[lldb/aarch64] Fix unwinding when signal interrupts a leaf function (#91321)

A leaf function may not store the link register to stack, but we it can
still end up being a non-zero frame if it gets interrupted by a signal.
Currently, we were unable to unwind past this function because we could
not read the link register value.

To make this work, this patch:
- changes the function-entry unwind plan to include the `fp|lr = <same>`
rules. This in turn necessitated an adjustment in the generic
instruction emulation logic to ensure that `lr=[sp-X]` can override the
`<same>` rule.
- allows the `<same>` rule for pc and lr in all
`m_all_registers_available` frames (and not just frame zero).

The test verifies that we can unwind in a situation like this, and that
the backtrace matches the one we computed before getting a signal.

Author: labath

lldb/source/Plugins/Instruction/ARM64/EmulateInstructionARM64.cpp

@@ -444,6 +444,8 @@ bool EmulateInstructionARM64::CreateFunctionEntryUnwind(
 
   // Our previous Call Frame Address is the stack pointer
   row->GetCFAValue().SetIsRegisterPlusOffset(gpr_sp_arm64, 0);
+  row->SetRegisterLocationToSame(gpr_lr_arm64, /*must_replace=*/false);
+  row->SetRegisterLocationToSame(gpr_fp_arm64, /*must_replace=*/false);
 
   unwind_plan.AppendRow(row);
   unwind_plan.SetSourceName("EmulateInstructionARM64");

lldb/source/Plugins/UnwindAssembly/InstEmulation/UnwindAssemblyInstEmulation.cpp

@@ -424,8 +424,6 @@ size_t UnwindAssemblyInstEmulation::WriteMemory(
     log->PutString(strm.GetString());
   }
 
-  const bool cant_replace = false;
-
   switch (context.type) {
   default:
   case EmulateInstruction::eContextInvalid:
@@ -467,7 +465,7 @@ size_t UnwindAssemblyInstEmulation::WriteMemory(
         m_pushed_regs[reg_num] = addr;
         const int32_t offset = addr - m_initial_sp;
         m_curr_row->SetRegisterLocationToAtCFAPlusOffset(reg_num, offset,
-                                                         cant_replace);
+                                                         /*can_replace=*/true);
         m_curr_row_modified = true;
       }
     }

lldb/source/Target/RegisterContextUnwind.cpp

@@ -1555,12 +1555,12 @@ RegisterContextUnwind::SavedLocationForRegister(
   }
 
   if (unwindplan_regloc.IsSame()) {
-    if (!IsFrameZero() &&
+    if (!m_all_registers_available &&
         (regnum.GetAsKind(eRegisterKindGeneric) == LLDB_REGNUM_GENERIC_PC ||
          regnum.GetAsKind(eRegisterKindGeneric) == LLDB_REGNUM_GENERIC_RA)) {
       UnwindLogMsg("register %s (%d) is marked as 'IsSame' - it is a pc or "
-                   "return address reg on a non-zero frame -- treat as if we "
-                   "have no information",
+                   "return address reg on a frame which does not have all "
+                   "registers available -- treat as if we have no information",
                    regnum.GetName(), regnum.GetAsKind(eRegisterKindLLDB));
       return UnwindLLDB::RegisterSearchResult::eRegisterNotFound;
     } else {

lldb/test/Shell/Unwind/Inputs/signal-in-leaf-function-aarch64.c

@@ -0,0 +1,15 @@
+#include <signal.h>
+#include <unistd.h>
+
+int __attribute__((naked)) signal_generating_add(int a, int b) {
+  asm("add w0, w1, w0\n\t"
+      "udf #0xdead\n\t"
+      "ret");
+}
+
+void sigill_handler(int) { _exit(0); }
+
+int main() {
+  signal(SIGILL, sigill_handler);
+  return signal_generating_add(42, 47);
+}

lldb/test/Shell/Unwind/signal-in-leaf-function-aarch64.test

@@ -0,0 +1,24 @@
+# REQUIRES: target-aarch64 && native
+# UNSUPPORTED: system-windows
+
+# RUN: %clang_host %S/Inputs/signal-in-leaf-function-aarch64.c -o %t
+# RUN: %lldb -s %s -o exit %t | FileCheck %s
+
+breakpoint set -n sigill_handler
+# CHECK: Breakpoint 1: where = {{.*}}`sigill_handler
+
+run
+# CHECK: thread #1, {{.*}} stop reason = signal SIGILL
+
+thread backtrace
+# CHECK: frame #0: [[ADD:0x[0-9a-fA-F]*]] {{.*}}`signal_generating_add
+# CHECK: frame #1: [[MAIN:0x[0-9a-fA-F]*]] {{.*}}`main
+
+continue
+# CHECK: thread #1, {{.*}} stop reason = breakpoint 1
+
+thread backtrace
+# CHECK: frame #0: {{.*}}`sigill_handler
+# Unknown number of signal trampoline frames
+# CHECK: frame #{{[0-9]+}}: [[ADD]] {{.*}}`signal_generating_add
+# CHECK: frame #{{[0-9]+}}: [[MAIN]] {{.*}}`main

lldb/unittests/UnwindAssembly/ARM64/TestArm64InstEmulation.cpp

@@ -77,7 +77,7 @@ TEST_F(TestArm64InstEmulation, TestSimpleDarwinFunction) {
 
   // UnwindPlan we expect:
 
-  // row[0]:    0: CFA=sp +0 =>
+  // row[0]:    0: CFA=sp +0 => fp= <same> lr= <same>
   // row[1]:    4: CFA=sp+16 => fp=[CFA-16] lr=[CFA-8]
   // row[2]:    8: CFA=fp+16 => fp=[CFA-16] lr=[CFA-8]
   // row[2]:   16: CFA=sp+16 => fp=[CFA-16] lr=[CFA-8]
@@ -88,13 +88,19 @@ TEST_F(TestArm64InstEmulation, TestSimpleDarwinFunction) {
   EXPECT_TRUE(engine->GetNonCallSiteUnwindPlanFromAssembly(
       sample_range, data, sizeof(data), unwind_plan));
 
-  // CFA=sp +0
+  // CFA=sp +0 => fp= <same> lr= <same>
   row_sp = unwind_plan.GetRowForFunctionOffset(0);
   EXPECT_EQ(0ull, row_sp->GetOffset());
   EXPECT_TRUE(row_sp->GetCFAValue().GetRegisterNumber() == gpr_sp_arm64);
   EXPECT_TRUE(row_sp->GetCFAValue().IsRegisterPlusOffset() == true);
   EXPECT_EQ(0, row_sp->GetCFAValue().GetOffset());
 
+  EXPECT_TRUE(row_sp->GetRegisterInfo(gpr_fp_arm64, regloc));
+  EXPECT_TRUE(regloc.IsSame());
+
+  EXPECT_TRUE(row_sp->GetRegisterInfo(gpr_lr_arm64, regloc));
+  EXPECT_TRUE(regloc.IsSame());
+
   // CFA=sp+16 => fp=[CFA-16] lr=[CFA-8]
   row_sp = unwind_plan.GetRowForFunctionOffset(4);
   EXPECT_EQ(4ull, row_sp->GetOffset());
@@ -146,6 +152,12 @@ TEST_F(TestArm64InstEmulation, TestSimpleDarwinFunction) {
   EXPECT_TRUE(row_sp->GetCFAValue().GetRegisterNumber() == gpr_sp_arm64);
   EXPECT_TRUE(row_sp->GetCFAValue().IsRegisterPlusOffset() == true);
   EXPECT_EQ(0, row_sp->GetCFAValue().GetOffset());
+
+  EXPECT_TRUE(row_sp->GetRegisterInfo(gpr_fp_arm64, regloc));
+  EXPECT_TRUE(regloc.IsSame());
+
+  EXPECT_TRUE(row_sp->GetRegisterInfo(gpr_lr_arm64, regloc));
+  EXPECT_TRUE(regloc.IsSame());
 }
 
 TEST_F(TestArm64InstEmulation, TestMediumDarwinFunction) {
@@ -381,8 +393,12 @@ TEST_F(TestArm64InstEmulation, TestFramelessThreeEpilogueFunction) {
   EXPECT_FALSE(row_sp->GetRegisterInfo(gpr_x26_arm64, regloc));
   EXPECT_FALSE(row_sp->GetRegisterInfo(gpr_x27_arm64, regloc));
   EXPECT_FALSE(row_sp->GetRegisterInfo(gpr_x28_arm64, regloc));
-  EXPECT_FALSE(row_sp->GetRegisterInfo(gpr_fp_arm64, regloc));
-  EXPECT_FALSE(row_sp->GetRegisterInfo(gpr_lr_arm64, regloc));
+
+  EXPECT_TRUE(row_sp->GetRegisterInfo(gpr_fp_arm64, regloc));
+  EXPECT_TRUE(regloc.IsSame());
+
+  EXPECT_TRUE(row_sp->GetRegisterInfo(gpr_lr_arm64, regloc));
+  EXPECT_TRUE(regloc.IsSame());
 
   row_sp = unwind_plan.GetRowForFunctionOffset(36);
   EXPECT_TRUE(row_sp->GetCFAValue().GetRegisterNumber() == gpr_sp_arm64);