[hwasan] Fix stack tag mismatch report

compiler-rt/lib/hwasan/hwasan_report.cpp

@@ -230,33 +230,68 @@ static void PrintStackAllocations(const StackAllocationsRingBuffer *sa,
       tag_t obj_tag = base_tag ^ local.tag_offset;
       if (obj_tag != addr_tag)
         continue;
-      // Guess top bits of local variable from the faulting address, because
-      // we only store bits 4-19 of FP (bits 0-3 are guaranteed to be zero).
-      uptr local_beg = (fp + local.frame_offset) |
-                       (untagged_addr & ~(uptr(kRecordFPModulus) - 1));
-      uptr local_end = local_beg + local.size;
+
+      // We only store bits 4-19 of FP (bits 0-3 are guaranteed to be zero).
+      // So we know only `FP % kRecordFPModulus`, and we can only calculate
+      // `local_beg % kRecordFPModulus`.
+      // Out of all possible `local_beg` we will only consider 2 candidates
+      // nearest to the `untagged_addr`.
+      uptr local_beg_mod = (fp + local.frame_offset) % kRecordFPModulus;
+      // Pick `local_beg` in the same 1 MiB block as `untagged_addr`.
+      uptr local_beg =
+          RoundDownTo(untagged_addr, kRecordFPModulus) + local_beg_mod;
+      // Pick the largest `local_beg <= untagged_addr`. It's either the current
+      // one or the one before.
+      if (local_beg > untagged_addr)
+        local_beg -= kRecordFPModulus;
+
+      uptr offset = -1ull;
+      const char *whence;
+      const char *cause = nullptr;
+      uptr best_beg;
+
+      // Try two 1 MiB blocks options and pick nearest one.
+      for (uptr i = 0; i < 2; ++i, local_beg += kRecordFPModulus) {
+        uptr local_end = local_beg + local.size;
+        if (local_beg > local_end)
+          continue;  // This is a wraparound.
+        if (local_beg <= untagged_addr && untagged_addr < local_end) {
+          offset = untagged_addr - local_beg;
+          whence = "inside";
+          cause = "use-after-scope";
+          best_beg = local_beg;
+          break;  // This is as close at it can be.
+        }
+
+        if (untagged_addr >= local_end) {
+          uptr new_offset = untagged_addr - local_end;
+          if (new_offset < offset) {
+            offset = new_offset;
+            whence = "after";
+            cause = "stack-buffer-overflow";
+            best_beg = local_beg;
+          }
+        } else {
+          uptr new_offset = local_beg - untagged_addr;
+          if (new_offset < offset) {
+            offset = new_offset;
+            whence = "before";
+            cause = "stack-buffer-overflow";
+            best_beg = local_beg;
+          }
+        }
+      }
+
+      // To fail the `untagged_addr` must be near nullptr, which is impossible
+      // with Linux user space memory layout.
+      if (!cause)
+        continue;
 
       if (!found_local) {
         Printf("\nPotentially referenced stack objects:\n");
         found_local = true;
       }
 
-      uptr offset;
-      const char *whence;
-      const char *cause;
-      if (local_beg <= untagged_addr && untagged_addr < local_end) {
-        offset = untagged_addr - local_beg;
-        whence = "inside";
-        cause = "use-after-scope";
-      } else if (untagged_addr >= local_end) {
-        offset = untagged_addr - local_end;
-        whence = "after";
-        cause = "stack-buffer-overflow";
-      } else {
-        offset = local_beg - untagged_addr;
-        whence = "before";
-        cause = "stack-buffer-overflow";
-      }
       Decorator d;
       Printf("%s", d.Error());
       Printf("Cause: %s\n", cause);
@@ -267,10 +302,11 @@ static void PrintStackAllocations(const StackAllocationsRingBuffer *sa,
           common_flags()->symbolize_vs_style,
           common_flags()->strip_path_prefix);
       Printf(
-          "%p is located %zd bytes %s a %zd-byte local variable %s [%p,%p) "
+          "%p is located %zd bytes %s a %zd-byte local variable %s "
+          "[%p,%p) "
           "in %s %s\n",
-          untagged_addr, offset, whence, local_end - local_beg, local.name,
-          local_beg, local_end, local.function_name, location.data());
+          untagged_addr, offset, whence, local.size, local.name, best_beg,
+          best_beg + local.size, local.function_name, location.data());
       location.clear();
       Printf("%s\n", d.Default());
     }

compiler-rt/test/hwasan/TestCases/stack-uas.c

@@ -1,6 +1,8 @@
 // Tests use-after-scope detection and reporting.
 // RUN: %clang_hwasan -O0 -g %s -o %t && not %run %t 2>&1 | FileCheck %s
 // RUN: %clang_hwasan -O2 -g %s -o %t && not %run %t 2>&1 | FileCheck %s
+// RUN: %clang_hwasan -O2 -g %s -DBUFFER_SIZE=1000000 -o %t && not %run %t 2>&1 | FileCheck %s
+// RUN: %clang_hwasan -O2 -g %s -DBUFFER_SIZE=2000000 -o %t && not %run %t 2>&1 | FileCheck %s
 // RUN: %clang_hwasan -g %s -o %t && not %env_hwasan_opts=symbolize=0 %run %t 2>&1 | FileCheck %s --check-prefix=NOSYM
 
 // RUN: %clang_hwasan -mllvm -hwasan-use-after-scope=false -g %s -o %t && %run %t 2>&1
@@ -18,6 +20,10 @@
 #include <sanitizer/hwasan_interface.h>
 #include <stdio.h>
 
+#ifndef BUFFER_SIZE
+#  define BUFFER_SIZE 0x800
+#endif
+
 void USE(void *x) { // pretend_to_do_something(void *x)
   __asm__ __volatile__(""
                        :
@@ -41,8 +47,8 @@ __attribute__((noinline)) void Unrelated3() {
 __attribute__((noinline)) char buggy() {
   char *volatile p;
   {
-    char zzz[0x800] = {};
-    char yyy[0x800] = {};
+    char zzz[BUFFER_SIZE] = {};
+    char yyy[BUFFER_SIZE] = {};
     // With -hwasan-generate-tags-with-calls=false, stack tags can occasionally
     // be zero, leading to a false negative
     // (https://github.com/llvm/llvm-project/issues/69221). Work around it by
@@ -71,7 +77,7 @@ int main() {
   // CHECK: is located in stack of thread
   // CHECK: Potentially referenced stack objects:
   // CHECK: Cause: use-after-scope
-  // CHECK-NEXT: 0x{{.*}} is located 0 bytes inside a 2048-byte local variable {{zzz|yyy}} [0x{{.*}}) in buggy {{.*}}stack-uas.c:
+  // CHECK-NEXT: 0x{{.*}} is located 0 bytes inside a {{.*}}-byte local variable {{zzz|yyy}} [0x{{.*}}) in buggy {{.*}}stack-uas.c:
   // CHECK: Memory tags around the buggy address
 
   // NOSYM: Previously allocated frames: