[ubsan] Fix print_stacktrace=1:fast_unwind_on_fatal=0 to correctly fallback to fast unwinder

ubsan_GetStackTrace (from 52b7510) called by
~ScopeReport leaves top/bottom zeroes in the
`!WillUseFastUnwind(request_fast_unwind)` code path.
When BufferedStackTrace::Unwind falls back to UnwindFast,
`if (stack_top < 4096) return;` will return early, leaving just one frame in the stack trace.

Fix this by always initializing top/bottom like 261d6e0.

Reviewed By: eugenis, yln

Differential Revision: https://reviews.llvm.org/D123562

Author: MaskRay

compiler-rt/lib/ubsan/ubsan_diag.cpp

@@ -32,15 +32,13 @@ using namespace __ubsan;
 // Windows.
 // TODO(yln): This is a temporary workaround. GetStackTrace functions will be
 // removed in the future.
-void ubsan_GetStackTrace(BufferedStackTrace *stack, uptr max_depth,
-                         uptr pc, uptr bp, void *context, bool fast) {
+void ubsan_GetStackTrace(BufferedStackTrace *stack, uptr max_depth, uptr pc,
+                         uptr bp, void *context, bool request_fast) {
   uptr top = 0;
   uptr bottom = 0;
-  if (StackTrace::WillUseFastUnwind(fast)) {
-    GetThreadStackTopAndBottom(false, &top, &bottom);
-    stack->Unwind(max_depth, pc, bp, nullptr, top, bottom, true);
-  } else
-    stack->Unwind(max_depth, pc, bp, context, 0, 0, false);
+  GetThreadStackTopAndBottom(false, &top, &bottom);
+  bool fast = StackTrace::WillUseFastUnwind(request_fast);
+  stack->Unwind(max_depth, pc, bp, context, top, bottom, fast);
 }
 
 static void MaybePrintStackTrace(uptr pc, uptr bp) {

compiler-rt/test/ubsan/TestCases/Misc/Linux/diag-stacktrace.cpp

@@ -0,0 +1,22 @@
+/// Fast unwinder does not work with Thumb code
+// UNSUPPORTED: thumb
+
+// RUN: %clangxx -fsanitize=return %gmlt -O2 -fno-omit-frame-pointer -fasynchronous-unwind-tables %s -o %t
+// RUN: %env_ubsan_opts=print_stacktrace=1:fast_unwind_on_fatal=0 not %run %t 2>&1 | FileCheck %s
+// RUN: %env_ubsan_opts=print_stacktrace=1:fast_unwind_on_fatal=1 not %run %t 2>&1 | FileCheck %s
+// RUN: %clangxx -fsanitize=return %gmlt -O2 -fno-omit-frame-pointer -fno-exceptions -fno-asynchronous-unwind-tables %s -o %t
+// RUN: %env_ubsan_opts=print_stacktrace=1:fast_unwind_on_fatal=0 not %run %t 2>&1 | FileCheck %s
+// RUN: %env_ubsan_opts=print_stacktrace=1:fast_unwind_on_fatal=1 not %run %t 2>&1 | FileCheck %s
+
+// CHECK:      runtime error: execution reached the end of a value-returning function without returning a value
+// CHECK-NEXT: #0 {{.*}}f() {{.*}}.cpp:[[#@LINE+1]]
+__attribute__((noinline)) int f() {}
+
+// CHECK-NEXT: #1 {{.*}}g() {{.*}}.cpp:[[#@LINE+1]]
+__attribute__((noinline)) void g() { f(); }
+
+// CHECK-NEXT: #2 {{.*}}h() {{.*}}.cpp:[[#@LINE+1]]
+__attribute__((noinline)) void h() { g(); }
+
+// CHECK-NEXT: #3 {{.*}}main {{.*}}.cpp:[[#@LINE+1]]
+int main() { h(); }