Changes

=== Overview ===

=== Overview ===

'''vDSO''' stands for [http://man7.org/linux/man-pages/man7/vdso.7.html ''virtual dynamic shared object''] and basically is a shared library exported by kernel into every userspace program. It usually export the following helper functions: gettimeofday, time, getcpu, clock_gettime. Userspace applications don't call them directly but make use of '''libc''' instead.

'''vDSO''' stands for [http://man7.org/linux/man-pages/man7/vdso.7.html ''virtual dynamic shared object''] and basically is a shared library exported by kernel into every userspace program. It usually exports a few frequently used functions (such as gettimeofday, getcpu and etc) and userspace applications don't call them directly but make use of '''libc''' instead.

=== A problem ===

=== A problem ===

While kernel guarantees backwards compatibility, i.e. these helpers won't suddenly disappear, the internal structure of the '''vDSO''' itself may vary the way it wants, userspace programs won't notice such changes but this make a huge problem for CRIU. The '''vDSO''' is a part of userspace application and its structure highly coupled with kernel internals.

While the kernel guarantees backwards compatibility (i.e. helper functions won't suddenly disappear), the internal structure of the '''vDSO''' itself may vary. Userspace programs won't notice such changes but it brings a huge problem to CRIU. The '''vDSO''' structure is highly coupled with the kernel internals. If an application is being migrated to a new kernel release (with a brand new '''vDSO''') the old '''vDSO''' (which is stored in the application's memory) is no longer usable.

 

 

To solve this problem CRIU does that named call proxification, where all functions from the original '''vDSO''' code are redirected to a new one provided by the kernel where application is restored. This done in a several steps on restore:

 

# Scan runtime environment for current '''vDSO''' and parse its structure

 

 

In case if an application is dumped and restored on same kernel CRIU detects that '''vDSO''' structure has not be changed and doesn't use calls proxification.