* [PATCH v1 1/2] Implement -run-with exit-with-parent=on
2025-10-03 13:24 [PATCH v1 0/2] Implement -run-with exit-with-parent=on Richard W.M. Jones
@ 2025-10-03 13:24 ` Richard W.M. Jones
2025-10-03 14:12 ` Richard W.M. Jones
2025-10-03 13:24 ` [PATCH v1 2/2] tests/qtest: Use exit-with-parent=on in qtest invocations Richard W.M. Jones
2025-10-03 14:11 ` [PATCH v1 0/2] Implement -run-with exit-with-parent=on Richard W.M. Jones
2 siblings, 1 reply; 5+ messages in thread
From: Richard W.M. Jones @ 2025-10-03 13:24 UTC (permalink / raw)
To: qemu-devel; +Cc: pbonzini, berrange
Libguestfs wants to use qemu to run a captive appliance. When the
program linked to libguestfs exits, we want qemu to be cleaned up.
Libguestfs goes to great lengths to do this at the moment: it either
forks a separate process to ensure clean-up is done, or it asks
libvirt to clean up the qemu process. However this is complicated and
not totally reliable.
On Linux, FreeBSD and macOS, there are mechanisms to ensure a signal
or message is delivered to a process when its parent process goes
away. The qemu test suite even uses this mechanism on Linux (see
PR_SET_PDEATHSIG in tests/qtest/libqtest.c).
In nbdkit we have long had the concept of running nbdkit captively,
and we have the nbdkit --exit-with-parent flag to help
(https://libguestfs.org/nbdkit-captive.1.html#EXIT-WITH-PARENT)
This commit adds the same mechanism. The syntax is:
qemu -run-with exit-with-parent=on [...]
This is not a feature that most typical users of qemu (for running
general purpose, long-lived VMs) should use, so it defaults to off.
The exit-with-parent.[ch] files are copied from nbdkit, where they
have a 3-clause BSD license which is compatible with qemu:
https://gitlab.com/nbdkit/nbdkit/-/tree/master/common/utils?ref_type=heads
Thanks: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
---
include/qemu/exit-with-parent.h | 47 +++++++++
qemu-options.hx | 13 ++-
system/exit-with-parent.c | 169 ++++++++++++++++++++++++++++++++
system/meson.build | 1 +
system/vl.c | 13 +++
5 files changed, 241 insertions(+), 2 deletions(-)
diff --git a/include/qemu/exit-with-parent.h b/include/qemu/exit-with-parent.h
new file mode 100644
index 0000000000..141f68d53b
--- /dev/null
+++ b/include/qemu/exit-with-parent.h
@@ -0,0 +1,47 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ * Originally derived from nbdkit common/utils/exit-with-parent.h
+ * Copyright Red Hat
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ * * Neither the name of Red Hat nor the names of its contributors may be
+ * used to endorse or promote products derived from this software without
+ * specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+ * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+ * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#ifndef NBDKIT_EXIT_WITH_PARENT_H
+#define NBDKIT_EXIT_WITH_PARENT_H
+
+/* Test if the feature is available on the platform. */
+extern bool can_exit_with_parent(void);
+
+/*
+ * --exit-with-parent: kill the current process if the parent exits.
+ * This may return -1 on error.
+ */
+extern int set_exit_with_parent(void);
+
+#endif /* NBDKIT_EXIT_WITH_PARENT_H */
diff --git a/qemu-options.hx b/qemu-options.hx
index 075f4be2e3..9ff0346afe 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -5437,15 +5437,18 @@ DEF("qtest-log", HAS_ARG, QEMU_OPTION_qtest_log, "", QEMU_ARCH_ALL)
#if defined(CONFIG_POSIX) && !defined(EMSCRIPTEN)
DEF("run-with", HAS_ARG, QEMU_OPTION_run_with,
- "-run-with [async-teardown=on|off][,chroot=dir][user=username|uid:gid]\n"
+ "-run-with [async-teardown=on|off][,chroot=dir]\n" \
+ " [,exit-with-parent=on|off][,user=username|uid:gid]\n"
" Set miscellaneous QEMU process lifecycle options:\n"
" async-teardown=on enables asynchronous teardown (Linux only)\n"
+ " exit-with-parent=on causes QEMU to exit if the parent\n"
+ " process of QEMU exits (Linux, FreeBSD, macOS only)\n"
" chroot=dir chroot to dir just before starting the VM\n"
" user=username switch to the specified user before starting the VM\n"
" user=uid:gid ditto, but use specified user-ID and group-ID instead\n",
QEMU_ARCH_ALL)
SRST
-``-run-with [async-teardown=on|off][,chroot=dir][user=username|uid:gid]``
+``-run-with [async-teardown=on|off][,chroot=dir][,exit-with-parent=on|off][,user=username|uid:gid]``
Set QEMU process lifecycle options.
``async-teardown=on`` enables asynchronous teardown. A new process called
@@ -5463,6 +5466,12 @@ SRST
immediately before starting the guest execution. This is especially useful
in combination with ``user=...``.
+ ``exit-with-parent=on`` causes QEMU to exit if the parent process of
+ QEMU exits. This can be used when QEMU runs a captive appliance,
+ where the lifetime of the appliance is scoped to the parent process.
+ In case the parent process crashes, QEMU is still cleaned up.
+ This only works on Linux, FreeBSD and macOS platforms.
+
``user=username`` or ``user=uid:gid`` can be used to drop root privileges
before starting guest execution. QEMU will use the ``setuid`` and ``setgid``
system calls to switch to the specified identity. Note that the
diff --git a/system/exit-with-parent.c b/system/exit-with-parent.c
new file mode 100644
index 0000000000..913439d553
--- /dev/null
+++ b/system/exit-with-parent.c
@@ -0,0 +1,169 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ * Originally derived from nbdkit common/utils/exit-with-parent.c
+ * Copyright Red Hat
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ * * Neither the name of Red Hat nor the names of its contributors may be
+ * used to endorse or promote products derived from this software without
+ * specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+ * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+ * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+/*
+ * Implement the --exit-with-parent feature on operating systems which
+ * support it.
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/exit-with-parent.h"
+
+#ifdef CONFIG_LINUX
+#include <sys/prctl.h>
+#endif
+
+#ifdef __FreeBSD__
+#include <sys/procctl.h>
+#endif
+
+#if defined(CONFIG_LINUX)
+
+/*
+ * Send SIGTERM to self when the parent exits. This will cause
+ * qemu_system_killed() to be called.
+ *
+ * PR_SET_PDEATHSIG has been defined since Linux 2.1.57.
+ */
+int
+set_exit_with_parent(void)
+{
+ return prctl(PR_SET_PDEATHSIG, SIGTERM);
+}
+
+bool
+can_exit_with_parent(void)
+{
+ return true;
+}
+
+#elif defined(__FreeBSD__)
+
+/*
+ * Send SIGTERM to self when the parent exits. This will cause
+ * qemu_system_killed() to be called.
+ *
+ * PROC_PDEATHSIG_CTL has been defined since FreeBSD 11.2.
+ */
+int
+set_exit_with_parent(void)
+{
+ const int sig = SIGTERM;
+ return procctl(P_PID, 0, PROC_PDEATHSIG_CTL, (void *) &sig);
+}
+
+bool
+can_exit_with_parent(void)
+{
+ return true;
+}
+
+#elif defined(__APPLE__)
+
+/* For macOS. */
+
+#include "qemu/thread.h"
+#include "qemu/error-report.h"
+#include "system/runstate.h"
+#include <sys/event.h>
+
+static void *
+exit_with_parent_loop(void *vp)
+{
+ const pid_t ppid = getppid();
+ int fd;
+ struct kevent kev, res[1];
+ int r;
+
+ /* Register the kevent to wait for ppid to exit. */
+ fd = kqueue();
+ if (fd == -1) {
+ error_report("exit_with_parent_loop: kqueue: %m");
+ return NULL;
+ }
+ EV_SET(&kev, ppid, EVFILT_PROC, EV_ADD | EV_ENABLE, NOTE_EXIT, 0, NULL);
+ if (kevent(fd, &kev, 1, NULL, 0, NULL) == -1) {
+ error_report("exit_with_parent_loop: kevent: %m");
+ close(fd);
+ return NULL;
+ }
+
+ /* Wait for the kevent to happen. */
+ r = kevent(fd, 0, 0, res, 1, NULL);
+ if (r == 1 && res[0].ident == ppid) {
+ /* Behave like Linux and FreeBSD above, as if SIGTERM was sent */
+ qemu_system_killed(SIGTERM, ppid);
+ }
+
+ return NULL;
+}
+
+int
+set_exit_with_parent(void)
+{
+ int r;
+ QemuThread exit_with_parent_thread;
+
+ /*
+ * We have to block waiting for kevent, so that requires that we
+ * start a background thread.
+ */
+ qemu_thread_create(&exit_with_parent_thread,
+ "exit-parent",
+ exit_with_parent_loop, NULL,
+ QEMU_THREAD_DETACHED);
+ return 0;
+}
+
+bool
+can_exit_with_parent(void)
+{
+ return true;
+}
+
+#else /* any platform that doesn't support this function */
+
+int
+set_exit_with_parent(void)
+{
+ g_assert_not_reached();
+}
+
+bool
+can_exit_with_parent(void)
+{
+ return false;
+}
+
+#endif
diff --git a/system/meson.build b/system/meson.build
index 6d21ff9faa..4b69ef0f5f 100644
--- a/system/meson.build
+++ b/system/meson.build
@@ -15,6 +15,7 @@ system_ss.add(files(
'datadir.c',
'dirtylimit.c',
'dma-helpers.c',
+ 'exit-with-parent.c',
'globals.c',
'ioport.c',
'ram-block-attributes.c',
diff --git a/system/vl.c b/system/vl.c
index 00f3694725..47b6e60b94 100644
--- a/system/vl.c
+++ b/system/vl.c
@@ -53,6 +53,7 @@
#include "qemu/sockets.h"
#include "qemu/accel.h"
#include "qemu/async-teardown.h"
+#include "qemu/exit-with-parent.h"
#include "hw/usb.h"
#include "hw/isa/isa.h"
#include "hw/scsi/scsi.h"
@@ -783,6 +784,10 @@ static QemuOptsList qemu_run_with_opts = {
.name = "chroot",
.type = QEMU_OPT_STRING,
},
+ {
+ .name = "exit-with-parent",
+ .type = QEMU_OPT_BOOL,
+ },
{
.name = "user",
.type = QEMU_OPT_STRING,
@@ -3690,6 +3695,14 @@ void qemu_init(int argc, char **argv)
if (str) {
os_set_chroot(str);
}
+ if (qemu_opt_get_bool(opts, "exit-with-parent", false)) {
+ if (!can_exit_with_parent()) {
+ error_report("exit-with-parent is not available"
+ " on this platform");
+ exit(1);
+ }
+ set_exit_with_parent();
+ }
str = qemu_opt_get(opts, "user");
if (str) {
if (!os_set_runas(str)) {
--
2.50.1
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