runtime: common handling of _AT_RANDOM auxv

The Linux kernel provides 16 bytes of random data via the auxv vector
at startup. Currently we consume this separately on 386, amd64, arm,
and arm64. Now that we have a common auxv parser, handle _AT_RANDOM in
the common path.

Change-Id: Ib69549a1d37e2d07a351cf0f44007bcd24f0d20d
Reviewed-on: https://go-review.googlesource.com/22062Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/os_linux.go

@@ -177,7 +177,8 @@ var failallocatestack = []byte("runtime: failed to allocate stack for the new OS
 var failthreadcreate = []byte("runtime: failed to create new OS thread\n")
 
 const (
-	_AT_NULL = 0 // End of vector
+	_AT_NULL   = 0  // End of vector
+	_AT_RANDOM = 25 // introduced in 2.6.29
 )
 
 func sysargs(argc int32, argv **byte) {
@@ -195,6 +196,12 @@ func sysargs(argc int32, argv **byte) {
 	auxv := (*[1 << 28]uintptr)(add(unsafe.Pointer(argv), uintptr(n)*sys.PtrSize))
 	for i := 0; auxv[i] != _AT_NULL; i += 2 {
 		tag, val := auxv[i], auxv[i+1]
+		switch tag {
+		case _AT_RANDOM:
+			// The kernel provides a pointer to 16-bytes
+			// worth of random data.
+			startupRandomData = (*[16]byte)(unsafe.Pointer(val))[:]
+		}
 		archauxv(tag, val)
 	}
 }

src/runtime/os_linux_386.go

@@ -4,16 +4,9 @@
 
 package runtime
 
-import "unsafe"
-
 const (
-	_AT_RANDOM  = 25
 	_AT_SYSINFO = 32
 )
 
 func archauxv(tag, val uintptr) {
-	switch tag {
-	case _AT_RANDOM:
-		startupRandomData = (*[16]byte)(unsafe.Pointer(val))[:]
-	}
 }

src/runtime/os_linux_arm.go

@@ -9,7 +9,6 @@ import "unsafe"
 const (
 	_AT_PLATFORM = 15 //  introduced in at least 2.6.11
 	_AT_HWCAP    = 16 // introduced in at least 2.6.11
-	_AT_RANDOM   = 25 // introduced in 2.6.29
 
 	_HWCAP_VFP   = 1 << 6  // introduced in at least 2.6.11
 	_HWCAP_VFPv3 = 1 << 13 // introduced in 2.6.30
@@ -34,10 +33,10 @@ func checkgoarm() {
 
 func archauxv(tag, val uintptr) {
 	switch tag {
-	case _AT_RANDOM: // kernel provides a pointer to 16-bytes worth of random data
-		startupRandomData = (*[16]byte)(unsafe.Pointer(val))[:]
-		// the pointer provided may not be word aligned, so we must treat it
-		// as a byte array.
+	case _AT_RANDOM:
+		// sysargs filled in startupRandomData, but that
+		// pointer may not be word aligned, so we must treat
+		// it as a byte array.
 		randomNumber = uint32(startupRandomData[4]) | uint32(startupRandomData[5])<<8 |
 			uint32(startupRandomData[6])<<16 | uint32(startupRandomData[7])<<24
 

src/runtime/os_linux_arm64.go

@@ -4,20 +4,14 @@
 
 package runtime
 
-import "unsafe"
-
-const (
-	_AT_RANDOM = 25 // introduced in 2.6.29
-)
-
 var randomNumber uint32
 
 func archauxv(tag, val uintptr) {
 	switch tag {
-	case _AT_RANDOM: // kernel provides a pointer to 16-bytes worth of random data
-		startupRandomData = (*[16]byte)(unsafe.Pointer(val))[:]
-		// the pointer provided may not be word aligned, so we must treat it
-		// as a byte array.
+	case _AT_RANDOM:
+		// sysargs filled in startupRandomData, but that
+		// pointer may not be word aligned, so we must treat
+		// it as a byte array.
 		randomNumber = uint32(startupRandomData[4]) | uint32(startupRandomData[5])<<8 |
 			uint32(startupRandomData[6])<<16 | uint32(startupRandomData[7])<<24
 	}

src/runtime/vdso_linux_amd64.go

@@ -18,7 +18,6 @@ import "unsafe"
 // http://refspecs.linuxfoundation.org/LSB_3.2.0/LSB-Core-generic/LSB-Core-generic/symversion.html
 
 const (
-	_AT_RANDOM       = 25
 	_AT_SYSINFO_EHDR = 33
 
 	_PT_LOAD    = 1 /* Loadable program segment */
@@ -303,8 +302,5 @@ func archauxv(tag, val uintptr) {
 		info1 := (*vdso_info)(noescape(unsafe.Pointer(&info)))
 		vdso_init_from_sysinfo_ehdr(info1, (*elf64Ehdr)(unsafe.Pointer(val)))
 		vdso_parse_symbols(info1, vdso_find_version(info1, &linux26))
-
-	case _AT_RANDOM:
-		startupRandomData = (*[16]byte)(unsafe.Pointer(val))[:]
 	}
 }