[base] Fix base::CallFtruncate and base::File::SetLength

Use ftruncate64 instead of ftruncate on POSIX systems supporting it.

This fixes an unchecked narrowing integer conversion from 64-bit length
to 32-bit off_t on systems where the type off_t has only 32 bits. On
those systems, trying to create a big file (4+GB) by using
File::SetLength resulted in a file of the wrong length due to the
integer overflow.

BUG=chromium:1236860
TEST=base_unittests --gtest_filter="FileTest.Length"

Change-Id: I3e0c4700a03ab0b556bc38e6a93d5188eb5debbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3070952
Commit-Queue: François Degros <[email protected]>
Auto-Submit: François Degros <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Cr-Commit-Position: refs/heads/main@{#916237}

base/files/file_posix.cc

@@ -41,7 +41,13 @@ bool IsOpenAppend(PlatformFile file) {
 }
 
 int CallFtruncate(PlatformFile file, int64_t length) {
+#if defined(OS_BSD) || defined(OS_APPLE) || defined(OS_FUCHSIA)
+  static_assert(sizeof(off_t) >= sizeof(int64_t),
+                "off_t is not a 64-bit integer");
   return HANDLE_EINTR(ftruncate(file, length));
+#else
+  return HANDLE_EINTR(ftruncate64(file, length));
+#endif
 }
 
 int CallFutimes(PlatformFile file, const struct timeval times[2]) {

base/files/file_unittest.cc

@@ -365,6 +365,15 @@ TEST(FileTest, Length) {
   for (int i = 0; i < file_size; i++)
     EXPECT_EQ(data_to_write[i], data_read[i]);
 
+#if !defined(OS_FUCHSIA)  // Fuchsia doesn't seem to support big files.
+  // Expand the file past the 4 GB limit.
+  const int64_t kBigFileLength = 5'000'000'000;
+  EXPECT_TRUE(file.SetLength(kBigFileLength));
+  EXPECT_EQ(kBigFileLength, file.GetLength());
+  EXPECT_TRUE(GetFileSize(file_path, &file_size));
+  EXPECT_EQ(kBigFileLength, file_size);
+#endif
+
   // Close the file and reopen with base::File::FLAG_CREATE_ALWAYS, and make
   // sure the file is empty (old file was overridden).
   file.Close();