To quote JaibraB:
EXPLAIN RTOC COBRA TRICK
The JIG Cobra has several protective measures to ensure that your code could not be used correctly even if your code could be dumped.
This trick rtoc in the registry is the first used for this purpose in addition to hinder analysis.
Registration is initially rtoc stored in the battery to keep the rtoc of lv2 and power it back later:
# =============== S U B R O U T I N E
cobra_syscall_sm_shutdown_hook: # CODE XREF: syscall_379 j
.set arg_20, 0x20
.set arg_28, 0x28
.set arg_30, 0x30
.set arg_38, 0x38
.set arg_40, 0x40
std %r0, arg_20(%sp)
std %rtoc, arg_28(%sp)
At this point we have to explain that the OFFSET DELTA.
DELTA OFFSET is a method used in the x86 in its original moments in the creation of computer viruses,
to calculate the memory address in which we are in the sea of bytes in RAM.
In the original time a computer virus when I did not know where he was pulled into an executable,
depending on the executable it could be an initial site or another, for it was invented DELTA OFFSET.
DELTA OFFSET can be used in any system, the procedure is:
- Using the record that indicates the current execution address (or the next depending on the system)
- Reducing the size of the previous code we use the value obtained from the registry.
Knowing this, and taking for example the x86 processor where the EIP register can not be read directly invented the trick
make a call to a "subfunction" which is simply the following line to the call:
X86 call instruction saves the top of the stack the address of the next instruction to itself.
Thus using pop draw from the top of the stack this value, and stored in eax for example, and having the memory address
where we only subtract the above would be missing and we have the exact calculation.
The PowerPC can use this trick using the BL instruction is equivalent (LINK BRANCH), which jumps to a "subfunction" but before you save
LR in the record the following address to BL.
At this point we see the trick used for the creation of the rtoc of charges at this time.
If you look both r0 and rtoc are passed to 0:
li %r0, 0
li %rtoc, 0
Subsequently, given the value 0x11DE0 to rtoc:
oris %rtoc, %rtoc, 1
ori %rtoc, %rtoc, 0x1DE0
A r0 is given the value 0x920:
oris %r0, %r0, 0
ori %r0, %r0, 0x920
R0 is subtracted from the value of rtoc:
subf %r0, %r0, %rtoc
Unlike the PowerPC x86 LR register can be read directly with mflr instruction, we put in RTOC the value obtained by the delta offset:
To calculate the delta offset subtract final instructions executed before the delta offset, which were 4, or 16 bytes:
addi %rtoc, %rtoc, -0x10
Finally we add the value of r0 at the end of the delta offset RTOC, storing the result in the RTOC and this already takes RTOC suitable for this hook:
add %rtoc, %rtoc, %r0
It takes having the RTOC stored in the stack 3 arguments that the hook received:
std %r3, arg_30(%sp)
std %r4, arg_38(%sp)
std %r5, arg_40(%sp)
You call the function of the charges where the first argument will check for command 0x8202 (a special command to the usual):
After making the necessary steps as charged, the battery recovers the original RTOC, like the arguments the hook received, it executes the original instruction
that was overwritten in the syscall entry 379 (in this case) to have our hook, and call the original syscall lv2:
ld %rtoc, arg_28(%sp)
ld %r3, arg_30(%sp)
ld %r4, arg_38(%sp)
ld %r5, arg_40(%sp)
Upon returning to retrieve the original LR from the stack and returns to the prompt,
ld %r0, arg_20(%sp)
# End of function cobra_syscall_sm_shutdown_hook
With the release of this information, we are now closer then ever to unlocking the secrets of the COBRA USB, and all of the capabilities it hides. Hopefully this will help solve the problem, and allow someone (or some people) to open source a working version, or bring about possible CFW and MFW features for all of us to enjoy!
Source : JaicraB Blog: PS3, Cobra
Thanks to PSX-SCENE member gamba for the news, and master for the translation.