syscall: add a Go directive "go:nocheckptr" to forkAndExecInChild1

forkAndExecInChild1 func must not acquire any locks in child, because
they might have been locked at the time of the fork. This implies to
no rescheduling, no malloc calls, and no new stack segments.

So, doing a "checkptrAlignment" is bad here, because checkptr
functions added by the instrumentation could grow the stack, which
should not be done between fork and exec calls.

Hence using a Go directive "go:nocheckptr" to forkAndExecInChild1
func,so that the compiler should not do "checkptrAlignment" when
functions marked with "go:norace".

This race detection bug was caught in go 1.21 on s390x.
Running a "./race.bash" script from "go/src" directory failed and the
bug details are provided in issue link mentioned below.

Fixes #58785

Change-Id: I254091368b0789d886acdf26f8aa8d8f5a986b24
Reviewed-on: https://go-review.googlesource.com/c/go/+/481415
Auto-Submit: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

src/syscall/exec_linux.go

@@ -203,6 +203,7 @@ type cloneArgs struct {
 //
 //go:noinline
 //go:norace
+//go:nocheckptr
 func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid uintptr, err1 Errno, mapPipe [2]int, locked bool) {
 	// Defined in linux/prctl.h starting with Linux 4.3.
 	const (