[FS#865] Upgrading Firmware is broken on Linksys EA9500

[LEDE Bugs]

The following task has a new comment added:

FS#865 - Upgrading Firmware is broken on Linksys EA9500
User who did this - monokoo (monokoo)

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I have the same issue when I test the BCM53XX for my router.

The ttl log shows below:
root at LEDE:/# Watchdog handover: fd=3
- watchdog -
killall: telnetd: no process killed
killall: dropbear: no process killed
Sending TERM to remaining processes ... logd rpcd netifd odhcpd crond hd-idle dbus-daemon smbd nmbd blockd dnsmasq device_online.s sleep sh sysupgrade ubus shellinaboxd shellinaboxd ubusd 
[  919.193846] device br-lan left promiscuous mode
Sending KILL to remaining processes ... dnsc 
Extracted 0x3fffe4 bytes into /tmp/sysupgrade-bcm53xx/kernel
Extracted 0x1e60000 bytes into /tmp/sysupgrade-bcm53xx/root
Unlocking firmware ...

Writing from /tmp/kernel.trx to firmware ...     
Switching to ramdisk...
[  926.699953] UBIFS (ubi0:1): background thread "ubifs_bgt0_1" stops
[  926.802945] UBIFS (ubi0:1): un-mount UBI device 0
Performing system upgrade...
removing ubiblock0_0
[  926.952099] block ubiblock0_0: released
ubimkvol: error!: bad volume size: "0"
cannot create rootfs volume
ubiupdatevol: error!: file size must be specified if input is stdin
tar: invalid tar magic
sysupgrade successful
umount: can't unmount /dev: Resource busy
umount: can't unmount /tmp: Resource busy
[  927.292174] reboot: Restarting system
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=865#comment2879