Get kernel version from generic EFI zboot image

This feature was added to the kernel since 6.1

https://elixir.bootlin.com/linux/v6.1/source/drivers/firmware/efi/libstub/Makefile.zboot
https://elixir.bootlin.com/linux/v6.1/source/drivers/firmware/efi/libstub/zboot-header.S

See discussion [Here](https://lists.gnu.org/archive/html/grub-devel/2022-12/msg00099.html)

1 files changed, 73 insertions, 0 deletions

diff --git a/functions b/functions
index a8c9103..ab929f2 100644
--- a/functions
+++ b/functions
@@ -210,9 +210,71 @@ detect_compression() {
         return
     fi
 
+    read -rd '' bytes < <(od -An -j0x04 -t c -N4 "$1" | tr -dc '[:alnum:]')
+    if [[ "$bytes" == 'zimg' ]]; then
+        echo 'zimg'
+        return
+    fi
+
     # out of ideas, assuming uncompressed
 }
 
+kver_zimage() {
+    # Generic EFI zboot added since kernel 6.1
+    # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/firmware/efi/libstub/Makefile.zboot?h=v6.1
+    # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/firmware/efi/libstub/zboot-header.S?h=v6.1
+
+    local kver='' reader start size comp_type
+
+    # Reading 4 bytes from address 0x08 is the starting offset of compressed data
+    start="$(od -An -j0x08 -t u4 -N4 "$1" | tr -dc '[:alnum:]')"
+
+    # Reading 4 bytes from address 0x0c is the size of compressed data,
+    # but it needs to be corrected according to the compressed type.
+    size="$(od -An -j0x0c -t u4 -N4 "$1" | tr -dc '[:alnum:]')"
+
+    # Read 36 bytes (before 0x3c) from address 0x18,
+    # which is a nul-terminated string representing the compressed type.
+    read -rd '' comp_type < <(od -An -j0x18 -t a -N32 "$1" | sed 's/ nul//g' | tr -dc '[:alnum:]')
+
+    [[ "$start" =~ ^[0-9]+$ ]] || return 1
+    [[ "$size" =~ ^[0-9]+$ ]] || return 1
+
+    case "$comp_type" in
+        'gzip')
+            reader='zcat'
+            ;;
+        'lz4')
+            reader='lz4cat'
+            size="$((size + 4))"
+            ;;
+        'lzma')
+            reader='xzcat'
+            size="$((size + 4))"
+            ;;
+        'lzo')
+            reader="lzop -d"
+            size="$((size + 4))"
+            ;;
+        'xzkern')
+            reader='xzcat'
+            size="$((size + 4))"
+            ;;
+        'zstd22')
+            reader='zstdcat'
+            size="$((size + 4))"
+            ;;
+        *)
+            reader="$comp_type"
+            size="$((size + 4))"
+            ;;
+    esac
+
+    read -r _ _ kver _ < <(dd if="$1" bs=1 count="$size" skip="$start" 2>/dev/null | $reader - | grep -m1 -aoE  'Linux version .(\.[-[:alnum:]+]+)+')
+
+    printf '%s' "$kver"
+}
+
 kver_generic() {
     # For unknown architectures, we can try to grep the uncompressed or gzipped
     # image for the boot banner.
@@ -223,6 +285,17 @@ kver_generic() {
     # Loosely grep for `linux_banner`:
     # https://elixir.bootlin.com/linux/v5.7.2/source/init/version.c#L46
     local kver='' reader='cat'
+    local comp_type=''
+
+    comp_type="$(detect_compression "$1")"
+
+    if [[ "$comp_type" == 'zimg' ]]; then
+        # Generic EFI zboot image
+        kver_zimage "$1"
+        return 0
+    elif [[ "$comp_type" == 'gzip' ]]; then
+        reader='zcat'
+    fi
 
     [[ "$(detect_compression "$1")" == 'gzip' ]] && reader='zcat'