ASan: add backtrace_symbols test and clarify code is correct

[thurston]

Summary

This is another patch for https://github.com/google/sanitizers/issues/321
(sanitizer interceptors can write to freed memory, causing corruption),
in this case for backtrace_symbols.

backtrace_symbols is already correct, hence this patch removes the
TODO note. Additionally, this patch adds a test case for it.

compiler-rt/lib/sanitizer_common/sanitizer_common_interceptors.inc

INTERCEPTOR(int, backtrace, void **buffer, int size) {
   return res;
 }
 
 INTERCEPTOR(char **, backtrace_symbols, void **buffer, int size) {
   void *ctx;
   COMMON_INTERCEPTOR_ENTER(ctx, backtrace_symbols, buffer, size);
   if (buffer && size)
     COMMON_INTERCEPTOR_READ_RANGE(ctx, buffer, size * sizeof(*buffer));
-  // FIXME: under ASan the call below may write to freed memory and corrupt
-  // its metadata. See
-  // https://github.com/google/sanitizers/issues/321.
+  // The COMMON_INTERCEPTOR_READ_RANGE above ensures that 'buffer' is
+  // valid for reading.
   char **res = REAL(backtrace_symbols)(buffer, size);
   if (res && size) {
     COMMON_INTERCEPTOR_WRITE_RANGE(ctx, res, size * sizeof(*res));
     for (int i = 0; i < size; ++i)
       COMMON_INTERCEPTOR_WRITE_RANGE(ctx, res[i], internal_strlen(res[i]) + 1);
   }
   return res;
 }

compiler-rt/test/asan/TestCases/backtrace_symbols_interceptor.cpp

+// RUN: %clangxx_asan -O0 %s -o %t && not %run %t 2>&1 | FileCheck %s
+
+// Test the backtrace_symbols() interceptor.
+
+#include <assert.h>
+#include <execinfo.h>
+#include <math.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#define MAX_BT 100
+
+int main() {
+  void **buffer = (void **)malloc(sizeof(void *) * MAX_BT);
+  assert(buffer != NULL);
+
+  int numEntries = backtrace(buffer, MAX_BT);
+  printf("backtrace returned %d entries\n", numEntries);
+
+  free(buffer);
+
+  // Deliberate use-after-free of 'buffer'. We expect ASan to
+  // catch this, without triggering internal sanitizer errors.
+  char **strings = backtrace_symbols(buffer, numEntries);
+  assert(strings != NULL);
+
+  for (int i = 0; i < numEntries; i++) {
+    printf("%s\n", strings[i]);
+  }
+
+  free(strings);
+
+  // CHECK: use-after-free
+  // CHECK: SUMMARY
+  return 0;
+}