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Thursday September 06, 2007

How do you access structure members in assembly files... If you look at the assembly files in OpenSolaris kernel [fortunately there are not many of them], you will see instructions like the one below at quite a few places :

          movq    T_PROCP(%r13), %r14     /* %r14 = proc */  <-- from swtch.s

In this particular case, %r13 holds the thread pointer and it is accessing the proc pointer from it and storing it in %r14. How it is accessing the member is the focus of this post.
If this was a c-source file, we could have done something similar to:
          proc_t *p = t->t_procp;
and compiler will take care of the rest.
But if we need to code it in assembly, we have to explicitly pass the 'offset' of the member [here, t_procp] to the instrcution. But if we were to hard code the 'offset', it would be a nightmare to maintain it, whenever structures changed.

So how OpenSolaris addresses this is pretty neat.
There is a file called offsets.in, which has entries as shown below :

_kthread                      THREAD_SIZE
        t_pcb                   T_LABEL
        t_lock
        .....
        t_procp
        t_link

This file gets compiled into a header file called assym.h and it can be included in any assembly file that makes references to these members. This Makefile takes care of building the header file. Looking at the assym.h, after building it, the entry for t_procp looks like this :

#define T_PROCP 0xdc

This is exactly what we need to feed to the assembly instruction !!

If we need to access a structure member that is not already present in the offsets.in, all we need to do is, just add the 'appropriate structure and its member', compile and start accessing in the assembly code.

What if the structure I am interested in is not of global signifcance - i.e no other module other than mine may care about it - in which case I don't want to clutter the offsets.in but want my own offsets.in which does the same job.
An example for this can be found here offsets.in - the structure and offsets that are declared here are of no significance to sparc so just confine it to x86.

You can use make(1S)'s AS_INC_PATH env varaible in your Makefiles to point to the location of where the generated heder files are present - with this varaible, you don't end up including the header file path for your c-files [confine it to assembly files].
( Sep 06 2007, 09:46:06 PM PDT ) Permalink