From that point on, that process will always use the patched code.KGraft uses an extra level of indirection called a “reality-check” to decide, at the entry of patched functions, which code to execute based on the process flag.This means that the old version of function setup ran, which did not set the variable global_ptr.

Vaughan-Nichols of ZDNet alerted us that Linux 4.0 will provide support for “no-reboot patching.” The gist: When a security patch or other critical OS update comes out, you can apply it .

While rebootless patching is convenient for everyone, it’s a game changer for some applications.

Programs typically have only a handful of update points, and they are naturally placed at the start of long-running loops, when invariants are established and/or events have been fully handled.

Second, the programmer must define which indicate how data from an old version of a type/class should be used to initialize the updated version.

Besides their flexibility, these systems do not impose any measurable overhead on normal execution.

This is because the whole-process approach allows the code to be fully optimized internally — such optimizations would be inhibited by added levels of indirection (like trampolines) and spatial non-locality common in per-function updating approaches. These are points at which the program polls to see whether a dynamic update is available, and if so will start applying it.

But Linux 4.0’s rebootless patching support limits flexibility for the sake of better performance, backward-compatibility, and ease of use by the kernel programmer, even though the latter is not quite satisfying, as we have discussed: ironically, a “rebootless patch” could result in a crash, defeating the point!

The research community has been looking at how to support highly-flexible DSU for many years now.

This problem would be particularly acute if the old and new version changed a data structure’s format. Given that the original data-structure has no space for new fields, the idea is to create a separate data-structure just for them and rewrite the original binary code to use the new structure when manipulating the new fields.

This approach, however, introduces a lot of complexity (and opportunities for bugs) as the code is maintained and patched further, going ahead.

In this post, co-authored with my Ph D student Luís Pina, I take a closer look at the challenge that DSU presents, showing that what Linux will support is still quite far from what we might hope for, but that ideas from the research community promise to get us closer to the ideal, both for operating systems and hopefully for many other applications as well. If in the updated program the entries of a hash table are extended with a timeout field, then a dynamic update needs to convert in-memory hashtable entries to now contain a timeout field; otherwise, when the updated code goes to access that field, it will behave unpredictably.