The Checksum VFS Shim

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The Checksum VFS Shim
Table Of Contents

# 1. Overview

The checksum VFS extension is a VFS shim that adds an 8-byte checksum to the end of every page in an SQLite database. The checksum is added as each page is written and verified as each page is read. The checksum is intended to help detect database corruption caused by random bit-flips in the mass storage device.

The checksum VFS extension requires SQLite version 3.32.0 (2020-05-22) or later. It will not work with earlier versions of SQLite.

# 2. Compiling

The checksum VFS module is a loadable extension. It is not included in the amalgamation. It must be added to SQLite either at compile-time or at run-time. The source code to the checksum VFS module is in the ext/misc/cksumvfs.c source file in the SQLite source tree.

To build the checksum VFS module into a run-time loadable extension, use commands similar to the following:

- (linux) → `gcc -fPIC -shared cksumvfs.c -o cksumvfs.so`
- (mac) → `clang -fPIC -dynamiclib cksumvfs.c -o cksumvfs.dylib`
- (windows) → `cl cksumvfs.c -link -dll -out:cksumvfs.dll`

You may want to add additional compiler options, of course, according to the needs of your project.

To statically link this extension with your product, compile it like any other C-language module but add the "-DSQLITE_CKSUMVFS_STATIC" option so that this module knows that it is being statically linked rather than dynamically linked.

# 3. Loading

To load this extension as a shared library, you first have to bring up a dummy SQLite database connection to use as the argument to the sqlite3_load_extension() API call. Then you invoke the sqlite3_load_extension() API and shutdown the dummy database connection. All subsequent database connections that are opened will include this extension. For example:

    sqlite3 *db;
    sqlite3_open(":memory:", &db);
    sqlite3_load_extension(db, "./cksumvfs");
    sqlite3_close(db);

If this extension is compiled with -DSQLITE_CKSUMVFS_STATIC and statically linked against the application, initialize it using a single API call as follows:

    sqlite3_cksumvfs_init();

Cksumvfs is a VFS shim. When loaded, "cksmvfs" becomes the new default VFS, and it uses the prior default VFS as the next VFS down in the stack. This is normally what you want. However, in complex situations where multiple VFS shims are being loaded, it might be important to ensure that cksumvfs is loaded in the correct order so that it sequences itself into the default VFS Shim stack in the right order.

# 4. Usage

Open database connections using the sqlite3_open() or sqlite3_open_v2() interfaces, as normal. Ordinary database files (without a checksum) will operate normally. Databases with checksums will return an SQLITE_IOERR_DATA error if a page is encountered that contains an invalid checksum.

Checksumming only works on databases that have a reserve bytes value of exactly 8. The default value for reserve-bytes is 0. Hence, newly created database files will omit the checksum by default. To create a database that includes a checksum, change the reserve-bytes value to 8 by running code similar to this:

    int n = 8;
    sqlite3_file_control(db, 0, SQLITE_FCNTL_RESERVE_BYTES, &n);

If you do this immediately after creating a new database file, before anything else has been written into the file, then that might be all that you need to do. Otherwise, the API call above should be followed by:

    sqlite3_exec(db, "VACUUM", 0, 0, 0);

It never hurts to run the VACUUM, even if you don't need it. If the database is in WAL mode, you should shutdown and reopen all database connections before continuing.

From the CLI, use the ".filectrl reserve_bytes 8" command, followed by "VACUUM;".

Note that SQLite allows the number of reserve-bytes to be increased but not decreased. So if a database file already has a reserve-bytes value greater than 8, there is no way to activate checksumming on that database, other than to dump and restore the database file. Note also that other extensions might also make use of the reserve-bytes. Checksumming will be incompatible with those other extensions.

# 5. Verification Of Checksums

If any checksum is incorrect, the "PRAGMA quick_check" command will find it. To verify that checksums are actually enabled and running, use SQL like the following:

    SELECT count(*), verify_checksum(data)
      FROM sqlite_dbpage
     GROUP BY 2;

There are three possible outputs from the verify_checksum() function: 1, 0, and NULL. 1 is returned if the checksum is correct. 0 is returned if the checksum is incorrect. NULL is returned if the page is unreadable. If checksumming is enabled, the read will fail if the checksum is wrong, so the usual result from verify_checksum() on a bad checksum is NULL.

If everything is OK, the query above should return a single row where the second column is 1. Any other result indicates either that there is a checksum error, or checksum validation is disabled.

# 6. Controlling Checksum Verification

The cksumvfs extension implements a new PRAGMA statement that can be used to disable, re-enable, or query the status of checksum verification:

    PRAGMA checksum_verification;          -- query status
    PRAGMA checksum_verification=OFF;      -- disable verification
    PRAGMA checksum_verification=ON;       -- re-enable verification

The "checksum_verification" pragma will return "1" (true) or "0" (false) if checksum verification is enabled or disabled, respectively. "Verification" in this context means the feature that causes SQLITE_IOERR_DATA errors if a checksum mismatch is detected while reading. Checksums are always kept up-to-date as long as the reserve bytes value of the database is 8, regardless of the setting of this pragma. Checksum verification can be disabled (for example) to do forensic analysis of a database that has previously reported a checksum error.

The "checksum_verification" pragma will always respond with "0" if the database file does not have a reserve bytes value of 8. The pragma will return no rows at all if the cksumvfs extension is not loaded.

This page was last updated on 2022-01-08 05:02:57Z