ds_identify: if /sys is not available use dmidecode (#42)

On non-Linux systems, `/sys` won't be available. In these cases, we can query `dmidecode(8)` directly.  This PR implements a dmi_decode function to query the same fields ds-identify
would otherwise read from /sys.  This path is taken when /sys isn't present.  In addition to
adding dmidecode support, non-Linux systems also need to map in virtualization detection
as systemd-detect-virt is not present; on FreeBSD, use sysctl kern.vm_guest and provide a
mapping[1] between BSD values and those that match with systemd-detect-virt[2].

1. https://github.com/freebsd/freebsd/blob/master/sys/kern/subr_param.c#L149-L157
2. https://www.freedesktop.org/software/systemd/man/systemd-detect-virt.html

LP: #1852442

1 files changed, 54 insertions, 3 deletions

diff --git a/tools/ds-identify b/tools/ds-identify
index 20a99ee9..c93d4a77 100755
--- a/tools/ds-identify
+++ b/tools/ds-identify
@@ -179,13 +179,39 @@ debug() {
     echo "$@" 1>&3
 }
 
+dmi_decode() {
+    local sys_field="$1" dmi_field="" val=""
+    command -v dmidecode >/dev/null 2>&1 || {
+        warn "No dmidecode program. Cannot read $sys_field."
+        return 1
+    }
+    case "$1" in
+        sys_vendor) dmi_field="system-manufacturer";;
+        product_name) dmi_field="system-product-name";;
+        product_uuid) dmi_field="system-uuid";;
+        product_serial) dmi_field="system-serial-number";;
+        chassis_asset_tag) dmi_field="chassis-asset-tag";;
+        *) error "Unknown field $sys_field. Cannot call dmidecode."
+           return 1;;
+    esac
+    val=$(dmidecode --quiet "--string=$dmi_field" 2>/dev/null) || return 1
+    _RET="$val"
+}
+
 get_dmi_field() {
     local path="${PATH_SYS_CLASS_DMI_ID}/$1"
-    if [ ! -f "$path" ] || [ ! -r "$path" ]; then
-        _RET="$UNAVAILABLE"
+    _RET="$UNAVAILABLE"
+    if [ -d "${PATH_SYS_CLASS_DMI_ID}" ]; then
+        if [ -f "$path" ] && [ -r "$path" ]; then
+            read _RET < "${path}" || _RET="$ERROR"
+            return
+        fi
+        # if `/sys/class/dmi/id` exists, but not the object we're looking for,
+        # do *not* fallback to dmidecode!
         return
     fi
-    read _RET < "${path}" || _RET="$ERROR"
+    dmi_decode "$1" || _RET="$ERROR"
+    return
 }
 
 block_dev_with_label() {
@@ -267,6 +293,31 @@ detect_virt() {
         if [ $r -eq 0 ] || { [ $r -ne 0 ] && [ "$out" = "none" ]; }; then
             virt="$out"
         fi
+    elif [ "$DI_UNAME_KERNEL_NAME" = "FreeBSD" ]; then
+        # Map FreeBSD's vm_guest names to those systemd-detect-virt that
+        # don't match up. See
+        # https://github.com/freebsd/freebsd/blob/master/sys/kern/subr_param.c#L144-L160
+        # https://www.freedesktop.org/software/systemd/man/systemd-detect-virt.html
+        #
+        #  systemd    | kern.vm_guest
+        # ---------------------+---------------
+        #  none       | none
+        #  kvm        | kvm
+        #  vmware     | vmware
+        #  microsoft  | hv
+        #  oracle     | vbox
+        #  xen        | xen
+        #  parallels  | parallels
+        #  bhyve      | bhyve
+        #  vm-other   | generic
+        out=$(sysctl -qn kern.vm_guest 2>/dev/null) && {
+            case "$out" in
+                hv) virt="microsoft" ;;
+                vbox) virt="oracle" ;;
+                generic) "vm-other";;
+                *) virt="$out"
+            esac
+        }
     fi
     _RET="$virt"
 }