json |  110 +++++++++++++++++++++++++++++++++++
 qerror.c         |    4 ++
 qerror.h         |    3 +
 qmp-commands.hx  |  131 ++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 415 insertions(+)

diff --git a/monitor.c b/monitor.c
index 49dccfe..04b86b7 100644
--- a/monitor.c
+++ b/monitor.c
@@ -140,6 +140,23 @@ struct mon_fd_t {
     QLIST_ENTRY(mon_fd_t) next;
 };
 
+/* file descriptor associated with a file descriptor set */
+typedef struct mon_fdset_fd_t mon_fdset_fd_t;
+struct mon_fdset_fd_t {
+    int fd;
+    bool removed;
+    QLIST_ENTRY(mon_fdset_fd_t) next;
+};
+
+/* file descriptor set containing fds passed via SCM_RIGHTS */
+typedef struct mon_fdset_t mon_fdset_t;
+struct mon_fdset_t {
+    int64_t id;
+    int refcount;
+    QLIST_HEAD(, mon_fdset_fd_t) fds;
+    QLIST_ENTRY(mon_fdset_t) next;
+};
+
 typedef struct MonitorControl {
     QObject *id;
     JSONMessageParser parser;
@@ -211,6 +228,8 @@ static inline int mon_print_count_get(const Monitor *mon) { 
return 0; }
 #define QMP_ACCEPT_UNKNOWNS 1
 
 static QLIST_HEAD(mon_list, Monitor) mon_list;
+static QLIST_HEAD(mon_fdsets, mon_fdset_t) mon_fdsets;
+static int mon_refcount;
 
 static mon_cmd_t mon_cmds[];
 static mon_cmd_t info_cmds[];
@@ -2389,6 +2408,154 @@ int monitor_get_fd(Monitor *mon, const char *fdname)
     return -1;
 }
 
+static void monitor_fdset_cleanup(mon_fdset_t *mon_fdset)
+{
+    mon_fdset_fd_t *mon_fdset_fd;
+    mon_fdset_fd_t *mon_fdset_fd_next;
+
+    if (mon_fdset->refcount != 0) {
+        return;
+    }
+
+    QLIST_FOREACH_SAFE(mon_fdset_fd, &mon_fdset->fds, next, mon_fdset_fd_next) 
{
+        if (mon_refcount == 0 || mon_fdset_fd->removed) {
+            close(mon_fdset_fd->fd);
+            QLIST_REMOVE(mon_fdset_fd, next);
+            g_free(mon_fdset_fd);
+        }
+    }
+
+    if (QLIST_EMPTY(&mon_fdset->fds)) {
+        QLIST_REMOVE(mon_fdset, next);
+        g_free(mon_fdset);
+    }
+}
+
+AddfdInfo *qmp_add_fd(bool has_fdset_id, int64_t fdset_id, Error **errp)
+{
+    int fd;
+    Monitor *mon = cur_mon;
+    mon_fdset_t *mon_fdset;
+    mon_fdset_fd_t *mon_fdset_fd;
+    AddfdInfo *fdinfo;
+
+    fd = qemu_chr_fe_get_msgfd(mon->chr);
+    if (fd == -1) {
+        qerror_report(QERR_FD_NOT_SUPPLIED);
+        return NULL;
+    }
+
+    if (has_fdset_id) {
+        QLIST_FOREACH(mon_fdset, &mon_fdsets, next) {
+            if (mon_fdset->id == fdset_id) {
+                break;
+            }
+        }
+        if (mon_fdset == NULL) {
+            qerror_report(QERR_FDSET_NOT_FOUND, fdset_id);
+            return NULL;
+        }
+    } else {
+        int64_t fdset_id_prev = -1;
+        mon_fdset_t *mon_fdset_cur = QLIST_FIRST(&mon_fdsets);
+
+        /* Use first available fdset ID */
+        QLIST_FOREACH(mon_fdset, &mon_fdsets, next) {
+            mon_fdset_cur = mon_fdset;
+            if (fdset_id_prev == mon_fdset_cur->id - 1) {
+                fdset_id_prev = mon_fdset_cur->id;
+                continue;
+            }
+            break;
+        }
+
+        mon_fdset = g_malloc0(sizeof(*mon_fdset));
+        mon_fdset->id = fdset_id_prev + 1;
+        mon_fdset->refcount = 0;
+
+        /* The fdset list is ordered by fdset ID */
+        if (mon_fdset->id == 0) {
+            QLIST_INSERT_HEAD(&mon_fdsets, mon_fdset, next);
+        } else if (mon_fdset->id < mon_fdset_cur->id) {
+            QLIST_INSERT_BEFORE(mon_fdset_cur, mon_fdset, next);
+        } else {
+            QLIST_INSERT_AFTER(mon_fdset_cur, mon_fdset, next);
+        }
+    }
+
+    mon_fdset_fd = g_malloc0(sizeof(*mon_fdset_fd));
+    mon_fdset_fd->fd = fd;
+    mon_fdset_fd->removed = false;
+    QLIST_INSERT_HEAD(&mon_fdset->fds, mon_fdset_fd, next);
+
+    fdinfo = g_malloc0(sizeof(*fdinfo));
+    fdinfo->fdset_id = mon_fdset->id;
+    fdinfo->fd = mon_fdset_fd->fd;
+
+    return fdinfo;
+}
+
+void qmp_remove_fd(int64_t fdset_id, bool has_fd, int64_t fd, Error **errp)
+{
+    mon_fdset_t *mon_fdset;
+    mon_fdset_fd_t *mon_fdset_fd;
+    char fd_str[20];
+
+    QLIST_FOREACH(mon_fdset, &mon_fdsets, next) {
+        if (mon_fdset->id != fdset_id) {
+            continue;
+        }
+        QLIST_FOREACH(mon_fdset_fd, &mon_fdset->fds, next) {
+            if (has_fd && mon_fdset_fd->fd != fd) {
+                continue;
+            }
+            mon_fdset_fd->removed = true;
+            if (has_fd) {
+                break;
+            }
+        }
+        monitor_fdset_cleanup(mon_fdset);
+        return;
+    }
+    snprintf(fd_str, sizeof(fd_str), "%ld", fd);
+    qerror_report(QERR_FD_NOT_FOUND, fd_str);
+}
+
+FdsetInfoList *qmp_query_fdsets(Error **errp)
+{
+    mon_fdset_t *mon_fdset;
+    mon_fdset_fd_t *mon_fdset_fd;
+    FdsetInfoList *fdset_list = NULL;
+
+    QLIST_FOREACH(mon_fdset, &mon_fdsets, next) {
+        FdsetInfoList *fdset_info = g_malloc0(sizeof(*fdset_info));
+        FdsetFdInfoList *fdsetfd_list = NULL;
+
+        fdset_info->value = g_malloc0(sizeof(*fdset_info->value));
+        fdset_info->value->fdset_id = mon_fdset->id;
+        fdset_info->value->refcount = mon_fdset->refcount;
+        fdset_info->value->in_use = mon_refcount > 0 ? true : false;
+
+        QLIST_FOREACH(mon_fdset_fd, &mon_fdset->fds, next) {
+            FdsetFdInfoList *fdsetfd_info = g_malloc0(sizeof(*fdsetfd_info));
+
+            fdsetfd_info->value = g_malloc0(sizeof(*fdsetfd_info->value));
+            fdsetfd_info->value->fd = mon_fdset_fd->fd;
+            fdsetfd_info->value->removed = mon_fdset_fd->removed;
+
+            fdsetfd_info->next = fdsetfd_list;
+            fdsetfd_list = fdsetfd_info;
+        }
+
+        fdset_info->value->fds = fdsetfd_list;
+
+        fdset_info->next = fdset_list;
+        fdset_list = fdset_info;
+    }
+
+    return fdset_list;
+}
+
 /* mon_cmds and info_cmds would be sorted at runtime */
 static mon_cmd_t mon_cmds[] = {
 #include "hmp-commands.h"
diff --git a/qapi-schema.json b/qapi-schema.json
index cddf63a..1a21bf8 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -2200,3 +2200,113 @@
 # Since: 0.14.0
 ##
 { 'command': 'closefd', 'data': {'fdname': 'str'} }
+
+# @AddfdInfo:
+#
+# Information about a file descriptor that was added to an fd set.
+#
+# @fdset_id: The ID of the fd set that @fd was added to.
+#
+# @fd: The file descriptor that was received via SCM rights and
+#      added to the fd set.
+#
+# Since: 1.2.0
+##
+{ 'type': 'AddfdInfo', 'data': {'fdset_id': 'int', 'fd': 'int'} }
+
+##
+# @add-fd:
+#
+# Add a file descriptor, that was passed via SCM rights, to an fd set.
+#
+# @fdset_id: #optional The ID of the fd set to add the file descriptor to.
+#
+# Returns: @AddfdInfo on success
+#          If file descriptor was not received, FdNotSupplied
+#          If @fdset_id does not exist, FdSetNotFound
+#
+# Notes: The list of fd sets is shared by all monitor connections.
+#
+#        If @fdset_id is not specified, a new fd set will be created.
+#
+# Since: 1.2.0
+##
+{ 'command': 'add-fd', 'data': {'*fdset_id': 'int'},
+  'returns': 'AddfdInfo' }
+
+##
+# @remove-fd:
+#
+# Remove a file descriptor from an fd set.
+#
+# @fdset_id: The ID of the fd set that the file descriptor belongs to.
+#
+# @fd: #optional The file descriptor that is to be removed.
+#
+# Returns: Nothing on success
+#          If @fdset_id or @fd is not found, FdNotFound
+#
+# Since: 1.2.0
+#
+# Notes: The list of fd sets is shared by all monitor connections.
+#
+#        File descriptors that are removed:
+#        o will not be closed until the reference count corresponding
+#          to @fdset_id reaches zero.
+#        o will not be available for use after successful completion
+#          of the remove-fd command.
+#
+#        If @fd is not specified, all file descriptors in @fdset_id
+#        will be removed.
+##
+{ 'command': 'remove-fd', 'data': {'fdset_id': 'int', '*fd': 'int'} }
+
+##
+# @FdsetFdInfo:
+#
+# Information about a file descriptor that belongs to an fd set.
+#
+# @fd: The file descriptor value.
+#
+# @removed: If true, the remove-fd command has been issued for this fd.
+#
+# Since: 1.2.0
+##
+{ 'type': 'FdsetFdInfo', 'data': {'fd': 'int', 'removed': 'bool'} }
+
+##
+# @FdsetInfo:
+#
+# Information about an fd set.
+#
+# @fdset_id: The ID of the fd set.
+#
+# @refcount: A count of the number of outstanding dup() references to
+#            this fd set.
+#
+# @in_use: If true, a monitor is connected and has access to this fd set.
+#
+# @fds: A list of file descriptors that belong to this fd set.
+#
+# Since: 1.2.0
+##
+{ 'type': 'FdsetInfo',
+  'data': {'fdset_id': 'int', 'refcount': 'int', 'in_use': 'bool',
+           'fds': ['FdsetFdInfo']} }
+
+##
+# @query-fdsets:
+#
+# Return information describing all fd sets.
+#
+# Returns: A list of @FdsetInfo
+#
+# Since: 1.2.0
+#
+# Notes: The list of fd sets is shared by all monitor connections.
+#
+#        File descriptors are not closed until @refcount is zero,
+#        and either @in_use is false or @removed is true.
+#
+##
+{ 'command': 'query-fdsets', 'returns': ['FdsetInfo'] }
diff --git a/qerror.c b/qerror.c
index 92c4eff..63a0aa1 100644
--- a/qerror.c
+++ b/qerror.c
@@ -148,6 +148,10 @@ static const QErrorStringTable qerror_table[] = {
         .desc      = "No file descriptor supplied via SCM_RIGHTS",
     },
     {
+        .error_fmt = QERR_FDSET_NOT_FOUND,
+        .desc      = "File descriptor set with ID '%(id)' not found",
+    },
+    {
         .error_fmt = QERR_FEATURE_DISABLED,
         .desc      = "The feature '%(name)' is not enabled",
     },
diff --git a/qerror.h b/qerror.h
index b4c8758..b908d3f 100644
--- a/qerror.h
+++ b/qerror.h
@@ -133,6 +133,9 @@ QError *qobject_to_qerror(const QObject *obj);
 #define QERR_FD_NOT_SUPPLIED \
     "{ 'class': 'FdNotSupplied', 'data': {} }"
 
+#define QERR_FDSET_NOT_FOUND \
+    "{ 'class': 'FdSetNotFound', 'data': { 'id': %ld } }"
+
 #define QERR_FEATURE_DISABLED \
     "{ 'class': 'FeatureDisabled', 'data': { 'name': %s } }"
 
diff --git a/qmp-commands.hx b/qmp-commands.hx
index ac46638..3c243d8 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -926,6 +926,137 @@ Example:
 
 EQMP
 
+     {
+        .name       = "add-fd",
+        .args_type  = "fdset_id:i?",
+        .params     = "add-fd fdset_id",
+        .help       = "Add a file descriptor, that was passed via SCM rights, 
to an fd set",
+        .mhandler.cmd_new = qmp_marshal_input_add_fd,
+    },
+
+SQMP
+add-fd
+-------
+
+Add a file descriptor, that was passed via SCM rights, to an fd set.
+
+Arguments:
+
+- "fdset_id": The ID of the fd set to add the file descriptor to.
+              (json-int, optional)
+
+Return a json-object with the following information:
+
+- "fdset_id": The ID of the fd set that the fd was added to. (json-int)
+- "fd": The file descriptor that was received via SCM rights and added to the
+        fd set. (json-int)
+
+Example:
+
+-> { "execute": "add-fd", "arguments": { "fdset_id": 1 } }
+<- { "return": { "fdset_id": 1, "fd": 3 } }
+
+Notes:
+
+(1) The list of fd sets is shared by all monitor connections.
+(2) If "fdset_id" is not specified, a new fd set will be created.
+
+EQMP
+
+     {
+        .name       = "remove-fd",
+        .args_type  = "fdset_id:i,fd:i?",
+        .params     = "remove-fd fdset_id fd",
+        .help       = "Remove a file descriptor from an fd set",
+        .mhandler.cmd_new = qmp_marshal_input_remove_fd,
+    },
+
+SQMP
+remove-fd
+---------
+
+Remove a file descriptor from an fd set.
+
+Arguments:
+
+- "fdset_id": The ID of the fd set that the file descriptor belongs to.
+              (json-int)
+- "fd": The file descriptor that is to be removed. (json-int, optional)
+
+Example:
+
+-> { "execute": "remove-fd", "arguments": { "fdset_id": 1, "fd": 3 } }
+<- { "return": {} }
+
+Notes:
+
+(1) The list of fd sets is shared by all monitor connections.
+(2) File descriptors that are removed:
+    (a) will not be closed until the reference count corresponding to fdset_id
+        reaches zero.
+    (b) will not be available for use after successful completion of the
+        remove-fd command.
+(3) If "fd" is not specified, all file descriptors in "fdset_id" will be
+    removed.
+
+EQMP
+
+    {
+        .name       = "query-fdsets",
+        .args_type  = "",
+        .help       = "Return information describing all fd sets",
+        .mhandler.cmd_new = qmp_marshal_input_query_fdsets,
+    },
+
+SQMP
+query-fdsets
+-------------
+
+Return information describing all fd sets.
+
+Arguments: None
+
+Example:
+
+-> { "execute": "query-fdsets" }
+<- { "return": [
+       {
+         "fdset_id": 1
+         "refcount": 0,
+         "in_use": true,
+         "fds": [
+           {
+             "fd": 21,
+             "removed": false
+           },
+           {
+             "fd": 23,
+             "removed": false
+           }
+         ],
+       },
+       {
+         "fdset_id": 2
+         "refcount": 0,
+         "in_use": true,
+         "fds": [
+           {
+             "fd": 22,
+             "removed": false
+           }
+         ],
+       }
+     ]
+   }
+
+Notes:
+
+(1) The list of fd sets is shared by all monitor connections.
+(2) File descriptors are not closed until refcount is zero, and
+    either in_use is false or removed is true.
+
+EQMP
+
     {
         .name       = "block_passwd",
         .args_type  = "device:B,password:s",
-- 
1.7.10.4