AU: Add a utility routine to get the filesystem size from a device.

utils.cc

Index: utils.cc
diff --git a/utils.cc b/utils.cc
index 6006eb85e736f63933380e613f26ebad509b75a6..c8a4da64e29cf1d23037a4b9c7c329ccc593ef72 100644
--- a/utils.cc
+++ b/utils.cc
@@ -18,6 +18,7 @@
 
 #include <algorithm>
 
+#include <base/eintr_wrapper.h>
 #include <base/file_path.h>
 #include <base/file_util.h>
 #include <base/rand_util.h>
@@ -421,6 +422,61 @@ bool UnmountFilesystem(const string& mountpoint) {
   return true;
 }
 
+bool GetFilesystemSize(const std::string& device,
+                       int* out_block_count,
+                       int* out_block_size) {
+  int fd = HANDLE_EINTR(open(device.c_str(), O_RDONLY));
+  TEST_AND_RETURN_FALSE(fd >= 0);
+  ScopedFdCloser fd_closer(&fd);
+  return GetFilesystemSizeFromFD(fd, out_block_count, out_block_size);
+}
+
+bool GetFilesystemSizeFromFD(int fd,
+                             int* out_block_count,
+                             int* out_block_size) {
+  TEST_AND_RETURN_FALSE(fd >= 0);
+
+  // Determine the ext3 filesystem size by directly reading the block count and
+  // block size information from the superblock. See include/linux/ext3_fs.h for
+  // more details on the structure.
+  ssize_t kBufferSize = 16 * sizeof(uint32_t);
+  char buffer[kBufferSize];
+  const int kSuperblockOffset = 1024;
+  if (HANDLE_EINTR(pread(fd, buffer, kBufferSize, kSuperblockOffset)) !=
+      kBufferSize) {
+    PLOG(ERROR) << "Unable to determine file system size:";
+    return false;
+  }
+  uint32_t block_count;  // ext3_fs.h: ext3_super_block.s_blocks_count
+  uint32_t log_block_size;  // ext3_fs.h: ext3_super_block.s_log_block_size
+  uint16_t magic;  // ext3_fs.h: ext3_super_block.s_magic
+  memcpy(&block_count, &buffer[1 * sizeof(int32_t)], sizeof(block_count));
+  memcpy(&log_block_size, &buffer[6 * sizeof(int32_t)], sizeof(log_block_size));
+  memcpy(&magic, &buffer[14 * sizeof(int32_t)], sizeof(magic));
+  block_count = le32toh(block_count);
+  const int kExt3MinBlockLogSize = 10;  // ext3_fs.h: EXT3_MIN_BLOCK_LOG_SIZE
+  log_block_size = le32toh(log_block_size) + kExt3MinBlockLogSize;
+  magic = le16toh(magic);
+
+  // Sanity check the parameters.
+  const uint16_t kExt3SuperMagic = 0xef53;  // ext3_fs.h: EXT3_SUPER_MAGIC
+  TEST_AND_RETURN_FALSE(magic == kExt3SuperMagic);
+  const int kExt3MinBlockSize = 1024;  // ext3_fs.h: EXT3_MIN_BLOCK_SIZE
+  const int kExt3MaxBlockSize = 4096;  // ext3_fs.h: EXT3_MAX_BLOCK_SIZE
+  int block_size = 1 << log_block_size;
+  TEST_AND_RETURN_FALSE(block_size >= kExt3MinBlockSize &&
+                        block_size <= kExt3MaxBlockSize);
+  TEST_AND_RETURN_FALSE(block_count > 0);
+
+  if (out_block_count) {
+    *out_block_count = block_count;
+  }
+  if (out_block_size) {
+    *out_block_size = block_size;
+  }
+  return true;
+}
+
 bool GetBootloader(BootLoader* out_bootloader) {
   // For now, hardcode to syslinux.
   *out_bootloader = BootLoader_SYSLINUX;