277
edits
Changes
Jump to navigation
Jump to search
mLine 1:
Line 1: − draft+ + + + + + + + + + + + + + + + + + + +
→A problem
=== 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 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:
# Scan runtime environment for current '''vDSO''' and parse its structure
# 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 <code>jmp</code> 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.
[[Category: Under the hood]]