From a82032351c921c3d5d57122aca57b47f83f9b9d1 Mon Sep 17 00:00:00 2001 From: Thiago Macieira Date: Thu, 10 Sep 2020 13:30:59 -0700 Subject: [PATCH] forkfd/Linux: ask clone() to use the SIGCHLD as the termination signal Because of these lines in the Linux kernel (kernel/fork.c, see [1][3]): if (clone_flags & CLONE_VFORK) trace = PTRACE_EVENT_VFORK; else if (args->exit_signal != SIGCHLD) trace = PTRACE_EVENT_CLONE; else trace = PTRACE_EVENT_FORK; Without CLONE_VFORK (which we can't use), if the exit signal isn't SIGCHLD, the debugger will get a PTRACE_EVENT_CLONE, which makes it think the process we're starting is a thread, not a new process. Both gdb and lldb remain attached to the child and when it later performs an execve(), they get mightily confused. See gdb bug report[5]. The idea of not having an exit_signal was so that no SIGCHLD would be delivered to the parent process in the first place. That way, some misguided SIGCHLD handler (*cough* GLib *cough*) wouldn't reap our processes. Unfortunately, what I didn't realize was that the kernel sends SIGCHLD anyway (see [2][4]), so this defensive measure didn't actually work. Consequently, we can pass SIGCHLD to clone() and get the debuggers working again. [ChangeLog][Linux] Fixed an issue that would cause debugging a Qt application that uses QProcess to confuse both gdb and lldb if the Linux kernel was version 5.4 or higher. Behavior outside of a debugging session was not affected. [1] https://code.woboq.org/linux/linux/kernel/fork.c.html#_do_fork [2] https://code.woboq.org/linux/linux/kernel/signal.c.html#do_notify_parent [3] https://elixir.bootlin.com/linux/v5.8/source/kernel/fork.c#L2432 [4] https://elixir.bootlin.com/linux/v5.8/source/kernel/signal.c#L1925 [5] https://sourceware.org/bugzilla/show_bug.cgi?id=26562 Fixes: QTBUG-86319 Change-Id: I2fc68c725ba649218bd9fffd1633863613537d42 Reviewed-by: Dimitrios Apostolou Reviewed-by: Oswald Buddenhagen Reviewed-by: hjk (cherry picked from commit d1fc991c6e3c45a5f93c925d49e3fe77ce6ce455) Reviewed-by: Qt Cherry-pick Bot --- src/3rdparty/forkfd/forkfd_linux.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/src/3rdparty/forkfd/forkfd_linux.c b/src/3rdparty/forkfd/forkfd_linux.c index 523fbca3ccc..ffe0e9a5e24 100644 --- a/src/3rdparty/forkfd/forkfd_linux.c +++ b/src/3rdparty/forkfd/forkfd_linux.c @@ -147,7 +147,7 @@ int system_forkfd(int flags, pid_t *ppid, int *system) } *system = 1; - unsigned long cloneflags = CLONE_PIDFD; + unsigned long cloneflags = CLONE_PIDFD | SIGCHLD; pid = sys_clone(cloneflags, &pidfd); if (ppid) *ppid = pid; @@ -171,7 +171,7 @@ int system_forkfd_wait(int ffd, struct forkfd_info *info, int ffdoptions, struct { siginfo_t si; int ret; - int options = __WALL | convertForkfdWaitFlagsToWaitFlags(ffdoptions); + int options = convertForkfdWaitFlagsToWaitFlags(ffdoptions); if ((options & WNOHANG) == 0) { /* check if the file descriptor is non-blocking */