[BOLT][BTI] Disassemble PLT entries when processing BTI binaries

PLT entries are PseudoFunctions, and are not disassembled or emitted. For BTI, we need to check the first MCInst of PLT entries, to see if indirectly calling them is safe or not. This patch disassembles PLTs for binaries using BTI, while not changing the behaviour for binaries without BTI. The PLTs are only disassembled, not emitted.

Author: bgergely0

bolt/lib/Core/BinaryFunction.cpp

@@ -147,6 +147,11 @@ static cl::opt<bool> TrapOnAVX512(
  cl::init(false), cl::ZeroOrMore, cl::Hidden, cl::cat(BoltCategory));
 
 bool shouldPrint(const BinaryFunction &Function) {
+ // PLT stubs are disassembled for BTI binaries, therefore they should be
+ // printed.
+ if (Function.getBinaryContext().usesBTI() && Function.isPLTFunction())
+ return true;
+
  if (Function.isIgnored())
  return false;
 

bolt/lib/Rewrite/RewriteInstance.cpp

@@ -461,6 +461,12 @@ Error RewriteInstance::setProfile(StringRef Filename) {
 
 /// Return true if the function \p BF should be disassembled.
 static bool shouldDisassemble(const BinaryFunction &BF) {
+
+ const BinaryContext &BC = BF.getBinaryContext();
+ // Disassemble PLT functions on AArch64 to check BTI landing pads.
+ if (BC.usesBTI() && BF.isPLTFunction())
+ return true;
+
  if (BF.isPseudo())
  return false;
 

bolt/test/runtime/AArch64/disassemble-plts.c

@@ -0,0 +1,31 @@
+// This test checks that BOLT disassembles PLT stubs in binaries using BTI,
+// while keeping them not disassembled in non-BTI binaries.
+
+// RUN: %clang -fuse-ld=lld --target=aarch64-unknown-linux-gnu %s -o %t.exe \
+// RUN: -Wl,-q
+// RUN: llvm-bolt %t.exe -o %t.bolt --print-disasm | FileCheck %s
+
+// RUN: %clang -fuse-ld=lld --target=aarch64-unknown-linux-gnu \
+// RUN: -mbranch-protection=standard %s -o %t.bti.exe -Wl,-q -Wl,-z,force-bti
+// RUN: llvm-bolt %t.bti.exe -o %t.bolt --print-disasm | FileCheck %s \
+// RUN: --check-prefix=CHECK-BTI
+
+// For the non-BTI binary, PLTs should not be disassembled.
+// CHECK-NOT: Binary Function "{{.*}}@PLT" after disassembly {
+
+// Check that PLTs are disassembled for the BTI binary.
+// CHECK-BTI: Binary Function "__libc_start_main@PLT" after disassembly {
+// CHECK-BTI: adrp
+// CHECK-BTI: ldr
+// CHECK-BTI: add
+// CHECK-BTI: br
+// CHECK-BTI: End of Function "__libc_start_main@PLT"
+
+#include <stdio.h>
+#include <stdlib.h>
+int main(int argc, char **argv) {
+ if (argc > 3)
+ exit(42);
+ else
+ printf("Number of args: %d\n", argc);
+}