On Aug 18, 2008, at 3:51 PM, Michael Neuling wrote:
In message <[EMAIL PROTECTED]
> you wrote:
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 with the emulator or the emulation environment. Have
you isolated the faulting instruction, verified that it's to a
reasonable address, and tried examining memory at the faulting address
using your emulator's command interface?
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.
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
Mikey
On Mon, Aug 18, 2008 at 10:19 PM, Michael Neuling
<[EMAIL PROTECTED]> wrote:
In message <[EMAIL PROTECTED]
> you
wrote:
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.
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
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