ctr: Identify the platform.variant for ARM platforms

In the commit "26329b2b8d7fd4e290b2b0f0163547f2d79bb817",
dmcgowan/fix-pull-multi-arch, PR#1535. The containerd has enabled the pull
multi-arch image support. But the platform.variant field of OCI for ARM
hadn't been ready at that time, so all ARM images could not pull successfully.

"
docker.io/library/hello-world:latest:
resolved       |++++++++++++++++++++++++++++++++++++++|
index-sha256:3644c0788e3d3823f9e97f757f01d2ddc6eb5458df9d801:
done           |++++++++++++++++++++++++++++++++++++++|
elapsed: 5.1 s
total:  2.7 Ki (533.0 B/s)
unpacking sha256:3d3823f9e97f757f01d2ddc6eb5458df9d801...
ctr: : manifest not found: not found
"

In this patch we'll detect the ARM variants from /proc/cpuinfo. Because Linux
kernel has already detected the ABI, ISA and Features for us. We don't need to
parse them from registers again.

Change-Id: I479b34bf3f52df9f7a6b3c77718b7d316dbf7f69
Signed-off-by: Wei Chen <Wei.Chen@arm.com>
Signed-off-by: Penny Zheng <Penny.Zheng@arm.com>

platforms/cpuinfo.go

@@ -0,0 +1,85 @@
+package platforms
+
+import (
+	"bufio"
+	"os"
+	"runtime"
+	"strings"
+
+	"github.com/containerd/containerd/errdefs"
+	"github.com/containerd/containerd/log"
+	"github.com/pkg/errors"
+)
+
+// Present the ARM instruction set architecture, eg: v7, v8
+var cpuVariant string
+
+func init() {
+	if isArmArch(runtime.GOARCH) {
+		cpuVariant = getCPUVariant()
+	} else {
+		cpuVariant = ""
+	}
+}
+
+// For Linux, the kernel has already detected the ABI, ISA and Features.
+// So we don't need to access the ARM registers to detect platform information
+// by ourselves. We can just parse these information from /proc/cpuinfo
+func getCPUInfo(pattern string) (info string, err error) {
+	if !isLinuxOS(runtime.GOOS) {
+		return "", errors.Wrapf(errdefs.ErrNotImplemented, "getCPUInfo for OS %s", runtime.GOOS)
+	}
+
+	cpuinfo, err := os.Open("/proc/cpuinfo")
+	if err != nil {
+		return "", err
+	}
+	defer cpuinfo.Close()
+
+	// Start to Parse the Cpuinfo line by line. For SMP SoC, we parse
+	// the first core is enough.
+	scanner := bufio.NewScanner(cpuinfo)
+	for scanner.Scan() {
+		newline := scanner.Text()
+		list := strings.Split(newline, ":")
+
+		if len(list) > 1 && strings.EqualFold(strings.TrimSpace(list[0]), pattern) {
+			return strings.TrimSpace(list[1]), nil
+		}
+	}
+
+	// Check whether the scanner encountered errors
+	err = scanner.Err()
+	if err != nil {
+		return "", err
+	}
+
+	return "", errors.Wrapf(errdefs.ErrNotFound, "getCPUInfo for pattern: %s", pattern)
+}
+
+func getCPUVariant() string {
+	variant, err := getCPUInfo("Cpu architecture")
+	if err != nil {
+		log.L.WithError(err).Error("failure getting variant")
+		return ""
+	}
+
+	switch variant {
+	case "8":
+		variant = "v8"
+	case "7", "7M", "?(12)", "?(13)", "?(14)", "?(15)", "?(16)", "?(17)":
+		variant = "v7"
+	case "6", "6TEJ":
+		variant = "v6"
+	case "5", "5T", "5TE", "5TEJ":
+		variant = "v5"
+	case "4", "4T":
+		variant = "v4"
+	case "3":
+		variant = "v3"
+	default:
+		variant = "unknown"
+	}
+
+	return variant
+}