[build] Allow genfsimg to be used on third party UEFI binaries

Extract the PE header offset from the MZ header rather than assuming a
fixed offset as used in the binaries created by the iPXE build system.

This allows genfsimg to be used to create bootable filesystem images
from third party UEFI binaries such as the UEFI shell.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

diff --git a/src/util/genfsimg b/src/util/genfsimg
index a1fad247ee0d10ec58f17e8e60967735fc058e38..5d0a2bdea91def244cd8e9f03a49c2eeac11ed8e 100755 (executable)
--- a/src/util/genfsimg
+++ b/src/util/genfsimg
@@ -16,6 +16,18 @@ help() {
     echo " -p PAD   pad filesystem (in kB)"
 }
 
+# Get hex byte from binary file
+#
+get_byte() {
+    local FILENAME
+    local OFFSET
+
+    FILENAME="${1}"
+    OFFSET="${2}"
+
+    od -j "${OFFSET}" -N 1 -A n -t x1 -- "${FILENAME}" | tr -d " "
+}
+
 # Get hex word from binary file
 #
 get_word() {
@@ -37,12 +49,18 @@ efi_boot_name() {
 
     FILENAME="${1}"
 
-    PESIG=$(get_word "${FILENAME}" 192)
+    MZSIG=$(get_word "${FILENAME}" 0)
+    if [ "${MZSIG}" != "4d5a" ] ; then
+       echo "${FILENAME}: invalid MZ header" >&2
+       exit 1
+    fi
+    PEOFF=$(get_byte "${FILENAME}" 0x3c)
+    PESIG=$(get_word "${FILENAME}" "0x${PEOFF}")
     if [ "${PESIG}" != "5045" ] ; then
-       echo "${FILENAME}: not an EFI executable" >&2
+       echo "${FILENAME}: invalid PE header" >&2
        exit 1
     fi
-    ARCH=$(get_word "${FILENAME}" 196)
+    ARCH=$(get_word "${FILENAME}" $(( "0x${PEOFF}" + 4 )) )
     case "${ARCH}" in
        "4c01" )
            echo "BOOTIA32.EFI"