[Xen-devel] Memory mapping

[Frederic Beck <frederic.beck@xxxxxxxx>]

Hello

I managed to do what i wanted with my module (finally gave up on the
syscall and did everything in the module init), and i foudn the offsets
i was looking for. I later found out that what i've done was already
available via XenAccess (tools/linux-offset-finder).

I have the following offsets:
linux_name = 0x225;
linux_tasks = 0xd0;
linux_mm = 0xe8;
linux_pid = 0x10c;
linux_pgd = 0x24;
linux_addr = 0x84;

After disabling fast system calls handling, i modified  do_guest_trap in
xen/arch/x86/traps.c. I get the user regs to have access to the CPU
registers EAX, EIP and ESP (info i wanna log via the tracing). I take
the ESP and apply the mask 0xFFFFF000 to get the base address of the
thread_info struct.

user_regs = guest_cpu_user_regs();
base_addr = (user_regs->esp & 0xFFFFF000);

Then the first 4 bytes after this base_addr are on a regular system a
pointer to a task_struct where the PId of the current task is stored,
which i wanna get. On a regular OS (i tested on Dom0 or DomU) I can
find easily the PID and print it out. I would like to do the same in
the hypervisor. 

I tried to map the same code without modification, but i begin reading
memory addresses that do not exist and the hypervisor crashes when i
create my DomU. So far, i'm not very surprised, as there must be some
virtual memory handling that i'm missing.

However, i do not understand well how i can do the mapping. how do i
map this kernel address in the hypervisor to read the right memory
address ?

I guess that this mapping will be different for HVM and PV domains ? Is
there a function that dynamically detects what kind of domain the guest
is ?

Moreover, i found out in XenAccess that the right formula to get the
PID would be 
memcpy(&pid, memory + offset + PID_OFFSET - TASKS_OFFSET,4);
I guess that memory is the base adress of the virtual stack. How can i
find that value ? in that case the base_addr i calculated earlier would
be the offset ?

Thanks for the help!

Regards
Fred

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