Vdso

Overview

vDSO stands for 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

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.

Calls proxification

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:

1. Scan runtime environment for current vDSO and parse its structure (size, sections, symbols, etc)
2. On restore of dumped vDSO memory area we patch function entry points so that they are redirected to current vDSO (for x86 architecture is simply a jmp instruction)

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

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.

Unsolved proxification problems

1. If an application in very unlikely case is Checkpointed over first bytes of vdso entry and vdso is different on the destination migration node, those might be the bytes that are being patched during proxification. If it's on vdso just after those entry-bytes, the stale data from vvar is taken (might be just fine afterall).