[asan] Fix handling of %m in printf interceptor.

llvm-svn: 290632

compiler-rt/lib/sanitizer_common/sanitizer_common_interceptors_format.inc

@@ -435,10 +435,6 @@ static const char *printf_parse_next(const char *p, PrintfDirective *dir) {
 }
 
 static int printf_get_value_size(PrintfDirective *dir) {
-  if (dir->convSpecifier == 'm') {
-    return sizeof(char *);
-  }
-
   if (char_is_one_of(dir->convSpecifier, "cCsS")) {
     unsigned charSize =
         format_get_char_size(dir->convSpecifier, dir->lengthModifier);
@@ -519,6 +515,9 @@ static void printf_common(void *ctx, const char *format, va_list aq) {
       // Dynamic precision
       SKIP_SCALAR_ARG(&aq, 'd', sizeof(int));
     }
+    // %m does not require an argument: strlen(errno).
+    if (dir.convSpecifier == 'm')
+      continue;
     int size = printf_get_value_size(&dir);
     if (size == FSS_INVALID) {
       Report("WARNING: unexpected format specifier in printf "

compiler-rt/lib/sanitizer_common/tests/sanitizer_format_interceptor_test.cc

@@ -256,4 +256,8 @@ TEST(SanitizerCommonInterceptors, Printf) {
 
   // Checks for wide-character strings are not implemented yet.
   testPrintf("%ls", 1, 0);
+
+  testPrintf("%m", 0);
+  testPrintf("%m%s", 1, test_buf_size);
+  testPrintf("%s%m%s", 2, test_buf_size, test_buf_size);
 }

compiler-rt/test/asan/TestCases/printf-m.c

@@ -0,0 +1,14 @@
+// RUN: %clang_asan -O2 %s -o %t && %run %t
+
+// FIXME: printf is not intercepted on Windows yet.
+// XFAIL: win32
+
+#include <stdio.h>
+
+int main() {
+  char s[5] = {'w', 'o', 'r', 'l', 'd'};
+  // Test that %m does not consume an argument. If it does, %s would apply to
+  // the 5-character buffer, resulting in a stack-buffer-overflow report.
+  printf("%m %s, %.5s\n", "hello", s);
+  return 0;
+}