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[X86][KCFI] Do not emit a type prefix for nocf_check functions #158133

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[X86][KCFI] Do not emit a type prefix for nocf_check functions #158133

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[X86][KCFI] Do not emit a type prefix for nocf_check functions
samitolvanen Sep 11, 2025
55c9356
Also allow nocf_check only with -fsanitize=kcfi
samitolvanen Sep 15, 2025
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5 changes: 3 additions & 2 deletions llvm/lib/Target/X86/X86AsmPrinter.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -170,8 +170,9 @@ void X86AsmPrinter::emitKCFITypeId(const MachineFunction &MF) {
Type = mdconst::extract<ConstantInt>(MD->getOperand(0));

// If we don't have a type to emit, just emit padding if needed to maintain
// the same alignment for all functions.
if (!Type) {
// the same alignment for all functions. Also skip `nocf_check` functions as
// they are not indirectly callable due to a missing ENDBR.
if (!Type || F.doesNoCfCheck()) {
EmitKCFITypePadding(MF, /*HasType=*/false);
return;
}
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39 changes: 39 additions & 0 deletions llvm/test/CodeGen/X86/kcfi-nocf-check.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,39 @@
; RUN: llc -mtriple=x86_64-unknown-unknown -x86-indirect-branch-tracking < %s | FileCheck %s

; CHECK-LABEL: __cfi_cf_check_func:
; CHECK: movl $12345678, %eax
define void @cf_check_func() !kcfi_type !2 {
; CHECK-LABEL: cf_check_func:
; CHECK: endbr64
; CHECK: retq
entry:
ret void
}

; CHECK-NOT: __cfi_notype_cf_check_func:
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We just need one CHECK-NOT: __cfi_ and the next ; CHECK-NOT: __cfi_nocf_check_func: can be removed.

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We just need one CHECK-NOT: __cfi_ and the next ; CHECK-NOT: __cfi_nocf_check_func: can be removed.

Do you mean something like this?

; CHECK-NOT:   __cfi_
define void @notype_cf_check_func() {
; CHECK-LABEL: notype_cf_check_func:
; CHECK:       endbr64
; CHECK:       retq
entry:
  ret void
}

define void @nocf_check_func() #0 !kcfi_type !2 {
; CHECK-LABEL: nocf_check_func:
; CHECK-NOT:   endbr64
; CHECK:       retq
entry:
  ret void
}

Perhaps I misunderstood your comment, because if we now end up emitting a __cfi_ type prefix before nocf_check_func, FileCheck is not going to detect that. We're still going to need a second CHECK-NOT: __cfi_ before nocf_check_func to catch this case.

Am I missing some clever FileCheck trick that would make this simpler?

; CHECK-NOT: movl
define void @notype_cf_check_func() {
; CHECK-LABEL: notype_cf_check_func:
; CHECK: endbr64
; CHECK: retq
entry:
ret void
}

; CHECK-NOT: __cfi_nocf_check_func:
; CHECK-NOT: movl
define void @nocf_check_func() #0 !kcfi_type !2 {
; CHECK-LABEL: nocf_check_func:
; CHECK-NOT: endbr64
; CHECK: retq
entry:
ret void
}

attributes #0 = { nocf_check }

!llvm.module.flags = !{!0, !1}

!0 = !{i32 8, !"cf-protection-branch", i32 1}
!1 = !{i32 4, !"kcfi", i32 1}
!2 = !{i32 12345678}
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