[Qemu-devel] [PATCH v0 3/7] threads: add infrastructure to process sigsegv

[Denis Plotnikov <dplotnikov@virtuozzo.com>]

Allows to define sigsegv handler temporary for all threads.
This is useful to implement copy-on-write logic while
linux usefaultfd doesn't support write-protected faults.
In the future, switch to using WP userfaultfd when it's
available.

It's going to be used on background snapshotting.

Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
---
 include/qemu/thread.h    |  5 ++++
 util/qemu-thread-posix.c | 50 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+)

diff --git a/include/qemu/thread.h b/include/qemu/thread.h
index 9910f49b3a..886985d289 100644
--- a/include/qemu/thread.h
+++ b/include/qemu/thread.h
@@ -210,4 +210,9 @@ void qemu_lockcnt_inc_and_unlock(QemuLockCnt *lockcnt);
  */
 unsigned qemu_lockcnt_count(QemuLockCnt *lockcnt);
 
+
+typedef void (*sigsegv_handler)(int v0, siginfo_t *v1, void *v2);
+void sigsegv_user_handler_set(sigsegv_handler handler);
+void sigsegv_user_handler_reset(void);
+
 #endif
diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index 7306475899..e51abc9275 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -489,6 +489,45 @@ static void qemu_thread_set_name(QemuThread *thread, const char *name)
 #endif
 }
 
+static sigsegv_handler sigsegv_user_handler;
+
+void sigsegv_user_handler_set(sigsegv_handler handler)
+{
+    assert(handler);
+    atomic_set(&sigsegv_user_handler, handler);
+}
+
+static sigsegv_handler sigsegv_user_handler_get(void)
+{
+    return atomic_read(&sigsegv_user_handler);
+}
+
+void sigsegv_user_handler_reset(void)
+{
+    atomic_set(&sigsegv_user_handler, NULL);
+}
+
+static void sigsegv_default_handler(int v0, siginfo_t *v1, void *v2)
+{
+    sigsegv_handler handler = sigsegv_user_handler_get();
+
+    if (!handler) {
+        // remove the sigsegv handler if it's not set by user
+        // this will lead to re-raising the error without a handler
+        // and exiting from the program with "Sigmentation fault"
+        int err;
+        struct sigaction act;
+        memset(&act, 0, sizeof(act));
+        act.sa_flags = SA_RESETHAND;
+        err = sigaction(SIGSEGV, &act, NULL);
+        if (err) {
+            error_exit(err, __func__);
+        }
+    } else {
+        handler(v0, v1, v2);
+    }
+}
+
 void qemu_thread_create(QemuThread *thread, const char *name,
                        void *(*start_routine)(void*),
                        void *arg, int mode)
@@ -496,14 +535,25 @@ void qemu_thread_create(QemuThread *thread, const char *name,
     sigset_t set, oldset;
     int err;
     pthread_attr_t attr;
+    struct sigaction act;
 
     err = pthread_attr_init(&attr);
     if (err) {
         error_exit(err, __func__);
     }
 
+    memset(&act, 0, sizeof(act));
+    act.sa_flags = SA_SIGINFO;
+    act.sa_sigaction = sigsegv_default_handler;
+    err = sigaction(SIGSEGV, &act, NULL);
+    if (err) {
+        error_exit(err, __func__);
+    }
+
     /* Leave signal handling to the iothread.  */
     sigfillset(&set);
+    // ...all but SIGSEGV
+    sigdelset(&set, SIGSEGV);
     pthread_sigmask(SIG_SETMASK, &set, &oldset);
     err = pthread_create(&thread->thread, &attr, start_routine, arg);
     if (err)
-- 
2.17.0