LivePatch

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Design

The patchset at xSplice v1 design and implementation includes all of the patches to make this work under Xen 4.7.

The design itself is [1].

The git code for hypervisor/toolstack currently is at: 

git://xenbits.xen.org/people/konradwilk/xen.git #xsplice.v9.1.rfc

The generation of payload is at: 

http://xenbits.xen.org/gitweb/?p=people/konradwilk/xsplice-build-tools.git

IRC

We will have monthly IRC meetings to discuss code/issues/etc.

They are on the fourth Tuesday of every month. At 10:00AM EST on #xsplice channel on irc.freenode.net

Toolstack

Things that need to be done in there:

  • xen-xsplice better smarts. Right now you need to do 'upload','check' and 'apply'. Three operations which could be folded in one - since the system admin probably want to have all of those done at once. The 'all' option does that.
  • Instead of using 'xen-xsplice' we should use 'xl'. That above mentioned logic could be part of 'xl' while 'xen-xsplice' is an accessory function for expert users?

Build

To build hot-patches aka payloads aka binary fixes against the hypervisor, you need two things:

  • Xen hypervisor which supports xSplice hypercalls. See above URL for the patches.
  • Tool to build binary fixes against hypervisor. See above URL for the tool. Note that the #xsplice.v4 patchset also includes three examples of payloads for testing/development.

Hypervisor

Currently the implementation has many TODOs implemented.

However we need to expand the hypervisor to include support for:

  • build_id support Done!
  • Need 'dl_sym' functionality in hypervisor to parse ELF payload. Do dynamic linking. (Ross) Done!
  • Lookup and insertion in the symbol table. Compute the proper offset and virtual address based on symbol name (<do_domctl>+0xf) (Ross) Done!
  • Need to either remove duplicate names from the symbol table or have the hypervisor implement proper ELF parsing - integrate the elf sections and parse that during runtime. Done!
  • Insertion of exceptions in the exception table. Also support resorting it. The hot-patch may have new exceptions that need to be dealt with.Done!
  • Code to verify signature of the ELF payload.
  • Checking the build-id against the hypervisor.Done!
  • Code to patch and revert the hot-patch (for simplicity we could boot Xen with 'cpus=1' so we don't have to worry about proper code to trigger patching)Done!
    • Patching requires saving the initial state. That needs to be saved for revert.Done!
    • Patching needs allocation of memory, putting in the code at memory.Done!
    • Doing the verification - checking if any code that uses the to-be-patched-function is in use. Save what the virtual address for the functions is - and verify the stack when patching (this incidentally is what .xsplice_section structures can have).
    • Deal with proper page table types for the modification (ro for .data, x for .text, etc).
    • Code to revert.Done!
  • Code to activate patching mechanism. On VMEXIT routine right before checking the do_softirq. Ditto for PV paths. Call the top patching mechanism - which can IPI other CPUs (or wait until all CPUs have come to the same point).Done!
  • The code for doing the patching - call from __do_softirq - can also be used for runtime microcode loading.
  • Add functionality to execute ELF code.Done!
  • Patch with NOPs if .xsplice.funcs->new_size has 0.
  • Seperate pages for .text; .rodata; .bss and .data
  • A better mechanism to "mask" NMIs during patching. The existing mechanism looses NMI if they have been sent and we don't have a mechanism to replay them. Note that this is also fixes alternative section patching.

ELF payload file

The spec details how the structures look.

The external tool: 

http://xenbits.xen.org/gitweb/?p=people/konradwilk/xsplice-build-tools.git

can generate the payloads against the hypervisor.

Create regression hot-patches

There are three regression/test-cases:

  • xen_hello_world: changes xen_extra_version() to return "Hello World".
  • xen_bye_world: over-writes the xen_hello_world patch so it returns "Bye World."
  • xen_replace_world: Used to replace the two other payloads and make xen_extra_version() print "Hello Again World".

The idea is to do: 

 # cd /usr/lib/debug
 # ls
 xen-syms-4.7-unstable  xen_bye_world.xsplice  xen_hello_world.xsplice  
 xen_replace_world.xsplice
 # xen-xsplice load xen_hello_world.xsplice 
 Uploading xen_hello_world.xsplice (12336 bytes)
 Performing apply:. completed
 # xl info | grep extra
 xen_extra              : Hello World
 # xen-xsplice load xen_bye_world.xsplice 
 Uploading xen_bye_world.xsplice (9244 bytes)
 Performing apply:. completed
 # xl info | grep extra
 xen_extra              : Bye World!
 # xen-xsplice upload replace_bye_and_hello xen_replace_world.xsplice 
 Uploading xen_replace_world.xsplice (9328 bytes)
 # xen-xsplice list
  ID                                     | status
 ----------------------------------------+------------
 xen_hello_world                         | APPLIED
 xen_bye_world                           | APPLIED
 replace_bye_and_hello                   | CHECKED
 # xen-xsplice replace replace_bye_and_hello
 Performing replace:. completed
 # xl info | grep extra
 xen_extra              : Hello Again World!
 # xen-xsplice list
  ID                                     | status
 ----------------------------------------+------------
 xen_hello_world                         | CHECKED
 xen_bye_world                           | CHECKED
 replace_bye_and_hello                   | APPLIED

Build hot-patches

This explanation was lifted from this link and modified a bit:

This transcript should demonstrate how to use the tool. 


 $ mkdir ~/work
 $ cd ~/work
 $ git clone git://xenbits.xen.org/people/konradwilk/xen.git
 $ cd xen
 # git checkout xsplice.v4
 $ # Build a debug Xen and tools (including misc/xen-xsplice), install on a host  and reboot
 
 $ # Write a patch
 $ git diff > ~/work/test1.patch
 $ git reset --hard
 $ # Write another patch
 $ git diff > ~/work/test2.patch
 $ git reset --hard
 $ # Write another patch
 $ git diff > ~/work/test3.patch
 $ git reset --hard
 
 $ cd ~/work
 $ git clone git://xenbits.xen.org/people/konradwilk/xsplice-build-tools.git
 $ cd xsplice-build
 $ make
 $ ./xsplice-build -s ~/work/xen -p ~/work/test1.patch -o out1 --xen-debug --debug $ ./xsplice-build -s ~/work/xen -p ~/work/test2.patch -o out2 --xen-debug --debug $ ./xsplice-build -s ~/work/xen -p ~/work/test3.patch -o out3 --xen-debug --debug
 $ # copy out*/test*.xsplice onto the host
 
 
 On the host:
 # xen-xsplice upload test1 test1.xsplice
 # xen-xsplice upload test2 test2.xsplice
 # xen-xsplice upload test3 test3.xsplice
 
 # xen-xsplice apply test1
 # # Verify test1 is applied
 # xen-xsplice apply test2
 # # Verify test2 is also applied
 # xen-xsplice replace test3
 # # Verify test3 is applied and test1 and test2 are not
 # xen-xsplice revert test3
 # # Verify test3 is not applied
 
 # xen-xsplice unload test1
 # xen-xsplice unload test2
 # xen-xsplice unload test3
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