[PATCH] D59780: Support Intel Control-flow Enforcement Technology

Tue Mar 26 02:27:34 PDT 2019

MaskRay added a comment.

> Is --force-cet the best option name? H. J. Lu pointed out that gold has -z ibtplt and other options, but I couldn't find these options in the binutils' repository. Is the option really implemneted to gold?

The binutils-gdb repository hosts two linkers: `ld/ld-new` (GNU ld) and `gold/ld-new` (GNU gold). `-z ibtplt` and `-z ibt` are ld.bfd options, not gold's. I asked in https://reviews.llvm.org/D58102 if there is a proposal to add support to gold but get no response so far...

I've done some experiments and let me clarify things. @xiangzhangllvm Please correct me I missed something.

On the compiler side
--------------------

Support for Intel CET requires GCC 8 (https://gcc.gnu.org/gcc-8/changes.html).

`-mibt` and `-mshstk` enable the macros `__IBT__` `__SHSTK__`, respectively. `-mcet` implies both `-mibt` and `-mshstk` and defines `__CET__`. `-mshstk` enables several builtin functions. These options do no affect codegen.

`-fcf-protection=` is used to emit `.note.gnu.property` in object files. It requires corresponding `-mibt` or `-mshstk`.

  gcc a.c -mibt -c # `__IBT__` is defined but no .note.gnu.property
  gcc a.c -fcf-protection=branch -mibt -c # NT_GNU_PROPERTY_X86_FEATURE_IBT tag
  gcc a.c -fcf-protection=return -mshstk -c # NT_GNU_PROPERTY_X86_FEATURE_SHSTK tag
  gcc a.c -fcf-protection=full -mcet -c # -mcet is short for -mibt -mshstk, IBT + SHSTK

On the linker side
------------------

`ibt` is what concerns linkers. `shstk` has no influence other than the output note section. The feature is only implemented in ld.bfd, not in gold.

`-z ibtplt` generates the second PLT `.plt.sec` (not `.splt`). `-z ibt` implies `-z ibtplt`.
The option does not check the input NT_GNU_PROPERTY_X86_FEATURE_IBT tag.

`-z ibt` emits `NT_GNU_PROPERTY_X86_FEATURE_IBT` but plain `-z ibtplt` doesn't.

GCC 5~8 support Intel MPX and you shall see the `bnd` prefix in some jump instructions.
Note that GCC 9/Linux kernel 4.18 drop MPX. Fortunately it seems MPX doesn't change the size of PLT entries. Thus in lld we don't have to support MPX, but we are still flexible enough to add related support if it revives in the future.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D59780/new/

https://reviews.llvm.org/D59780