The $2 chinese ST-Link clones that you can find on Aliexpress/eBay are based on a STM32F101 chip. They can be repurposed into a JTAG adapter using the DirtyJTAG firmware. However, due to the limited number of GPIO available to the user, only the bare minimum JTAG pins are available : TDI/TDO/TMS/TCK (no SRST or TRST).
Press firmly the USB connector on a flat surface while holding the outer aluminium casing. This will release the PCB from its casing.
In order to install a newer firmware, you will need a SWD programmer. I chose to use another $2 ST-Link programmer. Connect them together like this :
Do not plug your ST-Link (target) in a USB port yet. Install stlink on your computer, and download a compiled release of DirtyJTAG for ST-Link adapters (available here). Enter this command to program the STM32 :
st-flash write /path/to/dirtyjtag-stlink.bin 0x8000000
If for some reason the flashing process fails because the reported flash size is 0, there is a fix.
If you have pogopins around, you could make your own flashing jig by soldering 4 of those on 2 pieces of stripboard, and hold them with a clothespin:
Otherwise some male to female header cable works great, it fits the pad hole snugly and doesn't make a loose connection.
This is the new pinout for your DirtyJTAG dongle. You may want to print it and glue it to the case for practical reasons.
Install UrJTAG with DirtyJTAG support. Plug your DirtyJTAG dongle, and check with the lsusb command that a new device with 0x1209/0xC0CA as its USB VID/PID has appeared.
$ jtag
UrJTAG 0.10 #
Copyright (C) 2002, 2003 ETC s.r.o.
Copyright (C) 2007, 2008, 2009 Kolja Waschk and the respective authors
UrJTAG is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
There is absolutely no warranty for UrJTAG.
warning: UrJTAG may damage your hardware!
Type "quit" to exit, "help" for help.
jtag> cable dirtyjtag
jtag> detect