diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_common_interceptors_format.inc b/compiler-rt/lib/sanitizer_common/sanitizer_common_interceptors_format.inc
--- a/compiler-rt/lib/sanitizer_common/sanitizer_common_interceptors_format.inc
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_common_interceptors_format.inc
@@ -539,24 +539,26 @@
       continue;
     } else if (size == FSS_STRLEN) {
       if (void *argp = va_arg(aq, void *)) {
+        uptr len;
         if (dir.starredPrecision) {
           // FIXME: properly support starred precision for strings.
-          size = 0;
+          len = 0;
         } else if (dir.fieldPrecision > 0) {
           // Won't read more than "precision" symbols.
-          size = internal_strnlen((const char *)argp, dir.fieldPrecision);
-          if (size < dir.fieldPrecision) size++;
+          len = internal_strnlen((const char *)argp, dir.fieldPrecision);
+          if (len < (uptr)dir.fieldPrecision)
+            len++;
         } else {
           // Whole string will be accessed.
-          size = internal_strlen((const char *)argp) + 1;
+          len = internal_strlen((const char *)argp) + 1;
         }
-        COMMON_INTERCEPTOR_READ_RANGE(ctx, argp, size);
+        COMMON_INTERCEPTOR_READ_RANGE(ctx, argp, len);
       }
     } else if (size == FSS_WCSLEN) {
       if (void *argp = va_arg(aq, void *)) {
         // FIXME: Properly support wide-character strings (via wcsrtombs).
-        size = 0;
-        COMMON_INTERCEPTOR_READ_RANGE(ctx, argp, size);
+        uptr len = 0;
+        COMMON_INTERCEPTOR_READ_RANGE(ctx, argp, len);
       }
     } else {
       // Skip non-pointer args
diff --git a/compiler-rt/test/asan/TestCases/vsnprintf.cpp b/compiler-rt/test/asan/TestCases/vsnprintf.cpp
new file mode 100644
--- /dev/null
+++ b/compiler-rt/test/asan/TestCases/vsnprintf.cpp
@@ -0,0 +1,24 @@
+// RUN: %clangxx_asan -O2 %s -o %t
+// RUN: %env_asan_opts=check_printf=1 %run %t 2>&1
+
+// FIXME: printf is not intercepted on Windows yet.
+// XFAIL: target={{.*windows.*}}
+
+#include <stdarg.h>
+#include <stdio.h>
+#include <string>
+
+void write(char *buf, int buf_size, const char *fmt, ...) {
+  va_list args;
+  va_start(args, fmt);
+  vsnprintf(buf, buf_size, fmt, args);
+  va_end(args);
+}
+
+int main() {
+  char buffer[100];
+  std::string str(2147483648, '=');
+  write(buffer, 100, "%s\n", str.c_str());
+  printf("%s", buffer);
+  return 0;
+}