util/async-teardown.c: move to softmmu/, only build it when system build is requested

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-ID: <20230901101302.3618955-9-mjt@tls.msk.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 147 insertions, 0 deletions

diff --git a/softmmu/async-teardown.c b/softmmu/async-teardown.c
new file mode 100644
index 0000000..62cdeb0
--- /dev/null
+++ b/softmmu/async-teardown.c
@@ -0,0 +1,146 @@
+/*
+ * Asynchronous teardown
+ *
+ * Copyright IBM, Corp. 2022
+ *
+ * Authors:
+ *  Claudio Imbrenda <imbrenda@linux.ibm.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or (at your
+ * option) any later version.  See the COPYING file in the top-level directory.
+ *
+ */
+
+#include "qemu/osdep.h"
+#include <dirent.h>
+#include <sys/prctl.h>
+#include <sched.h>
+
+#include "qemu/async-teardown.h"
+
+#ifdef _SC_THREAD_STACK_MIN
+#define CLONE_STACK_SIZE sysconf(_SC_THREAD_STACK_MIN)
+#else
+#define CLONE_STACK_SIZE 16384
+#endif
+
+static pid_t the_ppid;
+
+/*
+ * Close all open file descriptors.
+ */
+static void close_all_open_fd(void)
+{
+    struct dirent *de;
+    int fd, dfd;
+    DIR *dir;
+
+#ifdef CONFIG_CLOSE_RANGE
+    int r = close_range(0, ~0U, 0);
+    if (!r) {
+        /* Success, no need to try other ways. */
+        return;
+    }
+#endif
+
+    dir = opendir("/proc/self/fd");
+    if (!dir) {
+        /* If /proc is not mounted, there is nothing that can be done. */
+        return;
+    }
+    /* Avoid closing the directory. */
+    dfd = dirfd(dir);
+
+    for (de = readdir(dir); de; de = readdir(dir)) {
+        fd = atoi(de->d_name);
+        if (fd != dfd) {
+            close(fd);
+        }
+    }
+    closedir(dir);
+}
+
+static void hup_handler(int signal)
+{
+    /* Check every second if this process has been reparented. */
+    while (the_ppid == getppid()) {
+        /* sleep() is safe to use in a signal handler. */
+        sleep(1);
+    }
+
+    /* At this point the parent process has terminated completely. */
+    _exit(0);
+}
+
+static int async_teardown_fn(void *arg)
+{
+    struct sigaction sa = { .sa_handler = hup_handler };
+    sigset_t hup_signal;
+    char name[16];
+
+    /* Set a meaningful name for this process. */
+    snprintf(name, 16, "cleanup/%d", the_ppid);
+    prctl(PR_SET_NAME, (unsigned long)name);
+
+    /*
+     * Close all file descriptors that might have been inherited from the
+     * main qemu process when doing clone, needed to make libvirt happy.
+     * Not using close_range for increased compatibility with older kernels.
+     */
+    close_all_open_fd();
+
+    /* Set up a handler for SIGHUP and unblock SIGHUP. */
+    sigaction(SIGHUP, &sa, NULL);
+    sigemptyset(&hup_signal);
+    sigaddset(&hup_signal, SIGHUP);
+    sigprocmask(SIG_UNBLOCK, &hup_signal, NULL);
+
+    /* Ask to receive SIGHUP when the parent dies. */
+    prctl(PR_SET_PDEATHSIG, SIGHUP);
+
+    /*
+     * Sleep forever, unless the parent process has already terminated. The
+     * only interruption can come from the SIGHUP signal, which in normal
+     * operation is received when the parent process dies.
+     */
+    if (the_ppid == getppid()) {
+        pause();
+    }
+
+    /* At this point the parent process has terminated completely. */
+    _exit(0);
+}
+
+/*
+ * Allocate a new stack of a reasonable size, and return a pointer to its top.
+ */
+static void *new_stack_for_clone(void)
+{
+    size_t stack_size = CLONE_STACK_SIZE;
+    char *stack_ptr;
+
+    /* Allocate a new stack and get a pointer to its top. */
+    stack_ptr = qemu_alloc_stack(&stack_size);
+#if !defined(HOST_HPPA)
+    /* The top is at the end of the area, except on HPPA. */
+    stack_ptr += stack_size;
+#endif
+
+    return stack_ptr;
+}
+
+/*
+ * Block all signals, start (clone) a new process sharing the address space
+ * with qemu (CLONE_VM), then restore signals.
+ */
+void init_async_teardown(void)
+{
+    sigset_t all_signals, old_signals;
+
+    the_ppid = getpid();
+
+    sigfillset(&all_signals);
+    sigprocmask(SIG_BLOCK, &all_signals, &old_signals);
+    clone(async_teardown_fn, new_stack_for_clone(), CLONE_VM, NULL);
+    sigprocmask(SIG_SETMASK, &old_signals, NULL);
+}
diff --git a/softmmu/meson.build b/softmmu/meson.build
index ea5603f..c18b7ad 100644
--- a/softmmu/meson.build
+++ b/softmmu/meson.build
@@ -37,3 +37,4 @@ endif
 
 system_ss.add(when: seccomp, if_true: files('qemu-seccomp.c'))
 system_ss.add(when: fdt, if_true: files('device_tree.c'))
+system_ss.add(when: 'CONFIG_LINUX', if_true: files('async-teardown.c'))