Merge SA_IMMUTABLE-fixes-for-v5.16-rc2

I completed the first batch of signal changes for v5.17 against
v5.16-rc1 before the SA_IMMUTABLE fixes where completed. Which leaves
me with two lines of development that I want on my signal development
branch both rooted at v5.16-rc1. Especially as I am hoping
to reach the point of being able to remove SA_IMMUTABLE.

Linus merged my SA_IMUTABLE fixes as:
7af959b5d5c8 ("Merge branch 'SA_IMMUTABLE-fixes-for-v5.16-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace")

To avoid rebasing the development changes that are currently complete I am
merging the work I sent upstream to Linus to make my life simpler.

The SA_IMMUTABLE changes as they are described in Linus's merge commit.

Pull exit-vs-signal handling fixes from Eric Biederman:
"This is a small set of changes where debuggers were no longer able to
intercept synchronous SIGTRAP and SIGSEGV, introduced by the exit
cleanups.

This is essentially the change you suggested with all of i's dotted
and the t's crossed so that ptrace can intercept all of the cases it
has been able to intercept the past, and all of the cases that made it
to exit without giving ptrace a chance still don't give ptrace a
chance"

* 'SA_IMMUTABLE-fixes-for-v5.16-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace:
signal: Replace force_fatal_sig with force_exit_sig when in doubt
signal: Don't always set SA_IMMUTABLE for forced signals

Signed-off-by: "Eric W. Biederman" <[e-mail]>

arch/m68k/kernel/traps.c

···
  */
 asmlinkage void fpsp040_die(void)
 {
-	force_fatal_sig(SIGSEGV);
+	force_exit_sig(SIGSEGV);
 }
 
 #ifdef CONFIG_M68KFPU_EMU

arch/powerpc/kernel/signal_32.c

···
 	 * We kill the task with a SIGSEGV in this situation.
 	 */
 	if (do_setcontext(new_ctx, regs, 0)) {
-		force_fatal_sig(SIGSEGV);
+		force_exit_sig(SIGSEGV);
 		return -EFAULT;
 	}
 

arch/powerpc/kernel/signal_64.c

···
 	 */
 
 	if (__get_user_sigset(&set, &new_ctx->uc_sigmask)) {
-		force_fatal_sig(SIGSEGV);
+		force_exit_sig(SIGSEGV);
 		return -EFAULT;
 	}
 	set_current_blocked(&set);
···
 		return -EFAULT;
 	if (__unsafe_restore_sigcontext(current, NULL, 0, &new_ctx->uc_mcontext)) {
 		user_read_access_end();
-		force_fatal_sig(SIGSEGV);
+		force_exit_sig(SIGSEGV);
 		return -EFAULT;
 	}
 	user_read_access_end();

arch/s390/kernel/traps.c

···
 {
 	if (user_mode(regs)) {
 		report_user_fault(regs, SIGSEGV, 0);
-		force_fatal_sig(SIGSEGV);
+		force_exit_sig(SIGSEGV);
 	} else
 		die(regs, "Unknown program exception");
 }

arch/sparc/kernel/signal_32.c

···
 		get_sigframe(ksig, regs, sigframe_size);
 
 	if (invalid_frame_pointer(sf, sigframe_size)) {
-		force_fatal_sig(SIGILL);
+		force_exit_sig(SIGILL);
 		return -EINVAL;
 	}
 
···
 	sf = (struct rt_signal_frame __user *)
 		get_sigframe(ksig, regs, sigframe_size);
 	if (invalid_frame_pointer(sf, sigframe_size)) {
-		force_fatal_sig(SIGILL);
+		force_exit_sig(SIGILL);
 		return -EINVAL;
 	}
 

arch/sparc/kernel/windows.c

···
 		if ((sp & 7) ||
 		    copy_to_user((char __user *) sp, &tp->reg_window[window],
 				 sizeof(struct reg_window32))) {
-			force_fatal_sig(SIGILL);
+			force_exit_sig(SIGILL);
 			return;
 		}
 	}

arch/x86/entry/vsyscall/vsyscall_64.c

···
 	if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
 		warn_bad_vsyscall(KERN_DEBUG, regs,
 				  "seccomp tried to change syscall nr or ip");
-		force_fatal_sig(SIGSYS);
+		force_exit_sig(SIGSYS);
 		return true;
 	}
 	regs->orig_ax = -1;

arch/x86/kernel/vm86_32.c

···
 	user_access_end();
 Efault:
 	pr_alert("could not access userspace vm86 info\n");
-	force_fatal_sig(SIGSEGV);
+	force_exit_sig(SIGSEGV);
 	goto exit_vm86;
 }
 

include/linux/sched/signal.h

···
 extern void __wake_up_parent(struct task_struct *p, struct task_struct *parent);
 extern void force_sig(int);
 extern void force_fatal_sig(int);
+extern void force_exit_sig(int);
 extern int send_sig(int, struct task_struct *, int);
 extern int zap_other_threads(struct task_struct *p);
 extern struct sigqueue *sigqueue_alloc(void);

kernel/entry/syscall_user_dispatch.c

···
 		 * the selector is loaded by userspace.
 		 */
 		if (unlikely(__get_user(state, sd->selector))) {
-			force_fatal_sig(SIGSEGV);
+			force_exit_sig(SIGSEGV);
 			return true;
 		}
 
···
 			return false;
 
 		if (state != SYSCALL_DISPATCH_FILTER_BLOCK) {
-			force_fatal_sig(SIGSYS);
+			force_exit_sig(SIGSYS);
 			return true;
 		}
 	}

kernel/signal.c

···
 	return ret;
 }
 
+enum sig_handler {
+	HANDLER_CURRENT, /* If reachable use the current handler */
+	HANDLER_SIG_DFL, /* Always use SIG_DFL handler semantics */
+	HANDLER_EXIT,	 /* Only visible as the process exit code */
+};
+
 /*
  * Force a signal that the process can't ignore: if necessary
  * we unblock the signal and change any SIG_IGN to SIG_DFL.
···
  * that is why we also clear SIGNAL_UNKILLABLE.
  */
 static int
-force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, bool sigdfl)
+force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t,
+	enum sig_handler handler)
 {
 	unsigned long int flags;
 	int ret, blocked, ignored;
···
 	action = &t->sighand->action[sig-1];
 	ignored = action->sa.sa_handler == SIG_IGN;
 	blocked = sigismember(&t->blocked, sig);
-	if (blocked || ignored || sigdfl) {
+	if (blocked || ignored || (handler != HANDLER_CURRENT)) {
 		action->sa.sa_handler = SIG_DFL;
-		action->sa.sa_flags |= SA_IMMUTABLE;
+		if (handler == HANDLER_EXIT)
+			action->sa.sa_flags |= SA_IMMUTABLE;
 		if (blocked) {
 			sigdelset(&t->blocked, sig);
 			recalc_sigpending_and_wake(t);
···
 
 int force_sig_info(struct kernel_siginfo *info)
 {
-	return force_sig_info_to_task(info, current, false);
+	return force_sig_info_to_task(info, current, HANDLER_CURRENT);
 }
 
 /*
···
 	info.si_code = SI_KERNEL;
 	info.si_pid = 0;
 	info.si_uid = 0;
-	force_sig_info_to_task(&info, current, true);
+	force_sig_info_to_task(&info, current, HANDLER_SIG_DFL);
+}
+
+void force_exit_sig(int sig)
+{
+	struct kernel_siginfo info;
+
+	clear_siginfo(&info);
+	info.si_signo = sig;
+	info.si_errno = 0;
+	info.si_code = SI_KERNEL;
+	info.si_pid = 0;
+	info.si_uid = 0;
+	force_sig_info_to_task(&info, current, HANDLER_EXIT);
 }
 
 /*
···
 	info.si_flags = flags;
 	info.si_isr = isr;
 #endif
-	return force_sig_info_to_task(&info, t, false);
+	return force_sig_info_to_task(&info, t, HANDLER_CURRENT);
 }
 
 int force_sig_fault(int sig, int code, void __user *addr
···
 	info.si_errno = reason;
 	info.si_arch = syscall_get_arch(current);
 	info.si_syscall = syscall;
-	return force_sig_info_to_task(&info, current, force_coredump);
+	return force_sig_info_to_task(&info, current,
+		force_coredump ? HANDLER_EXIT : HANDLER_CURRENT);
 }
 
 /* For the crazy architectures that include trap information in