syscall: exec_linux.go: support platforms without SYS_DUP2

Updates #9974

This change is in preparation for merging the arm64 platform.

Arm64 does not support SYS_DUP2 at all, so define a new constant to be
the minimum dup(2) version supported. This constant defaults to SYS_DUP2
on all existing platforms.

Change-Id: If405878105082c7c880f8541c1491970124c9ce4
Reviewed-on: https://go-review.googlesource.com/7123Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

src/syscall/exec_linux.go

@@ -214,7 +214,7 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 	// Pass 1: look for fd[i] < i and move those up above len(fd)
 	// so that pass 2 won't stomp on an fd it needs later.
 	if pipe < nextfd {
-		_, _, err1 = RawSyscall(SYS_DUP2, uintptr(pipe), uintptr(nextfd), 0)
+		_, _, err1 = RawSyscall(_SYS_dup, uintptr(pipe), uintptr(nextfd), 0)
 		if err1 != 0 {
 			goto childerror
 		}
@@ -224,7 +224,7 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 	}
 	for i = 0; i < len(fd); i++ {
 		if fd[i] >= 0 && fd[i] < int(i) {
-			_, _, err1 = RawSyscall(SYS_DUP2, uintptr(fd[i]), uintptr(nextfd), 0)
+			_, _, err1 = RawSyscall(_SYS_dup, uintptr(fd[i]), uintptr(nextfd), 0)
 			if err1 != 0 {
 				goto childerror
 			}
@@ -254,7 +254,7 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 		}
 		// The new fd is created NOT close-on-exec,
 		// which is exactly what we want.
-		_, _, err1 = RawSyscall(SYS_DUP2, uintptr(fd[i]), uintptr(i), 0)
+		_, _, err1 = RawSyscall(_SYS_dup, uintptr(fd[i]), uintptr(i), 0)
 		if err1 != 0 {
 			goto childerror
 		}

src/syscall/syscall_linux_386.go

@@ -9,6 +9,8 @@ package syscall
 
 import "unsafe"
 
+const _SYS_dup = SYS_DUP2
+
 func Getpagesize() int { return 4096 }
 
 func TimespecToNsec(ts Timespec) int64 { return int64(ts.Sec)*1e9 + int64(ts.Nsec) }

src/syscall/syscall_linux_amd64.go

@@ -4,6 +4,8 @@
 
 package syscall
 
+const _SYS_dup = SYS_DUP2
+
 //sysnb	Dup2(oldfd int, newfd int) (err error)
 //sys	Fchown(fd int, uid int, gid int) (err error)
 //sys	Fstat(fd int, stat *Stat_t) (err error)

src/syscall/syscall_linux_arm.go

@@ -6,6 +6,8 @@ package syscall
 
 import "unsafe"
 
+const _SYS_dup = SYS_DUP2
+
 func Getpagesize() int { return 4096 }
 
 func TimespecToNsec(ts Timespec) int64 { return int64(ts.Sec)*1e9 + int64(ts.Nsec) }

src/syscall/syscall_linux_ppc64x.go

@@ -7,6 +7,8 @@
 
 package syscall
 
+const _SYS_dup = SYS_DUP2
+
 //sysnb	Dup2(oldfd int, newfd int) (err error)
 //sys	Fchown(fd int, uid int, gid int) (err error)
 //sys	Fstat(fd int, stat *Stat_t) (err error)