sh: fix syscall tracing

Addition of SECCOMP_FILTER exposed a longstanding bug in
do_syscall_trace_enter, whereby r0 (the 5th argument register) was
mistakenly used where r3 (syscall_nr) was intended. By overwriting r0
rather than r3 with -1 when attempting to block a syscall, the
existing code would instead have caused the syscall to execute with an
argument clobbered.

Commit 0bb605c2 then introduced skipping of the syscall when
do_syscall_trace_enter returns -1, so that the return value set by
seccomp filters would not be clobbered by -ENOSYS. This eliminated the
clobbering of the 5th argument register, but instead caused syscalls
made with a 5th argument of -1 to be misinterpreted as a request by
do_syscall_trace_enter to suppress the syscall.

Fixes: 0bb605c2 ("sh: Add SECCOMP_FILTER")
Fixes: ab99c733 ("sh: Make syscall tracer use tracehook notifiers, add TIF_NOTIFY_RESUME.")
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Rich Felker <dalias@libc.org>

arch/sh/kernel/entry-common.S

@@ -370,7 +370,6 @@ syscall_trace_entry:
 	 nop
 	cmp/eq	#-1, r0
 	bt	syscall_exit
-	mov.l	r0, @(OFF_R0,r15)	! Save return value
 	!			Reload R0-R4 from kernel stack, where the
 	!   	    	    	parent may have modified them using
 	!   	    	    	ptrace(POKEUSR).  (Note that R0-R2 are

arch/sh/kernel/ptrace_32.c

@@ -455,16 +455,11 @@ long arch_ptrace(struct task_struct *child, long request,
 
 asmlinkage long do_syscall_trace_enter(struct pt_regs *regs)
 {
-	long ret = 0;
-
 	if (test_thread_flag(TIF_SYSCALL_TRACE) &&
-	    tracehook_report_syscall_entry(regs))
-		/*
-		 * Tracing decided this syscall should not happen.
-		 * We'll return a bogus call number to get an ENOSYS
-		 * error, but leave the original number in regs->regs[0].
-		 */
-		ret = -1L;
+	    tracehook_report_syscall_entry(regs)) {
+		regs->regs[0] = -ENOSYS;
+		return -1;
+	}
 
 	if (secure_computing() == -1)
 		return -1;
@@ -475,7 +470,7 @@ asmlinkage long do_syscall_trace_enter(struct pt_regs *regs)
 	audit_syscall_entry(regs->regs[3], regs->regs[4], regs->regs[5],
 			    regs->regs[6], regs->regs[7]);
 
-	return ret ?: regs->regs[0];
+	return 0;
 }
 
 asmlinkage void do_syscall_trace_leave(struct pt_regs *regs)