Avoiding retpolines with static calls

Posted Mar 29, 2020 3:18 UTC (Sun) by ncm (guest, #165)
In reply to: Avoiding retpolines with static calls by nivedita76
Parent article: Avoiding retpolines with static calls

It seems like a virtual call would be closely akin to the static mechanism, as the vtable array of function pointers is in the object-code, "text" section.

But of course C++ code is hard to get into the kernel. Linus's deliberate choice to use C++ keywords in kernel headers is one barrier to overcome.

Avoiding retpolines with static calls

Posted Mar 29, 2020 4:06 UTC (Sun) by Cyberax (✭ supporter ✭, #52523) [Link]

Virtual calls in C++ defeat the whole purpose of the exercise. They still require an indirect jump and so can be exploited by Spectre.

Avoiding retpolines with static calls

Posted Mar 29, 2020 10:41 UTC (Sun) by ballombe (subscriber, #9523) [Link] (4 responses)

You are missing th historical perspective.
Stroupstrup deliberate choice to break C compatibility by adding new keyword to C++ is one barrier to overcome.

Avoiding retpolines with static calls

Posted Mar 29, 2020 12:28 UTC (Sun) by mpr22 (subscriber, #60784) [Link]

The historical perspective includes the fact that Linux is newer than C++.

Avoiding retpolines with static calls

Posted Apr 3, 2020 18:07 UTC (Fri) by adobriyan (subscriber, #30858) [Link]

In a perfect world (lets call it TexC) keywords would start with backslash...

\let foo: u32 = 0;

Avoiding retpolines with static calls

Posted Apr 4, 2020 17:18 UTC (Sat) by nix (subscriber, #2304) [Link] (1 responses)

I think that's overdoing it. Even C's done that, repeatedly. Do you blame the ANSI C committee for adding new keywords? (They added almost as many as C++ did.)

Avoiding retpolines with static calls

Posted Apr 6, 2020 15:03 UTC (Mon) by mathstuf (subscriber, #69389) [Link]

Aren't they all in the `_Upper` namespace (namepattern?) though? C reserved that long ago (and is why C++ can't use it either; C may add names there at any time).

Avoiding retpolines with static calls

Posted Mar 29, 2020 15:11 UTC (Sun) by nivedita76 (subscriber, #121790) [Link]

I'm not sure what you mean by that. The "static" mechanism discussed in the article is self-modifying code that patches the text to make a direct call. The vtable array is just a const array of function pointers, but is still accessed via an indirect call, which would have all the overhead of retpoline blocking branch prediction. The "closely akin" method would be if the compiler optimizes the virtual function call to a couple of test-and-direct-branch based on knowledge of what types could be involved.