From: Theodore Ts'o > Sent: 07 October 2022 02:41 > > On Thu, Oct 06, 2022 at 01:20:35PM -0700, Kees Cook wrote: > > > > So the question, then, is "why are they trying to exec while actively > > spawning new threads?" That appears to be the core problem here, and as > > far as I can tell, the kernel has behaved this way for a very long time. > > I don't think the kernel should fix this, either, because it leads to a > > very weird state for userspace, where the thread spawner may suddenly > > die due to the exec happening in another thread. This really looks like > > something userspace needs to handle correctly (i.e. don't try to exec > > while actively spawning threads). > > One of the classic failure modes is when a threaded program calls a > library, and that library might try to do a fork/exec (or call > system(3) to run some command. e.g., such as running "lvm create ..." > or to spawn some kind of helper daemon. > > There are a number of stack overflow questions about this, and there > are some solutions to _some_ of the problems, such as using > pthread_atfork(), and knowing that you are about to call fork/exec, > and use some out of band mechanism to to make sure no threads get > spawned until the fork/exec is completed --- but if you don't know > that a library is going to do a fork/exec, well, life is tough. Or that a library thread is about to create a new thread. > One technique even advocated by a stack overflow article is "avoid > using threads whenver possible". :-/ Doesn't fork() only create the current thread in the new process? So by the time exec() runs there is a nice single threaded process with an fd table that isn't shared. For helpers there is always (a properly implemented) posix_spawn() :-) David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
