[PATCH] D118355: Add -mmanual-endbr switch to allow manual selection of control-flow protection

[Xiang Zhang via Phabricator via cfe-commits]

xiangzhangllvm added inline comments.


================
Comment at: llvm/lib/Target/X86/X86IndirectBranchTracking.cpp:156-161
   if (needsPrologueENDBR(MF, M)) {
-    auto MBB = MF.begin();
-    Changed |= addENDBR(*MBB, MBB->begin());
+    if (!ManualENDBR || MF.getFunction().doesCfCheck()) {
+      auto MBB = MF.begin();
+      Changed |= addENDBR(*MBB, MBB->begin());
+    } else {
+      // When -mmanual-endbr is in effect, the compiler does not
----------------
I am a little puzzle here. Let me copy your patch description here:

```
>When -mmanual-endbr is set, llvm refrains from automatically adding
>ENDBR instructions to functions' prologues, which would have been
>automatically added by -fcf-protection=branch. Although this works
>correctly, missing ENDBR instructions where they are actually needed
>could lead to broken binaries, which would fail only in running time.
```
I think the purpose of "-mmanual-endbr" should be "Supplementary Way" for the cases which the CET can't correctly insert endbr in automatic way.
Here (in if (needsPrologueENDBR(MF, M)) ) the automatic way will insert the endbr. So I think the job for "-mmanual-endbr" should be done in parallel with the old condition. Some like:
```
if (ManualENDBR ){
  do something
}else { // automatic
  if (needsPrologueENDBR(MF, M)) {insert endbr}
 }
}
```


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D118355/new/

https://reviews.llvm.org/D118355