unix: add Major, Minor and Mkdev functions on Linux

Add Major, Minor and Mkdev functions for converting devices numbers to
their major/minor components and vice versa.

The functions follow the behavior of glibc's corresponding macros. Also
add an explanatory comment about the device number format, so the magic
numbers make more sense.

Test the conversion macros with some well-known device numbers for
devices which should be present on any Linux system.

Fixes golang/go#8106

Change-Id: Id336317985d6ac85ee83bc54e5f23703257c9121
Reviewed-on: https://go-review.googlesource.com/50550
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: tklauser

unix/dev_linux.go

@@ -0,0 +1,42 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Functions to access/create device major and minor numbers matching the
+// encoding used by the Linux kernel and glibc.
+//
+// The information below is extracted and adapted from bits/sysmacros.h in the
+// glibc sources:
+//
+// dev_t in glibc is 64-bit, with 32-bit major and minor numbers. glibc's
+// default encoding is MMMM Mmmm mmmM MMmm, where M is a hex digit of the major
+// number and m is a hex digit of the minor number. This is backward compatible
+// with legacy systems where dev_t is 16 bits wide, encoded as MMmm. It is also
+// backward compatible with the Linux kernel, which for some architectures uses
+// 32-bit dev_t, encoded as mmmM MMmm.
+
+package unix
+
+// Major returns the major component of a Linux device number.
+func Major(dev uint64) uint32 {
+	major := uint32((dev & 0x00000000000fff00) >> 8)
+	major |= uint32((dev & 0xfffff00000000000) >> 32)
+	return major
+}
+
+// Minor returns the minor component of a Linux device number.
+func Minor(dev uint64) uint32 {
+	minor := uint32((dev & 0x00000000000000ff) >> 0)
+	minor |= uint32((dev & 0x00000ffffff00000) >> 12)
+	return minor
+}
+
+// Mkdev returns a Linux device number generated from the given major and minor
+// components.
+func Mkdev(major, minor uint32) uint64 {
+	dev := uint64((major & 0x00000fff) << 8)
+	dev |= uint64((major & 0xfffff000) << 32)
+	dev |= uint64((minor & 0x000000ff) << 0)
+	dev |= uint64((minor & 0xffffff00) << 12)
+	return dev
+}

unix/dev_linux_test.go

@@ -0,0 +1,51 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix_test
+
+import (
+	"fmt"
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestDevices(t *testing.T) {
+	testCases := []struct {
+		path  string
+		major uint32
+		minor uint32
+	}{
+		// well known major/minor numbers according to
+		// https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/admin-guide/devices.txt
+		{"/dev/null", 1, 3},
+		{"/dev/zero", 1, 5},
+		{"/dev/random", 1, 8},
+		{"/dev/full", 1, 7},
+		{"/dev/urandom", 1, 9},
+		{"/dev/tty", 5, 0},
+	}
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("%s %v:%v", tc.path, tc.major, tc.minor), func(t *testing.T) {
+			var stat unix.Stat_t
+			err := unix.Stat(tc.path, &stat)
+			if err != nil {
+				t.Errorf("failed to stat device: %v", err)
+				return
+			}
+
+			dev := uint64(stat.Rdev)
+			if unix.Major(dev) != tc.major {
+				t.Errorf("for %s Major(%#x) == %d, want %d", tc.path, dev, unix.Major(dev), tc.major)
+			}
+			if unix.Minor(dev) != tc.minor {
+				t.Errorf("for %s Minor(%#x) == %d, want %d", tc.path, dev, unix.Minor(dev), tc.minor)
+			}
+			if unix.Mkdev(tc.major, tc.minor) != dev {
+				t.Errorf("for %s Mkdev(%d, %d) == %#x, want %#x", tc.path, tc.major, tc.minor, unix.Mkdev(tc.major, tc.minor), dev)
+			}
+		})
+
+	}
+}