Handle case where a file is mapped both private and shared and is wri…

…tten to

CMakeLists.txt

@@ -775,7 +775,6 @@ set(BASIC_TESTS
   mmap_short_file
   mmap_tmpfs
   mmap_write_complex
-  mmap_write_private
   mmap_zero_size_fd
   modify_ldt
   mount_ns_exec
@@ -1077,6 +1076,7 @@ set(TESTS_WITH_PROGRAM
   mmap_replace_most_mappings
   mmap_shared_prot
   mmap_write
+  mmap_write_private
   mprotect_growsdown
   mprotect_syscallbuf_overflow
   mutex_pi_stress

src/MmappedFileMonitor.cc

@@ -58,6 +58,15 @@ void MmappedFileMonitor::did_write(Task* t, const std::vector<Range>& ranges,
         if (km.device() != device_ || km.inode() != inode_) {
           continue;
         }
+        // If the mapping is MAP_PRIVATE then this write is dangerous
+        // because it's unpredictable what will be seen in the mapping.
+        // However, it could be OK if the application doesn't read from
+        // this part of the mapping. Just optimistically assume this mapping
+        // is not affected.
+        if (!(km.flags() & MAP_SHARED)) {
+          LOG(warn) << "MAP_PRIVATE mapping affected by write";
+          continue;
+        }
       }
 
       // We're discovering a mapping we care about
@@ -67,7 +76,6 @@ void MmappedFileMonitor::did_write(Task* t, const std::vector<Range>& ranges,
       }
 
       // stat matches.
-      ASSERT(t, km.flags() & MAP_SHARED);
       uint64_t mapping_offset = km.file_offset_bytes();
       int64_t local_offset = realized_offset;
       for (auto r : ranges) {

src/test/mmap_write_private.c

@@ -5,6 +5,9 @@
 int main(void) {
   int fd = open("output", O_RDWR | O_CREAT, 0777);
   int fd2;
+  int fd3;
+  ssize_t ret;
+  char* p2;
   char* p = (char*)mmap(NULL, 10, PROT_READ,
                         MAP_PRIVATE, fd, 0);
   test_assert(p != MAP_FAILED);
@@ -15,6 +18,17 @@ int main(void) {
   fd2 = open("output", O_RDWR, 0777);
   test_assert(fd2 >= 0);
 
+  p2 = (char*)mmap(NULL, 10, PROT_READ, MAP_SHARED, fd, 0);
+  test_assert(p2 != MAP_FAILED);
+
+  /* Test what happens if the file is mapped private AND shared and
+     we write to it */
+  fd3 = open("output", O_RDWR, 0777);
+  test_assert(fd3 >= 0);
+
+  ret = write(fd3, "x", 1);
+  test_assert(ret == 1);
+
   atomic_puts("EXIT-SUCCESS");
   return 0;
 }

src/test/mmap_write_private.run

@@ -0,0 +1,2 @@
+source `dirname $0`/util.sh
+RR_COPY_ALL_FILES=1 compare_test EXIT-SUCCESS