MappedFile and FileMap should support zero-length mappings.

Bug: http://b/119818070 "app crashes when reading asset of zero length"
Test: ran tests
Change-Id: Idd2ad6f6e72c8e445aff78a460fac96dea41c950

2 files changed, 20 insertions, 2 deletions

diff --git a/base/mapped_file.cpp b/base/mapped_file.cpp
index f7901afa4..faa845d14 100644
--- a/base/mapped_file.cpp
+++ b/base/mapped_file.cpp
@@ -16,6 +16,8 @@
 
 #include "android-base/mapped_file.h"
 
+#include <errno.h>
+
 namespace android {
 namespace base {
 
@@ -50,7 +52,14 @@ std::unique_ptr<MappedFile> MappedFile::FromFd(int fd, off64_t offset, size_t le
       new MappedFile{static_cast<char*>(base), length, slop, handle});
 #else
   void* base = mmap(nullptr, file_length, prot, MAP_SHARED, fd, file_offset);
-  if (base == MAP_FAILED) return nullptr;
+  if (base == MAP_FAILED) {
+    // http://b/119818070 "app crashes when reading asset of zero length".
+    // mmap fails with EINVAL for a zero length region.
+    if (errno == EINVAL && length == 0) {
+      return std::unique_ptr<MappedFile>(new MappedFile{nullptr, 0, 0});
+    }
+    return nullptr;
+  }
   return std::unique_ptr<MappedFile>(new MappedFile{static_cast<char*>(base), length, slop});
 #endif
 }
diff --git a/base/mapped_file_test.cpp b/base/mapped_file_test.cpp
index 7e89723c6..cfde73c74 100644
--- a/base/mapped_file_test.cpp
+++ b/base/mapped_file_test.cpp
@@ -25,7 +25,6 @@
 #include <string>
 
 #include "android-base/file.h"
-#include "android-base/unique_fd.h"
 
 TEST(mapped_file, smoke) {
   TemporaryFile tf;
@@ -37,3 +36,13 @@ TEST(mapped_file, smoke) {
   ASSERT_EQ('l', m->data()[0]);
   ASSERT_EQ('o', m->data()[1]);
 }
+
+TEST(mapped_file, zero_length_mapping) {
+  // http://b/119818070 "app crashes when reading asset of zero length".
+  // mmap fails with EINVAL for a zero length region.
+  TemporaryFile tf;
+  ASSERT_TRUE(tf.fd != -1);
+
+  auto m = android::base::MappedFile::FromFd(tf.fd, 4096, 0, PROT_READ);
+  ASSERT_EQ(0u, m->size());
+}