mkinitcpio: avoid using 'file' to get kernel versions

The kernel defines an offset at a fixed location within the bzImage
which we can use to find the kernel version string. Since this is fairly
important, reimplement the algorithm (which file almost surely uses)
using low level tools to avoid the possibility that file may break or
change its output, critically wounding mkinitcpio.

Note that this change leaves 'file' in use for lsinitcpio compression
detection.

Signed-off-by: Dave Reisner <dreisner@archlinux.org>

1 files changed, 17 insertions, 2 deletions

diff --git a/mkinitcpio b/mkinitcpio
index 013bc46..752107b 100755
--- a/mkinitcpio
+++ b/mkinitcpio
@@ -82,7 +82,13 @@ cleanup() {
 }
 
 resolve_kernver() {
-    local kernel=$1
+    local kernel=$1 offset kver
+
+    # this is intentionally very loose. only ensure that we're
+    # dealing with some sort of string that starts with something
+    # resembling dotted decimal notation. remember that there's no
+    # requirement for CONFIG_LOCALVERSION to be set.
+    local kver_validator='^[[:digit:]]+(\.[[:digit:]]+)+'
 
     if [[ -z $kernel ]]; then
         uname -r
@@ -99,7 +105,16 @@ resolve_kernver() {
         return 1
     fi
 
-    if file -Lb "$kernel" | grep -oP '(?<=version )[^ ]+'; then
+    # scrape the version out of the kernel image. locate the offset
+    # to the version string by reading 2 bytes out of image at at
+    # address 0x20E. this leads us to a string of, at most, 128 bytes.
+    # read the first word from this string as the kernel version.
+    offset=$(hexdump -s 526 -n 2 -e '"%0d"' "$kernel")
+    read kver _ < \
+        <(dd if="$kernel" bs=1 count=127 skip=$(( offset + 0x200 )) 2>/dev/null)
+
+    if [[ $kver =~ $kver_validator ]]; then
+        printf '%s' "$kver"
         return 0
     fi