[PATCH] D41723: Introduce the "retpoline" x86 mitigation technique for variant #2 of the speculative execution vulnerabilities disclosed today, specifically identified by CVE-2017-5715, "Branch Target Injection", and is one of the two halves to Spectre..

[James Y Knight via Phabricator via llvm-commits]

jyknight added a comment.

Looks like this uses a different command-line argument than the preliminary patchset for GCC, which is rather unfortunate.
http://git.infradead.org/users/dwmw2/gcc-retpoline.git/shortlog/refs/heads/gcc-7_2_0-retpoline-20171219
It'd be awesome if both compilers would somehow end up using the same command-line spelling. Maybe GCC would like to switch to "-mretpoline"? Have we been talking to them?

However, I do suspect the upstream kernel folks will need the equivalent of "-mindirect-branch=thunk-extern" mode (where the compiler doesn't actually emit the thunks, only the calls to them). This lets them define their own thunks which have "alternatives" annotations in them to allow them to be runtime patched out. (e.g. as in https://lkml.org/lkml/2018/1/4/419). That should be easy to support -- just don't do the thunk-emission -- but it does then mean the need to standardize on the names and semantics of the required thunks. And it would be good if the same function-names were used as GCC. Fine to do as a followup patch, but maybe at least have an idea what the command-line UI will be for it.


https://reviews.llvm.org/D41723