CFI: Add a salt attribute to Clang to differentiate identical function prototypes

[samitolvanen]

@lvwr reminded me about adding a way to Clang to differentiate between identical function prototypes:

As suggested by Andy Cooper, add a salt attribute that allows to differentiate hashes of functions with the same prototype.

A simple type attribute along the lines of __attribute__((cfi_salt("string"))) would do here, for example. The same attribute could be used for both Clang CFI and KCFI.

cc @kees

[kees]

Excellent, yes! I'd like to have automatic salting too -- i.e. the call graph analysis that splits buckets automatically. This is something @lvwr has worked on before. :)
https://www.blackhat.com/docs/asia-17/materials/asia-17-Moreira-Drop-The-Rop-Fine-Grained-Control-Flow-Integrity-For-The-Linux-Kernel.pdf

[bwendling]

See: KSPP#365

[bwendling]

Would it be okay to restrict this feature to C only (not C++)? The problem is with virtual functions. It feels like an ugly quagmire with no real solution.

[samitolvanen]

Would it be okay to restrict this feature to C only (not C++)?

I think restricting to C should be fine.