[OpenBIOS] Q: How to "squeeze" C functions in between two symbols?
andreas.faerber at web.de
Fri Oct 8 15:23:09 CEST 2010
Am 08.10.2010 um 14:54 schrieb Alexander Graf:
> On 08.10.2010, at 14:44, Andreas Färber wrote:
>> I've gotten AIX 6/7 to instantiate RTAS (patches upcoming) and
>> would like to trace what it's trying to do. I probably need to
>> implement the display-character token.
> As you're running in qemu, the gdbstub is very helpful at times.
Not too familiar with gdb, can I use Apple's host gdb for that? I
believe Blue once said something about gdb not working in *-elf
configuration and requiring *-linux instead?
>> The RTAS code in arch/ppc/qemu/start.S currently looks like this:
>> ...and I would like to branch to C code from there.
>> Is there a way to have code from, say, rtas.c go between the blr
>> and of_rtas_end symbol?
>> Or do I need to move the symbols to the ldscript and place the code
>> in a special section? If yes, how?
> Why do you want to have the code in between? You can just branch to
> the C code:
> b c_rtas_function
> The only thing you need to make sure is that you follow the ABI :).
> Input parameters go in r3-rsomething, output is in r3, stack pointer
> (r1) has to be valid.
> Also by only doing the b instead of blr you jump to the C function
> directly, so a return from there actually returns from the rtas
> If the rtas function follows a different ABI, better set up a stack
> frame and blr into the C function.
I assumed the latter is what I should do, following the CIF example.
>> Those symbols are being used for code size calculation and
>> relocation in arch/ppc/qemu/methods.c.
> Maybe I don't really understand the question though.
Thanks for promptly trying!
The code in methods.c basically does a memcpy() of
of_rtas_start..of_rtas_end to memory that AIX allocates for us. The
code is never called at its original location so needs to be PIC.
Thus, if the C functions are not copied along, any relative addressing
wouldn't work and absolute addressing would break as soon as the
functions get unmapped. You might think of it as a "separate"
executable that OpenBIOS loads and sets up.
Therefore I need a way to get one piece of memory containing the entry
point, data storage and all helper functions called.
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