[RFC] [COFF] [AArch64] Avoid crashing on .seh directives in assembly

[mstorsjo]

Summary

Not sure if this really does the right thing, but it kind of matches a similar if directive in the corresponding x86_64 code:

if (info->PrologEnd)
  EmitAbsDifference(streamer, info->PrologEnd, info->Begin);
else
  streamer.EmitIntValue(0, 1);

This at least avoids crashing on cases of a few .seh_* directives in (inline) assembly (which is present in a few places in mingw-w64).

[mstorsjo]

Ping @mgrang @rnk

[rnk]

lgtm

test/MC/AArch64/seh.s

75

This is the one that misbehaves, because it has no end prologue, right?

[mstorsjo]

test/MC/AArch64/seh.s

75

Not entirely sure, or if there's sonething else that isn't properly hooked up wrt SEH in the MC layer for aarch64. I noticed that there's lots of .seh_* directives for x86_64 that aren't available for aarch64 yet (although I guess some of them are arch specific).

Adding .seh_endprologue here isn't enough to make it not crash before this patch at least.

The real case where I ran into it was https://github.com/mirror/mingw-w64/blob/master/mingw-w64-crt/crt/crtexe.c#L185 though.

lib/MC/MCWin64EH.cpp

static void ARM64EmitUnwindInfo(MCStreamer &streamer, WinEH::FrameInfo *info) {
   MCContext &context = streamer.getContext();
   MCSymbol *Label = context.createTempSymbol();
 
   streamer.EmitValueToAlignment(4);
   streamer.EmitLabel(Label);
   info->Symbol = Label;
 
   uint32_t FuncLength = 0x0;
+  if (info->FuncletOrFuncEnd)
     FuncLength = (uint32_t)GetAbsDifference(streamer, info->FuncletOrFuncEnd,
                                             info->Begin);
-  if (FuncLength)
-    FuncLength /= 4;
   uint32_t PrologCodeBytes = ARM64CountOfUnwindCodes(info->Instructions);
   uint32_t TotalCodeBytes = PrologCodeBytes;
 
   // Process epilogs.
   MapVector<MCSymbol *, uint32_t> EpilogInfo;
   for (auto &I : info->EpilogMap) {
     MCSymbol *EpilogStart = I.first;
     auto &EpilogInstrs = I.second;

test/MC/AArch64/seh.s

+// This test checks that the SEH directives emit the correct unwind data.
+
+// RUN: llvm-mc -triple aarch64-pc-win32 -filetype=obj %s | llvm-readobj -s -r | FileCheck %s
+
+// CHECK:      Sections [
+// CHECK:        Section {
+// CHECK:          Name: .text
+// CHECK:          RelocationCount: 0
+// CHECK:          Characteristics [
+// CHECK-NEXT:       ALIGN_4BYTES
+// CHECK-NEXT:       CNT_CODE
+// CHECK-NEXT:       MEM_EXECUTE
+// CHECK-NEXT:       MEM_READ
+// CHECK-NEXT:     ]
+// CHECK-NEXT:   }
+// CHECK:        Section {
+// CHECK:          Name: .xdata
+// CHECK:          RawDataSize: 24
+// CHECK:          RelocationCount: 1
+// CHECK:          Characteristics [
+// CHECK-NEXT:       ALIGN_4BYTES
+// CHECK-NEXT:       CNT_INITIALIZED_DATA
+// CHECK-NEXT:       MEM_READ
+// CHECK-NEXT:     ]
+// CHECK-NEXT:   }
+// CHECK:        Section {
+// CHECK:          Name: .pdata
+// CHECK:          RelocationCount: 6
+// CHECK:          Characteristics [
+// CHECK-NEXT:       ALIGN_4BYTES
+// CHECK-NEXT:       CNT_INITIALIZED_DATA
+// CHECK-NEXT:       MEM_READ
+// CHECK-NEXT:     ]
+// CHECK-NEXT:   }
+// CHECK-NEXT: ]
+
+// CHECK-NEXT: Relocations [
+// CHECK-NEXT:   Section (4) .xdata {
+// CHECK-NEXT:     0x8 IMAGE_REL_ARM64_ADDR32NB __C_specific_handler
+// CHECK-NEXT:   }
+// CHECK-NEXT:   Section (5) .pdata {
+// CHECK-NEXT:     0x0 IMAGE_REL_ARM64_ADDR32NB func
+// CHECK-NEXT:     0x4 IMAGE_REL_ARM64_ADDR32NB .xdata
+// CHECK-NEXT:     0x8 IMAGE_REL_ARM64_ADDR32NB func
+// CHECK-NEXT:     0xC IMAGE_REL_ARM64_ADDR32NB .xdata
+// CHECK-NEXT:     0x10 IMAGE_REL_ARM64_ADDR32NB smallFunc
+// CHECK-NEXT:     0x14 IMAGE_REL_ARM64_ADDR32NB .xdata
+// CHECK-NEXT:   }
+// CHECK-NEXT: ]
+
+
+    .text
+    .globl func
+    .def func
+    .scl 2
+    .type 32
+    .endef
+    .seh_proc func
+func:
+    sub sp, sp, #24
+    .seh_stackalloc 24
+    mov x29, sp
+    .seh_endprologue
+    .seh_handler __C_specific_handler, @except
+    .seh_handlerdata
+    .long 0
+    .text
+    .seh_startchained
+    .seh_endprologue
+    .seh_endchained
+    add sp, sp, #24
+    ret
+    .seh_endproc
+
+// Test emission of small functions.
+    .globl smallFunc
+    .def smallFunc
+    .scl 2
+    .type 32
+    .endef
+    .seh_proc smallFunc
+smallFunc:
+    ret
+    .seh_endproc