Re: Qemu plugin vs. valgrind

[Marc Hacin]

Le 31/03/2020 à 02:28, Benjamin a écrit :
> Perhaps you're looking for something like this?
> https://wiki.qemu.org/Internships/ProjectIdeas/CacheModelling

I don't think so.
I made a few tries of a plugin, made from copy/pasting existing ones in 
QEMU's github.

I would like to monitor how data, in guest code, obtained by read() 
[user] or I/O registers [system], scatters in guest memory by side-effects.
One side-effect I'm thinking of is when a guest core register holding 
such data because of calculations on it, is saved on the stack because 
of a call. And then in another call an uninitialized local variable 
unlikely gets its value from this register saving.

Something like https://github.com/wmkhoo/taintgrind

After the tries I made with my QEMU plugin, and use of "-d in_asm,op 
-dfilter x+n", I think that these plugins cannot serve my need (but 
global stats).
I should have to dig into in_asm->op conversions, probably a too complex 
task. To add tracking code of a "tainted" attribute to every byte of 
guest memory and registers.

From -d, I guess that guest registers are emulated by standard host 
variables ?

With my understanding of the TCG based emulation, the supercalar / 
pipelines properties of the guest processor may be not emulated, but 
conforms to the host. Am I right ?
On my tests, mono-core, it seems that the load/store callback is called 
just after the call of the instruction-before callback which is the 
source of the load/store. I didn't see any intermix between memory and 
instruction callback, as it is the case on the real guest core.

It would be great for users (like me) that QEMU provide another plugin 
stage, at the "in_asm->op" process, as there are useful possible 
analysers features here. Same as the popular valgrind's ones (and 
valgrind is not cross-arch/OS as QEMU is).
A bigger dream would be plugins compatibility to inherit valgrind ones 
(but IRs are not the same I think).


> QEMU also supports a GDB stub, which might help with your last question.
> https://wiki.qemu.org/Features/gdbstub

Eventually, I have tested my plugin with a user mode guest statically 
and absolutely linked ELF (with no relocations).
Then I passed trigger information to my plugin via the argv feature.


Thanks for your answers.
--
Thierry Bernier