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What's the advantage of IFUNCS, over putting a function in the library that selects the implementation, either via function pointers or a switch/if? In particular given that they seem to be quite fragile and exploitable too.

I don't have much experience in low-level optimization, but would a modern CPU not be able to predict the path taken by a brach that tests the CPU features.

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> either via function pointers or a switch/if?

> but would a modern CPU not be able to predict the path taken by a brach that tests the CPU features.

That's true, but the CPU has finite branch predictor state, and now you've wasted some of it. Indirect calls hurt too, especially in you need retpolines.

This is a great read: https://www.agner.org/optimize/microarchitecture.pdf

The Linux kernel has interfaces for doing the same thing, more explicitly than ifunc:

https://docs.kernel.org/next/staging/static-keys.html

https://lwn.net/Articles/815908/

The advantage is that it avoids an extra function pointer indirection when the function you are trying to swap out is already an exported library function. For purely internal functions like in this case the only advantage is that it allows this exploit.

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