Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes: > On Tue, Jun 15, 2021 at 12:32 PM Eric W. Biederman > <ebiederm@xxxxxxxxxxxx> wrote: >> >> I had to update ret_from_kernel_thread to pop that state to get Linus's >> change to boot. Apparently kernel_threads exiting needs to be handled. > > You are very right. > > That, btw, seems to be a horrible design mistake, but I think it's how > "kernel_execve()" works - both for the initial "init", but also for > user-mode helper processes. > > Both of those cases do "kernel_thread()" to create a new thread, and > then that new kernel thread does kernel_execve() to create the user > space image for that thread. And that act of "execve()" clears > PF_KTHREAD from the thread, and then the final return from the kernel > thread function returns to that new user space. > > Or something like that. It's been ages since I looked at that code, > and your patch initially confused the heck out of me because I went > "that can't _possibly_ be needed". > > But yes, I think your patch is right. > > And I think our horrible "kernel threads return to user space when > done" is absolutely horrifically nasty. Maybe of the clever sort, but > mostly of the historical horror sort. > > Or am I mis-rememberting how this ends up working? Did you look at > exactly what it was that returned from kernel threads? > > This might be worth commenting on somewhere. But your patch for alpha > looks correct to me. Did you have some test-case to verify ptrace() on > io worker threads? At this point I just booted an alpha image and on qemu-system-alpha. I do have gdb in my kernel image so I can test that. I haven't yet but I can and should. Sleeping on it I came up with a plan to add TF_SWITCH_STACK_SAVED to indicate if the registers have been saved. The DO_SWITCH_STACK and UNDO_SWITCH_STACK helpers (except in alpha_switch_to) can test that. The ptrace helpers can test that and turn an access of random kernel stack contents into something well behaved that does WARN_ON_ONCE because we should not get there. I suspect adding TF_SWITCH_STACK_SAVED should come first so it is easy to verify the problem behavior, before I fix it. My real goal is to find a pattern that architectures whose register saves are structured like alphas can emulate, to minimize problems in the future. Plus I would really like to get the last handful of architectures updated so we can remove CONFIG_HAVE_ARCH_TRACEHOOK. I think we can do that on alpha because we save all of the system call arguments in pt_regs and that is all the other non-ptrace code paths care about. AKA I am trying to move the old architectures forward so we can get rid of unnecessary complications in the core code. Eric
