[StackProtector] don't check stack protector before calling nounwind functions

https://reviews.llvm.org/rGd656ae28095726830f9beb8dbd4d69f5144ef821
introduced a additional checks before calling noreturn functions in
response to this security paper related to Catch Handler Oriented
Programming (CHOP):
https://download.vusec.net/papers/chop_ndss23.pdf
See also:
https://bugs.chromium.org/p/llvm/issues/detail?id=30

This causes stack canaries to be inserted in C code which was
unexpected; we noticed certain Linux kernel trees stopped booting after
this (in functions trying to initialize the stack canary itself).
https://github.com/ClangBuiltLinux/linux/issues/1815

There is no point checking the stack canary like this when exceptions
are disabled (-fno-exceptions or function is marked noexcept) or for C
code.  The GCC patch for this issue does something similar:
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=a25982ada523689c8745d7fb4b1b93c8f5dab2e7

Android measured a 2% regression in RSS as a result of d656ae280957 and
undid it globally:
https://android-review.googlesource.com/c/platform/build/soong/+/2524336

Reviewed By: xiangzhangllvm

Differential Revision: https://reviews.llvm.org/D147975

1 files changed, 9 insertions, 12 deletions

diff --git a/llvm/lib/CodeGen/StackProtector.cpp b/llvm/lib/CodeGen/StackProtector.cpp
index 05ac176..387b653 100644
--- a/llvm/lib/CodeGen/StackProtector.cpp
+++ b/llvm/lib/CodeGen/StackProtector.cpp
@@ -482,18 +482,15 @@ bool StackProtector::InsertStackProtectors() {
     if (&BB == FailBB)
       continue;
     Instruction *CheckLoc = dyn_cast<ReturnInst>(BB.getTerminator());
-    if (!CheckLoc && !DisableCheckNoReturn) {
-      for (auto &Inst : BB) {
-        auto *CB = dyn_cast<CallBase>(&Inst);
-        if (!CB)
-          continue;
-        if (!CB->doesNotReturn())
-          continue;
-        // Do stack check before non-return calls (e.g: __cxa_throw)
-        CheckLoc = CB;
-        break;
-      }
-    }
+    if (!CheckLoc && !DisableCheckNoReturn)
+      for (auto &Inst : BB)
+        if (auto *CB = dyn_cast<CallBase>(&Inst))
+          // Do stack check before noreturn calls that aren't nounwind (e.g:
+          // __cxa_throw).
+          if (CB->doesNotReturn() && !CB->doesNotThrow()) {
+            CheckLoc = CB;
+            break;
+          }
 
     if (!CheckLoc)
       continue;