syscall: use rawSyscall6 to call ptrace in forkAndExecInChild

On darwin and openbsd, the autogenerated ptrace wrapper is
nosplit because it is called from forkAndExecInChild.

This makes it difficult to modify and improve the underlying
syscall mechanism, as ptrace is almost over the nosplit limit.

We better call ptrace directly using rawSyscall6 in
forkAndExecInChild so that we can lift the ptrace nosplit
restriction to.

Doing so also fixes a long-standing inconsistency:
forkAndExecInChild is documented to only allow rawSyscall, but
the ptrace wrapper is using non-raw syscalls.

Updates #64113

Change-Id: Ibbbb218511561c1a5cb5b6d288a691f9738b14a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/708575
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: qmuntal

src/syscall/exec_libc2.go

@@ -59,7 +59,6 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 		r1              uintptr
 		nextfd          int
 		i               int
-		err             error
 		pgrp            _C_int
 		cred            *Credential
 		ngroups, groups uintptr
@@ -99,8 +98,12 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 
 	// Enable tracing if requested.
 	if sys.Ptrace {
-		if err = ptrace(PTRACE_TRACEME, 0, 0, 0); err != nil {
-			err1 = err.(Errno)
+		if runtime.GOOS == "ios" {
+			err1 = ENOSYS
+			goto childerror
+		}
+		_, _, err1 = rawSyscall6(abi.FuncPCABI0(libc_ptrace_trampoline), PTRACE_TRACEME, 0, 0, 0, 0, 0)
+		if err1 != 0 {
 			goto childerror
 		}
 	}

src/syscall/mksyscall.pl

@@ -141,12 +141,6 @@ ($)
 	# without reading the header.
 	$text .= "// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT\n\n";
 
-	if ((($darwin || ($openbsd && $libc)) && $func =~ /^ptrace(Ptr)?$/)) {
-		# The ptrace function is called from forkAndExecInChild where stack
-		# growth is forbidden.
-		$text .= "//go:nosplit\n"
-	}
-
 	# Go function header.
 	my $out_decl = @out ? sprintf(" (%s)", join(', ', @out)) : "";
 	$text .= sprintf "func %s(%s)%s {\n", $func, join(', ', @in), $out_decl;