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Re: [Xen-devel] [PATCH v6 11/24] x86/xen_hello_world.xsplice: Test payload for patching 'xen_extra_version'.

Hi Konrad,

On 07/04/16 04:49, Konrad Rzeszutek Wilk wrote:
This change demonstrates how to generate an xSplice ELF payload.

The idea here is that we want to patch in the hypervisor
the 'xen_version_extra' function with an function that will
return 'Hello World'. The 'xl info | grep extraversion'
will reflect the new value after the patching.

To generate this ELF payload file we need:
  - C code of the new code (xen_hello_world_func.c).
  - C code generating the .xsplice.funcs structure
  - The address of the old code (xen_extra_version). We
    retrieve it by  using 'nm --defined' on xen-syms.
  - The size of the new and old code for which we use
    nm --defined -S on our code and xen-syms respectively.

There are two C files and one header files generated
during build. One could make this one C file if the
size of the newly patched function size was known in
advance (or an random value was choosen).

There is also a strict order of compiling:
  1) xen_hello_world_func.c
  2) config.h - extract the size of the new function,
     the old function and the old function address.
  3) xen_hello_world.c - which contains the .xsplice.funcs
  4) Link the object files in an xen_hello_world.xsplice file.

The use-case is simple:

$xen-xsplice load /usr/lib/debug/xen_hello_world.xsplice
$xen-xsplice list
  ID                                     | status
xen_hello_world                           APPLIED
$xl info | grep extra
xen_extra              : Hello World
$xen-xsplice revert xen_hello_world
Performing revert: completed
$xen-xsplice unload xen_hello_world
Performing unload: completed
$xl info | grep extra
xen_extra              : -unstable

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

For the ARM bits:

Acked-by: Julien Grall <julien.grall@xxxxxxx>


Julien Grall

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