AIX

Hi guys,

I use AIX version 5 on IBM Power 5+ machine. I am currently trying to experiment with sort of self-modifying code, like this:

Code:

ucontext_t ut;
getcontext(&ut);
int iar = ut.uc_mcontext.jmp_context.iar;
int pageSize = getpagesize();
int rest = iar % pageSize;
void *ptr = iar - rest;
mprotect(ptr,pageSize,PROT_READ | PROT_WRITE | PROT_EXEC);

However, mprotect fails with ENOMEM ("Not enough space"). The above code works perfectly on Linux machine.

I also set environment variables MPROTECT_TXT and XPG_SUS_ENV to ON, with no effect.

Is there a way to make it work, or is there any other way to make changes to program's own text segment?

Regards

Registers are os architecture specific?

Pointers are not integers. On some platforms integers can hold pointers safely but that's just coincidence; here, you're cramming 64-bit values into 32-bit variables and hence truncating them quite a lot. If you include stdint.h I think you can use the pointer-safe ptrdiff_t type instead of int

I've been trying to get over this problem for some time now, without success. I use type intptr_t as you advised:

Code:

uintptr_t ptr2 = iar;

Then I have a loop with mprotect:

Code:

mprotect(ptr2+i,pageSize,PROT_READ | PROT_WRITE | PROT_EXEC);

It failes with "Invalid argument" every time the address is not aligned to page size (as expected) but also fails with "Not enough memory" when the (ptr2-i) is properly aligned. Any more ideas?

That is still an unsigned int pointer type, implying you are assigning only 32 bits. IBM says some pages are not mprotect friendly this way: pSeries and AIX Information Center

You might need that env variable. I am familiar with self-modifying code from machine language, what do you want it to do?

Thank you for a quick reply.

I don't think that's the problem since the routine works well when performed on non-text segment (for example on page which contains area acquired via malloc).

I have these in my ~/.profile:

Code:

MPROTECT_TXT=ON
XPG_SUS_ENV=ON

Is this enough for these env vars to be 'seen'? I tried different values of those, no effect.

I am trying to write code to reload the program text from the original executable file in case an error occurs (e.g. a hardware error which scrambled some memory). Is there another way of doing it, instead of mprotect/memset ?

If anyone was wandering, switching to AIX 6.1 did the trick. Documentation for 6.1 states that mprotect for text segment will work with MPROTECT_TXT env variable set to ON and it does. Documentation for 5.3 does not say such thing apparently, I primarily thought that the call in both versions will have the same effect, but apparently it doesn't.

Anyway, thanks for help!