Re: self-modifying code in 2.6 kernel for ppc writes into readonly section

In message [ref] you wr
ote:

On Tue, Aug 19, 2008 at 12:25 AM, Becky Bruce [off-list ref] wro

te:

quoted

On Aug 18, 2008, at 3:51 PM, Michael Neuling wrote:

quoted

In message [ref]
you wrote:

quoted

The mmu is still disabled at this point.

What is marked as readonly is the text section of the vmlinux file
generated when compiling the kernel. And since the code tries to write
to the text section, I assumed it was the reason for the segmentation
fault.

Seriously, a seg fault in your emulator is a bug in the emulator!

Mikey is likely right here. I've (unfortunately) done a lot of emulator
work, and every time I've hit a problem like this, the problem has been wit

h

quoted

the emulator or the emulation environment.  Have you isolated the faulting
instruction, verified that it's to a reasonable address, and tried examinin

g

quoted

memory at the faulting address using your emulator's command interface?

yes, it's a store instruction. the value to be stored is a nop
instruction and the
address is inside the text section (it is writing over existing code that
is intended for other cpus).

quoted

quoted

quoted

I'm not sure how this is dealt with on real hardware.

The CPU seg faults... :-P

But only if the page is mapped non-writeable.  Even with the MMU on, Linux
maps itself in as writeable.  It's the OS, it can do whatever it wants.  So
it just works on real hardware, and it should just work in your emulator.

I forgot to mention that I'm trying to run directly the vmlinux image
in psim emulator.
I'm not sure it's even supposed to work this way.

Looking at the psim web page quickly, it seems to be for userspace
binaries.

So yeah, I don't think it's designed to be used like you are try to use
it.

quoted

quoted

quoted

Can somebody please explain how is it supposed to work ? Is it ok to
write to text section that you load on real hardware as readonly ?
(again, no mmu involved, as it is still turned off, so i'm not sure
who's guarding this section against writing)

I'm not sure how this works for 32 bit CPUs, so I can't speak to the
details of it.

For the 64bit MMU, if you're in real mode (MMU off), nothing can stop
this from being written.  The kernel ignores the elf sections
permissions and does it's own mapping but this can only be enforced once
the MMU is on.

The same is true on 32-bit ppc - the basic MMU architecture is very similar
if you have a part that has "real mode" (i.e. non-booke).  There is no way
to restrict stores in real mode.

-Becky

quoted

Mikey

quoted

On Mon, Aug 18, 2008 at 10:19 PM, Michael Neuling [off-list ref]
wrote:

quoted

In message [ref]
you

wrote:

quoted

quoted

quoted

Hello,

First, I'm talkin about the 2.6.11 version. I know arch/ppc is gone in
latest versions,
but i assume the code is still the same and just moved to powerpc.

There is a piece of code in the early initialization of the 2.6 kernel
that identifies the cpu type and then tries to eliminate code that
does not apply to the current cpu. This is done by writing nop's over
sections of code that are not needed (do_cpu_ftr_fixups in
arch/ppc/kernel/misc.S)

When I try to run the kernel in a ppc emulator, I get a segmentation
fault in do_cpu_ftr_fixups. From examining the section headers of the
vmlinux, the text section is marked as readonly. The piece of code
above mentioned is trying to write a nop to memory location inside the
text section which is readonly, so that explains the sigsegv  error.

Any segv in the emulator sounds like a bug in the emulator.

If the page really is marked read only, then writing to it should cause
a page fault.

quoted

Since the kernel does run on boards with ppc cpu's, can somebody
explain how come this is actually working ? Or if/where I am mistaking
with my assumptions ?

Thank you

P.S. please add me in cc in a reply to this message
_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@ozlabs.org
https://ozlabs.org/mailman/listinfo/linuxppc-dev

_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@ozlabs.org
https://ozlabs.org/mailman/listinfo/linuxppc-dev

_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@ozlabs.org
https://ozlabs.org/mailman/listinfo/linuxppc-dev