kern: syscall_thread_enter() cannot fail

Attempting to handle the error gracefully can easily result in missing
SIGSYS, so this was made to always succeed in
39024a8914 ("syscalls: fix missing SIGSYS for several ENOSYS errors")
and returns the nosys entry on failure.

Drop the pretense of returning an error and clean up a few dead error
paths.

Reviewed by:	kib, markj
Differential Revision:	https://reviews.freebsd.org/D57848

sys/kern/kern_sig.c

@@ -2690,7 +2690,7 @@ ptrace_syscallreq(struct thread *td, struct proc *p,
 	struct sysent *se;
 	register_t rv_saved[2];
 	unsigned int sc;
-	int error, nerror;
+	int nerror;
 	bool audited, sy_thr_static;
 
 	sc = tsr->ts_sa.code;
@@ -2734,12 +2734,8 @@ ptrace_syscallreq(struct thread *td, struct proc *p,
 	audited = AUDIT_SYSCALL_ENTER(sc, td) != 0;
 
 	if (!sy_thr_static) {
-		error = syscall_thread_enter(td, &se);
+		syscall_thread_enter(td, &se);
 		sy_thr_static = (se->sy_thrcnt & SY_THR_STATIC) != 0;
-		if (error != 0) {
-			tsr->ts_ret.sr_error = error;
-			return;
-		}
 	}
 
 	rv_saved[0] = td->td_retval[0];

sys/kern/kern_syscalls.c

@@ -88,7 +88,7 @@ syscall_thread_drain(struct sysent *se)
 		pause("scdrn", hz/2);
 }
 
-int
+void
 syscall_thread_enter(struct thread *td, struct sysent **se)
 {
 	uint32_t cnt, oldcnt;
@@ -100,11 +100,10 @@ syscall_thread_enter(struct thread *td, struct sysent **se)
 		oldcnt = (*se)->sy_thrcnt;
 		if ((oldcnt & (SY_THR_DRAINING | SY_THR_ABSENT)) != 0) {
 			*se = &nosys_sysent;
-			return (0);
+			break;
 		}
 		cnt = oldcnt + SY_THR_INCR;
 	} while (atomic_cmpset_acq_32(&(*se)->sy_thrcnt, oldcnt, cnt) == 0);
-	return (0);
 }
 
 void

sys/kern/subr_syscall.c

@@ -146,12 +146,8 @@ syscallenter(struct thread *td)
 	if (__predict_false(AUDIT_SYSCALL_ENABLED() ||
 	    SYSTRACE_ENABLED() || !sy_thr_static)) {
 		if (!sy_thr_static) {
-			error = syscall_thread_enter(td, &se);
+			syscall_thread_enter(td, &se);
 			sy_thr_static = (se->sy_thrcnt & SY_THR_STATIC) != 0;
-			if (error != 0) {
-				td->td_errno = error;
-				goto retval;
-			}
 		}
 
 #ifdef KDTRACE_HOOKS

sys/sys/sysent.h

@@ -328,7 +328,7 @@ struct nosys_args;
 int	lkmnosys(struct thread *, struct nosys_args *);
 int	lkmressys(struct thread *, struct nosys_args *);
 
-int	syscall_thread_enter(struct thread *td, struct sysent **se);
+void	syscall_thread_enter(struct thread *td, struct sysent **se);
 void	syscall_thread_exit(struct thread *td, struct sysent *se);
 
 int shared_page_alloc(int size, int align);