Add fallback kernel version scraper

The kernel version scraper included in mkinitcpio works only on
x86 kernel images.
This commit implements kernel version scraping on a generic
architecture if an uncompressed image is provided.

Signed-off-by: Davide Depau <davide@depau.eu>

functions

@@ -133,23 +133,51 @@ parseopts() {
     return 0
 }
 
-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 re='^[[:digit:]]+(\.[[:digit:]]+)+'
-
+kver_x86() {
     # 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.
-    local offset=$(hexdump -s 526 -n 2 -e '"%0d"' "$1")
+    local kver offset=$(hexdump -s 526 -n 2 -e '"%0d"' "$1")
     [[ $offset = +([0-9]) ]] || return 1
 
     read kver _ < \
         <(dd if="$1" bs=1 count=127 skip=$(( offset + 0x200 )) 2>/dev/null)
 
+    printf '%s' "$kver"
+}
+
+kver_generic() {
+    # For unknown architectures, we can try to grep the uncompressed
+    # image for the boot banner.
+    # This should work at least for ARM when run on /boot/Image. On
+    # other architectures it may be worth trying rather than bailing,
+    # and inform the user if none was found.
+
+    # Loosely grep for `linux_banner`:
+    # https://elixir.bootlin.com/linux/v5.7.2/source/init/version.c#L46
+    # Banner ends with a new line so we can safely grep `.*`
+    local kver=
+
+    read _ _ kver _ < <(grep -m1 -aoE 'Linux version .(\.[-[:alnum:]]+)+' "$1")
+
+    printf '%s' "$kver"
+}
+
+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 re='^[[:digit:]]+(\.[[:digit:]]+)+'
+
+    local arch=$(uname -m)
+    if [[ $arch == @(i?86|x86_64) ]]; then
+        kver=$(kver_x86 "$1")
+    else
+        kver=$(kver_generic "$1")
+    fi
+
     [[ $kver =~ $re ]] || return 1
 
     printf '%s' "$kver"

mkinitcpio

@@ -101,12 +101,6 @@ resolve_kernver() {
         return 0
     fi
 
-    arch=$(uname -m)
-    if [[ $arch != @(i?86|x86_64) ]]; then
-        error "kernel version extraction from image not supported for \`%s' architecture" "$arch"
-        return 1
-    fi
-
     if [[ ! -e $kernel ]]; then
         error "specified kernel image does not exist: \`%s'" "$kernel"
         return 1
@@ -116,6 +110,12 @@ resolve_kernver() {
 
     error "invalid kernel specified: \`%s'" "$1"
 
+    arch=$(uname -m)
+    if [[ $arch != @(i?86|x86_64) ]]; then
+        error "kernel version extraction from image not supported for \`%s' architecture" "$arch"
+        error "there's a chance the generic version extractor may work with a valid uncompressed kernel image"
+    fi
+
     return 1
 }