osdep: add qemu_fork() wrapper for safely handling signals

When using regular fork() the child process of course inherits
all the parents' signal handlers. If the child then proceeds
to close() any open file descriptors, it may break some of those
registered signal handlers. The child generally does not want to
ever run any of the signal handlers that the parent may have
installed in the short time before it exec's. The parent may also
have blocked various signals which the child process will want
enabled.

This introduces a wrapper qemu_fork() that takes care to sanitize
signal handling across fork. Before forking it blocks all signals
in the parent thread. After fork returns, the parent unblocks the
signals and carries on as usual. The child, however, resets all the
signal handlers back to their defaults before it unblocks signals.
The child process can now exec the binary in a "clean" signal
environment.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

include/qemu/osdep.h

@@ -69,6 +69,8 @@
 #include "sysemu/os-posix.h"
 #endif
 
+#include "qapi/error.h"
+
 #if defined(CONFIG_SOLARIS) && CONFIG_SOLARIS_VERSION < 10
 /* [u]int_fast*_t not in <sys/int_types.h> */
 typedef unsigned char           uint_fast8_t;
@@ -286,4 +288,18 @@ void os_mem_prealloc(int fd, char *area, size_t sz);
 
 int qemu_read_password(char *buf, int buf_size);
 
+/**
+ * qemu_fork:
+ *
+ * A version of fork that avoids signal handler race
+ * conditions that can lead to child process getting
+ * signals that are otherwise only expected by the
+ * parent. It also resets all signal handlers to the
+ * default settings.
+ *
+ * Returns 0 to child process, pid number to parent
+ * or -1 on failure.
+ */
+pid_t qemu_fork(Error **errp);
+
 #endif

util/oslib-posix.c

@@ -490,3 +490,74 @@ int qemu_read_password(char *buf, int buf_size)
     printf("\n");
     return ret;
 }
+
+
+pid_t qemu_fork(Error **errp)
+{
+    sigset_t oldmask, newmask;
+    struct sigaction sig_action;
+    int saved_errno;
+    pid_t pid;
+
+    /*
+     * Need to block signals now, so that child process can safely
+     * kill off caller's signal handlers without a race.
+     */
+    sigfillset(&newmask);
+    if (pthread_sigmask(SIG_SETMASK, &newmask, &oldmask) != 0) {
+        error_setg_errno(errp, errno,
+                         "cannot block signals");
+        return -1;
+    }
+
+    pid = fork();
+    saved_errno = errno;
+
+    if (pid < 0) {
+        /* attempt to restore signal mask, but ignore failure, to
+         * avoid obscuring the fork failure */
+        (void)pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
+        error_setg_errno(errp, saved_errno,
+                         "cannot fork child process");
+        errno = saved_errno;
+        return -1;
+    } else if (pid) {
+        /* parent process */
+
+        /* Restore our original signal mask now that the child is
+         * safely running. Only documented failures are EFAULT (not
+         * possible, since we are using just-grabbed mask) or EINVAL
+         * (not possible, since we are using correct arguments).  */
+        (void)pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
+    } else {
+        /* child process */
+        size_t i;
+
+        /* Clear out all signal handlers from parent so nothing
+         * unexpected can happen in our child once we unblock
+         * signals */
+        sig_action.sa_handler = SIG_DFL;
+        sig_action.sa_flags = 0;
+        sigemptyset(&sig_action.sa_mask);
+
+        for (i = 1; i < NSIG; i++) {
+            /* Only possible errors are EFAULT or EINVAL The former
+             * won't happen, the latter we expect, so no need to check
+             * return value */
+            (void)sigaction(i, &sig_action, NULL);
+        }
+
+        /* Unmask all signals in child, since we've no idea what the
+         * caller's done with their signal mask and don't want to
+         * propagate that to children */
+        sigemptyset(&newmask);
+        if (pthread_sigmask(SIG_SETMASK, &newmask, NULL) != 0) {
+            Error *local_err = NULL;
+            error_setg_errno(&local_err, errno,
+                             "cannot unblock signals");
+            error_report_err(local_err);
+            _exit(1);
+        }
+    }
+    return pid;
+}

util/oslib-win32.c

@@ -496,3 +496,12 @@ int qemu_read_password(char *buf, int buf_size)
     buf[i] = '\0';
     return 0;
 }
+
+
+pid_t qemu_fork(Error **errp)
+{
+    errno = ENOSYS;
+    error_setg_errno(errp, errno,
+                     "cannot fork child process");
+    return -1;
+}