1 files changed, 50 insertions, 0 deletions
diff --git a/libelffile/stream/file_output_stream.cc b/libelffile/stream/file_output_stream.cc
index bbfbdfdca8..5afe366636 100644
--- a/libelffile/stream/file_output_stream.cc
+++ b/libelffile/stream/file_output_stream.cc
@@ -16,6 +16,8 @@
 
 #include "file_output_stream.h"
 
+#include <android-base/logging.h>
+#include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
 
@@ -30,6 +32,54 @@ bool FileOutputStream::WriteFully(const void* buffer, size_t byte_count) {
 }
 
 off_t FileOutputStream::Seek(off_t offset, Whence whence) {
+  static const bool allow_sparse_files = unix_file::AllowSparseFiles();
+  // If we are not allowed to generate sparse files, write zeros instead.
+  if (UNLIKELY(!allow_sparse_files)) {
+    // Check the current file size.
+    int fd = file_->Fd();
+    struct stat sb;
+    if (fstat(fd, &sb) == -1) {
+      return -1;
+    }
+    off_t file_size = sb.st_size;
+    // Calculate new desired offset.
+    switch (whence) {
+      case kSeekSet:
+        break;
+      case kSeekCurrent: {
+        off_t curr_offset = lseek(fd, 0, SEEK_CUR);
+        if (curr_offset == -1) {
+          return -1;
+        }
+        offset += curr_offset;
+        whence = kSeekSet;
+        break;
+      }
+      case kSeekEnd:
+        offset += file_size;
+        whence = kSeekSet;
+        break;
+      default:
+        LOG(FATAL) << "Unsupported seek type: " << whence;
+        UNREACHABLE();
+    }
+    // Write zeros if we are extending the file.
+    if (offset > file_size) {
+      off_t curr_offset = lseek(fd, 0, SEEK_END);
+      if (curr_offset == -1) {
+        return -1;
+      }
+      static const std::array<uint8_t, 1024> buffer{};
+      while (curr_offset < offset) {
+        size_t size = std::min<size_t>(offset - curr_offset, buffer.size());
+        ssize_t bytes_written = write(fd, buffer.data(), size);
+        if (bytes_written < 0) {
+          return -1;
+        }
+        curr_offset += bytes_written;
+      }
+    }
+  }
   return lseek(file_->Fd(), offset, static_cast<int>(whence));
 }