Re: [Qemu-devel] [libvirt] [PATCH v4 0/7] file descriptor passing using pass-fd

[Daniel P. Berrange]

On Tue, Jun 26, 2012 at 03:11:40PM +0100, Daniel P. Berrange wrote:
> On Tue, Jun 26, 2012 at 09:52:51AM -0400, Corey Bryant wrote:
> > >So now from a client's POV you'd have a flow like
> > >
> > >    * drive_add "file=/dev/fd/N"  FDSET={N}
> > 
> > IIUC then drive_add would loop and pass each fd in the set via SCM_RIGHTS?
> 
> Yes, you'd probably use the JSON to tell QEMU exactly
> how many FDs you're intending to pass with the command,
> and then once the command is received, you'd have N*SCM_RIGHTS
> messages sent/received.
> 
> > >
> > >And in QEMU you'd have something like
> > >
> > >    * handle_monitor_command
> > >         - recvmsg all FDs, and stash them in a thread local "FDContext"
> > >           context
> > >         - invoke monitor command handler
> > >               - Sees file=/dev/fd/N
> > >               - Fetch /dev/fd/N from "FDContext"
> > >               - If success remove /dev/fd/N from "FDContext"
> > >         - close() all FDs left in "FDContext"
> > >
> > >The key point with this is that because the FDs are directly
> > >associated with a monitor command, QEMU can /guarantee/ that
> > >FDs are never leaked, regardless of client behaviour.
> > 
> > Wouldn't this leak fds if libvirt crashed part way through sending
> > the set of fds?
> 
> No, because you've scoped the FDs to the current monitor instance,
> and the current command being processed you know to close all FDs
> when the associated command has finished, as well as closing them
> all if you see EOF on the monitor socket while in the middle of
> receiving a command.

Here is a quick proof of concept (ie untested) patch to demonstrate
what I mean. It relies on Cory's patch which converts everything
to use qemu_open. It is also still valuable to make the change
to qemu_open() to support "/dev/fd/N" for passing FDs during
QEMU initial startup for CLI args.

IMHO, what I propose here is preferrable for QMP clients that
our current plan of requiring use of 3 monitor commands (passfd,
XXXXX, closefd).

>From 55a264b647d90a30c1cc00cb1896535246bcd9b7 Mon Sep 17 00:00:00 2001
From: "Daniel P. Berrange" <address@hidden>
Date: Tue, 26 Jun 2012 15:48:13 +0100
Subject: [PATCH] Support passing FDs with the monitor command that uses them

Add support for a syntax "/dev/fdlist/N", where 'N' refers to
the N'th FD that was passed along with the currently processing
command in this thread.

The QMP client sends

        {
          "execute": "drive_add",
          "arguments": {
               "filename": "/dev/fdlist/0",
               "backingfilename": "/dev/fdlist/1" },
          "nfds": 2
        }

followed by "nfds" * SCM_RIGHTS packets

The FDs received along with the "drive_add" command are placed
in a thread-local list, then the QMP handler function is invoked.
The qemu_open() function will resolve any path "/dev/fdlist/0"
to use an FD stored in the  thread-local list, and dup() it.
One the QMP handler function is finished, all FDs in the thread
local list are closed.

THe reason for choice of "/dev/fdlist/N", is to allow the
"/dev/fd/N" syntax to be used for FDs that are passed to QEMU
at startup time, which will be in the global FD number namespace,
not the thread-local index.

The reason for using a thread local is to ensure the FDs are
only made available to the currently executing monitor command
handler in this thread, and not any other threads.

The advantage over using a separate 'passfd' command, is that
this guarentees all FDs are closed in QEMU once the QMP command
handler is finished. There is no risk of leaks being triggered
by the client either intentionally, or via it crashing.

NB: this is a proof of concept demonstration of the overall
architectural design. The patch would need more work before
it was suitable for merging

 - Pick a better place/file for the fdlist APIs
 - Do proper error reporting in various places
 - Check for possibility that qemu_chr_fe_get_msgfd
   can block or return EAGAIN.

diff --git a/monitor.c b/monitor.c
index f6107ba..a3a4bd4 100644
--- a/monitor.c
+++ b/monitor.c
@@ -4490,6 +4490,8 @@ static void handle_qmp_command(JSONMessageParser *parser, 
QList *tokens)
     const mon_cmd_t *cmd;
     const char *cmd_name;
     Monitor *mon = cur_mon;
+    int nfds;
+    size_t i;
 
     args = input = NULL;
 
@@ -4535,6 +4537,17 @@ static void handle_qmp_command(JSONMessageParser 
*parser, QList *tokens)
         goto err_out;
     }
 
+    nfds = qdict_get_int(input, "nfds");
+    for (i = 0 ; i < nfds ; i++) {
+       int fd = qemu_chr_fe_get_msgfd(mon->chr);
+       if (fd == -1) {
+           qerror_report(QERR_FD_NOT_SUPPLIED);
+           goto err_out;
+       }
+
+       qemu_fdlist_add(fd);
+    }
+
     if (handler_is_async(cmd)) {
         err = qmp_async_cmd_handler(mon, cmd, args);
         if (err) {
@@ -4552,6 +4565,7 @@ err_out:
 out:
     QDECREF(input);
     QDECREF(args);
+    qemu_fdlist_closeall();
 }
 
 /**
diff --git a/osdep.c b/osdep.c
index 03817f0..0849cb6 100644
--- a/osdep.c
+++ b/osdep.c
@@ -75,6 +75,81 @@ int qemu_madvise(void *addr, size_t len, int advice)
 #endif
 }
 
+typedef struct QEMUFDList {
+    int *fds;
+    size_t nfds;
+} QEMUFDList;
+
+static pthread_key_t fdlist_key;
+
+static void qemu_fdlist_cleanup(void *data)
+{
+    size_t i;
+    QEMUFDList *fdlist = data;
+
+    if (!fdlist)
+       return;
+
+    for (i = 0 ; i < fdlist->nfds ; i++) {
+       close(fdlist->fds[i]);
+    }
+}
+
+int qemu_fdlist_init(void)
+{
+    if (pthread_key_create(&fdlist_key,
+                          qemu_fdlist_cleanup) < 0)
+       return -1;
+
+    return 0;
+}
+
+static QEMUFDList *qemu_fdlist_for_thread(void)
+{
+    QEMUFDList *fdlist = pthread_getspecific(fdlist_key);
+
+    if (fdlist == NULL) {
+       fdlist = g_new0(QEMUFDList, 1);
+       pthread_setspecific(fdlist_key, fdlist);
+    }
+
+    return fdlist;
+}
+
+void qemu_fdlist_add(int fd)
+{
+    QEMUFDList *fdlist = qemu_fdlist_for_thread();
+
+    fdlist->fds = g_renew(int, fdlist->fds, fdlist->nfds + 1);
+    fdlist->fds[fdlist->nfds++] = fd;
+}
+
+int qemu_fdlist_dup(int idx)
+{
+    QEMUFDList *fdlist = qemu_fdlist_for_thread();
+
+    if (idx >= fdlist->nfds) {
+       errno = EINVAL;
+       return -1;
+    }
+
+    return dup(fdlist->fds[idx]);
+}
+
+void qemu_fdlist_closeall(void)
+{
+    QEMUFDList *fdlist = qemu_fdlist_for_thread();
+    size_t i;
+
+    for (i = 0 ; i < fdlist->nfds ; i++) {
+       close(fdlist->fds[i]);
+    }
+    g_free(fdlist->fds);
+    fdlist->fds = NULL;
+    fdlist->nfds = 0;
+}
+
+
 
 /*
  * Opens a file with FD_CLOEXEC set
@@ -83,6 +158,26 @@ int qemu_open(const char *name, int flags, ...)
 {
     int ret;
     int mode = 0;
+    const char *p;
+
+    /* Attempt dup of fd for pre-opened file */
+    if (strstart(name, "/dev/fdlist/", &p)) {
+        int idx;
+       int fd;
+
+        idx = qemu_parse_fd(p);
+        if (idx == -1) {
+            return -1;
+        }
+
+       fd = qemu_fdlist_dup(idx);
+       if (fd == -1) {
+           return -1;
+       }
+
+       /* XXX verify flags match */
+       return fd;
+    }
 
     if (flags & O_CREAT) {
         va_list ap;
diff --git a/qemu-common.h b/qemu-common.h
index 8f87e41..b6b7203 100644
--- a/qemu-common.h
+++ b/qemu-common.h
@@ -194,6 +194,11 @@ ssize_t qemu_send_full(int fd, const void *buf, size_t 
count, int flags)
 ssize_t qemu_recv_full(int fd, void *buf, size_t count, int flags)
     QEMU_WARN_UNUSED_RESULT;
 
+int qemu_fdlist_init(void);
+void qemu_fdlist_add(int fd);
+int qemu_fdlist_dup(int idx);
+void qemu_fdlist_closeall(void);
+
 #ifndef _WIN32
 int qemu_eventfd(int pipefd[2]);
 int qemu_pipe(int pipefd[2]);
diff --git a/vl.c b/vl.c
index 1329c30..e515c84 100644
--- a/vl.c
+++ b/vl.c
@@ -2321,6 +2321,11 @@ int main(int argc, char **argv, char **envp)
     QLIST_INIT (&vm_change_state_head);
     os_setup_early_signal_handling();
 
+    if (qemu_fdlist_init() < 0) {
+       perror("fdlist");
+       exit(EXIT_FAILURE);
+    }
+
     module_call_init(MODULE_INIT_MACHINE);
     machine = find_default_machine();
     cpu_model = NULL;
-- 
1.7.10.2


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