Description:

When I use syslinux-install_update on a local image file attached to /dev/loop0 it fails with :

/usr/bin/syslinux-install_update: line 116: warning: command substitution: ignored null byte in input
sfdisk: toggle boot flags is supported for MBR only # added log
FAILED to Set the boot flag on /dev/loop0p1

My partitions are gpt and attached respectively to /dev/loop0p{1,2,3}

Additional info:
* syslinux (6.03-7)

Steps to reproduce:

I want to use syslinux-install_update to add a boot loader to a local image file.

I use the following script

# First I create an image file:
truncate -s 3G disk.img

# Create 3 gpt partitions for the system
cat <<EOF | sfdisk disk.img
label: gpt

start=2048, size=100M, bootable, name=boot
size=+500M, name=swap, type=0657FD6D-A4AB-43C4-84E5-0933C84B4F4F
name=root
EOF

losetup --partscan /dev/loop0 disk.img

The last command attaches my partitions to /dev/loop0p{1,2,3}.
After creating my Archlinux install on root I want to install syslinux as bootloader, so:

mount /dev/loop0p3 archlinux
mount /dev/loop0p1 archlinux/boot

syslinux-install_update -c archlinux -i -a -m

But it fails with the above errors.

I fix the problem by patching function "get_disk" with the following patch.

--- a/syslinux-install_update 2017-08-23 14:23:28.109304021 +0200
+++ b/syslinux-install_update 2017-08-23 14:24:57.451532689 +0200
@@ -60,7 +60,7 @@

case "$part" in
*[[:digit:]]p[[:digit:]]*)
- local disk="${part%%p${partnum}}" # get everything before p1
+ local disk="${part%%p[[:digit:]]}" # get everything before p1
;;
*)
local disk="${part%%[[:digit:]]*}"