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 kernel guarantees backwards compatibility, i.e. helper functions 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 it brings a huge problem to CRIU. The '''vDSO''' structure is highly coupled with the kernel internals so that if application is migrating to a new kernel release with brand new '''vDSO''' old one (which is still sitting in application memory) strictly speaking is no longer usable, because the new kernel implies new '''vDSO''' internal structure.

 

 

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

 

After that an restored application can continue using original '''vDSO''' helpers without problems.