runtime: fix plan9 monotonic time, crypto randomness

Open /dev/bintime at process start on Plan 9,
marked close-on-exec, hold it open for the duration of the
process, and use it for obtaining time.

The change to using /dev/bintime also sets up for an upcoming
Plan 9 change to add monotonic time to that file. If the monotonic
field is available, then nanotime1 and time.now use that field.
Otherwise they fall back to using Unix nanoseconds as "monotonic",
as they always have.

Before this CL, monotonic time went backward any time
aux/timesync decided to adjust the system's time-of-day backward.

Also use /dev/random for randomness (once at startup).
Before this CL, there was no real randomness in the runtime
on Plan 9 (the crypto/rand package still had some). Now there will be.

Change-Id: I0c20ae79d3d96eff1a5f839a56cec5c4bc517e61
Reviewed-on: https://go-review.googlesource.com/c/go/+/656755
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Bypass: Russ Cox <rsc@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>

src/runtime/env_plan9.go

@@ -30,7 +30,7 @@ const (
 func goenvs() {
 	buf := make([]byte, envBufSize)
 	copy(buf, envDir)
-	dirfd := open(&buf[0], _OREAD, 0)
+	dirfd := open(&buf[0], _OREAD|_OCEXEC, 0)
 	if dirfd < 0 {
 		return
 	}
@@ -40,7 +40,7 @@ func goenvs() {
 		buf = buf[:len(envDir)]
 		copy(buf, envDir)
 		buf = append(buf, name...)
-		fd := open(&buf[0], _OREAD, 0)
+		fd := open(&buf[0], _OREAD|_OCEXEC, 0)
 		if fd < 0 {
 			return
 		}

src/runtime/os_plan9.go

@@ -18,6 +18,7 @@ type mOS struct {
 	ignoreHangup  bool
 }
 
+func dupfd(old, new int32) int32
 func closefd(fd int32) int32
 
 //go:noescape
@@ -226,7 +227,7 @@ var sysstat = []byte("/dev/sysstat\x00")
 
 func getproccount() int32 {
 	var buf [2048]byte
-	fd := open(&sysstat[0], _OREAD, 0)
+	fd := open(&sysstat[0], _OREAD|_OCEXEC, 0)
 	if fd < 0 {
 		return 1
 	}
@@ -255,7 +256,7 @@ var pagesize = []byte(" pagesize\n")
 func getPageSize() uintptr {
 	var buf [2048]byte
 	var pos int
-	fd := open(&devswap[0], _OREAD, 0)
+	fd := open(&devswap[0], _OREAD|_OCEXEC, 0)
 	if fd < 0 {
 		// There's not much we can do if /dev/swap doesn't
 		// exist. However, nothing in the memory manager uses
@@ -314,11 +315,36 @@ func getpid() uint64 {
 	return uint64(_atoi(c))
 }
 
+var (
+	bintimeFD int32 = -1
+
+	bintimeDev = []byte("/dev/bintime\x00")
+	randomDev  = []byte("/dev/random\x00")
+)
+
 func osinit() {
 	physPageSize = getPageSize()
 	initBloc()
 	ncpu = getproccount()
 	getg().m.procid = getpid()
+
+	fd := open(&bintimeDev[0], _OREAD|_OCEXEC, 0)
+	if fd < 0 {
+		fatal("cannot open /dev/bintime")
+	}
+	bintimeFD = fd
+
+	// Move fd high up, to avoid conflicts with smaller ones
+	// that programs might hard code, and to make exec's job easier.
+	// Plan 9 allocates chunks of DELTAFD=20 fds in a row,
+	// so 18 is near the top of what's possible.
+	if bintimeFD < 18 {
+		if dupfd(bintimeFD, 18) < 0 {
+			fatal("cannot dup /dev/bintime onto 18")
+		}
+		closefd(bintimeFD)
+		bintimeFD = 18
+	}
 }
 
 //go:nosplit
@@ -329,7 +355,13 @@ func crash() {
 
 //go:nosplit
 func readRandom(r []byte) int {
-	return 0
+	fd := open(&randomDev[0], _OREAD|_OCEXEC, 0)
+	if fd < 0 {
+		fatal("cannot open /dev/random")
+	}
+	n := int(read(fd, unsafe.Pointer(&r[0]), int32(len(r))))
+	closefd(fd)
+	return n
 }
 
 func initsig(preinit bool) {
@@ -362,17 +394,6 @@ func usleep_no_g(usec uint32) {
 	usleep(usec)
 }
 
-//go:nosplit
-func nanotime1() int64 {
-	var scratch int64
-	ns := nsec(&scratch)
-	// TODO(aram): remove hack after I fix _nsec in the pc64 kernel.
-	if ns == 0 {
-		return scratch
-	}
-	return ns
-}
-
 var goexits = []byte("go: exit ")
 var emptystatus = []byte("\x00")
 var exiting uint32
@@ -530,3 +551,59 @@ func preemptM(mp *m) {
 	//
 	// TODO: Use a note like we use signals on POSIX OSes
 }
+
+//go:nosplit
+func readtime(t *uint64, min, n int) int {
+	if bintimeFD < 0 {
+		fatal("/dev/bintime not opened")
+	}
+	const uint64size = 8
+	r := pread(bintimeFD, unsafe.Pointer(t), int32(n*uint64size), 0)
+	if int(r) < min*uint64size {
+		fatal("cannot read /dev/bintime")
+	}
+	return int(r) / uint64size
+}
+
+// timesplit returns u/1e9, u%1e9
+func timesplit(u uint64) (sec int64, nsec int32)
+
+func frombe(u uint64) uint64 {
+	b := (*[8]byte)(unsafe.Pointer(&u))
+	return uint64(b[7]) | uint64(b[6])<<8 | uint64(b[5])<<16 | uint64(b[4])<<24 |
+		uint64(b[3])<<32 | uint64(b[2])<<40 | uint64(b[1])<<48 | uint64(b[0])<<56
+}
+
+//go:nosplit
+func nanotime1() int64 {
+	var t [4]uint64
+	if readtime(&t[0], 1, 4) == 4 {
+		// long read indicates new kernel sending monotonic time
+		// (https://github.com/rsc/plan9/commit/baf076425).
+		return int64(frombe(t[3]))
+	}
+	// fall back to unix time
+	return int64(frombe(t[0]))
+}
+
+//go:nosplit
+func walltime() (sec int64, nsec int32) {
+	var t [1]uint64
+	readtime(&t[0], 1, 1)
+	return timesplit(frombe(t[0]))
+}
+
+// Do not remove or change the type signature.
+// See comment in timestub.go.
+//
+//go:linkname time_now time.now
+func time_now() (sec int64, nsec int32, mono int64) {
+	var t [4]uint64
+	if readtime(&t[0], 1, 4) == 4 {
+		mono = int64(frombe(t[3])) // new kernel, use monotonic time
+	} else {
+		mono = int64(frombe(t[0])) // old kernel, fall back to unix time
+	}
+	sec, nsec = timesplit(frombe(t[0]))
+	return
+}

src/runtime/sys_plan9_386.s

@@ -58,6 +58,12 @@ TEXT runtime·closefd(SB),NOSPLIT,$0
 	MOVL	AX, ret+4(FP)
 	RET
 
+TEXT runtime·dupfd(SB),NOSPLIT,$0
+	MOVL	$5, AX
+	INT	$64
+	MOVL	AX, ret+8(FP)
+	RET
+
 TEXT runtime·exits(SB),NOSPLIT,$0
 	MOVL    $8, AX
 	INT     $64
@@ -87,32 +93,15 @@ TEXT runtime·plan9_tsemacquire(SB),NOSPLIT,$0
 	MOVL	AX, ret+8(FP)
 	RET
 
-TEXT nsec<>(SB),NOSPLIT,$0
+// func timesplit(u uint64) (sec int64, nsec int32)
-	MOVL	$53, AX
+TEXT runtime·timesplit(SB),NOSPLIT,$0
-	INT	$64
+	MOVL	u_lo+0(FP), AX
-	RET
+	MOVL	u_hi+4(FP), DX
-
-TEXT runtime·nsec(SB),NOSPLIT,$8
-	LEAL	ret+4(FP), AX
-	MOVL	AX, 0(SP)
-	CALL	nsec<>(SB)
-	CMPL	AX, $0
-	JGE	3(PC)
-	MOVL	$-1, ret_lo+4(FP)
-	MOVL	$-1, ret_hi+8(FP)
-	RET
-
-// func walltime() (sec int64, nsec int32)
-TEXT runtime·walltime(SB),NOSPLIT,$8-12
-	CALL	runtime·nanotime1(SB)
-	MOVL	0(SP), AX
-	MOVL	4(SP), DX
-
 	MOVL	$1000000000, CX
 	DIVL	CX
-	MOVL	AX, sec_lo+0(FP)
+	MOVL	AX, sec_lo+8(FP)
-	MOVL	$0, sec_hi+4(FP)
+	MOVL	$0, sec_hi+12(FP)
-	MOVL	DX, nsec+8(FP)
+	MOVL	DX, nsec+16(FP)
 	RET
 
 TEXT runtime·notify(SB),NOSPLIT,$0

src/runtime/sys_plan9_amd64.s

@@ -53,6 +53,17 @@ TEXT runtime·closefd(SB),NOSPLIT,$0
 	MOVL	AX, ret+8(FP)
 	RET
 
+TEXT runtime·dupfd(SB),NOSPLIT,$0
+	MOVQ	$5, BP
+	// Kernel expects each int32 arg to be 64-bit-aligned.
+	// The return value slot is where the kernel
+	// expects to find the second argument, so copy it there.
+	MOVL	new+4(FP), AX
+	MOVL	AX, ret+8(FP)
+	SYSCALL
+	MOVL	AX, ret+8(FP)
+	RET
+
 TEXT runtime·exits(SB),NOSPLIT,$0
 	MOVQ	$8, BP
 	SYSCALL
@@ -82,17 +93,9 @@ TEXT runtime·plan9_tsemacquire(SB),NOSPLIT,$0
 	MOVL	AX, ret+16(FP)
 	RET
 
-TEXT runtime·nsec(SB),NOSPLIT,$0
+// func timesplit(u uint64) (sec int64, nsec int32)
-	MOVQ	$53, BP
+TEXT runtime·timesplit(SB),NOSPLIT,$0
-	SYSCALL
+	MOVQ	u+0(FP), AX
-	MOVQ	AX, ret+8(FP)
-	RET
-
-// func walltime() (sec int64, nsec int32)
-TEXT runtime·walltime(SB),NOSPLIT,$8-12
-	CALL	runtime·nanotime1(SB)
-	MOVQ	0(SP), AX
-
 	// generated code for
 	//	func f(x uint64) (uint64, uint64) { return x/1000000000, x%1000000000 }
 	// adapted to reduce duplication
@@ -102,10 +105,10 @@ TEXT runtime·walltime(SB),NOSPLIT,$8-12
 	ADDQ	CX, DX
 	RCRQ	$1, DX
 	SHRQ	$29, DX
-	MOVQ	DX, sec+0(FP)
+	MOVQ	DX, sec+8(FP)
 	IMULQ	$1000000000, DX
 	SUBQ	DX, CX
-	MOVL	CX, nsec+8(FP)
+	MOVL	CX, nsec+16(FP)
 	RET
 
 TEXT runtime·notify(SB),NOSPLIT,$0

src/runtime/sys_plan9_arm.s

@@ -93,6 +93,13 @@ TEXT runtime·closefd(SB),NOSPLIT,$0-8
 	MOVW	R0, ret+4(FP)
 	RET
 
+//func dupfd(old, new int32) int32
+TEXT runtime·dupfd(SB),NOSPLIT,$0-12
+	MOVW	$SYS_DUP, R0
+	SWI	$0
+	MOVW	R0, ret+8(FP)
+	RET
+
 //func exits(msg *byte)
 TEXT runtime·exits(SB),NOSPLIT,$0-4
 	MOVW    $SYS_EXITS, R0
@@ -127,26 +134,10 @@ TEXT runtime·plan9_tsemacquire(SB),NOSPLIT,$0-12
 	MOVW	R0, ret+8(FP)
 	RET
 
-//func nsec(*int64) int64
+// func timesplit(u uint64) (sec int64, nsec int32)
-TEXT runtime·nsec(SB),NOSPLIT|NOFRAME,$0-12
+TEXT runtime·timesplit(SB),NOSPLIT,$0
-	MOVW	$SYS_NSEC, R0
+	MOVW	u_lo+0(FP), R1	// R1:R2 = nsec
-	SWI	$0
+	MOVW	u_hi+4(FP), R2
-	MOVW	arg+0(FP), R1
-	MOVW	0(R1), R0
-	MOVW	R0, ret_lo+4(FP)
-	MOVW	4(R1), R0
-	MOVW	R0, ret_hi+8(FP)
-	RET
-
-// func walltime() (sec int64, nsec int32)
-TEXT runtime·walltime(SB),NOSPLIT,$12-12
-	// use nsec system call to get current time in nanoseconds
-	MOVW	$sysnsec_lo-8(SP), R0	// destination addr
-	MOVW	R0,res-12(SP)
-	MOVW	$SYS_NSEC, R0
-	SWI	$0
-	MOVW	sysnsec_lo-8(SP), R1	// R1:R2 = nsec
-	MOVW	sysnsec_hi-4(SP), R2
 
 	// multiply nanoseconds by reciprocal of 10**9 (scaled by 2**61)
 	// to get seconds (96 bit scaled result)
@@ -173,9 +164,9 @@ TEXT runtime·walltime(SB),NOSPLIT,$12-12
 	SUB.HS	R5,R1			//    remainder -= 10**9
 	ADD.HS	$1,R6			//    sec += 1
 
-	MOVW	R6,sec_lo+0(FP)
+	MOVW	R6,sec_lo+8(FP)
-	MOVW	R7,sec_hi+4(FP)
+	MOVW	R7,sec_hi+12(FP)
-	MOVW	R1,nsec+8(FP)
+	MOVW	R1,nsec+16(FP)
 	RET
 
 //func notify(fn unsafe.Pointer) int32

src/runtime/timestub.go

@@ -5,7 +5,7 @@
 // Declarations for operating systems implementing time.now
 // indirectly, in terms of walltime and nanotime assembly.
 
-//go:build !faketime && !windows && !(linux && amd64)
+//go:build !faketime && !windows && !(linux && amd64) && !plan9
 
 package runtime
 

src/runtime/timestub2.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !aix && !darwin && !freebsd && !openbsd && !solaris && !wasip1 && !windows && !(linux && amd64)
+//go:build !aix && !darwin && !freebsd && !openbsd && !solaris && !wasip1 && !windows && !(linux && amd64) && !plan9
 
 package runtime