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forkfd/Linux: ask clone() to use the SIGCHLD as the termination signal

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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
Pick-to: 5.15
Change-Id: I2fc68c725ba649218bd9fffd1633863613537d42
Reviewed-by: Dimitrios Apostolou <jimis@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: hjk <hjk@qt.io>
This commit is contained in:
Thiago Macieira 2020-09-10 13:30:59 -07:00
parent 2ed99ff5ca
commit d1fc991c6e
1 changed files with 2 additions and 2 deletions
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4
src/3rdparty/forkfd/forkfd_linux.c vendored
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@ -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 (pid < 0)
return pid;
@ -173,7 +173,7 @@ 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 */