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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-13 14:07:11 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-13 14:07:11 +0000
commit63847496f14c813a5d80efd5b7de0f1294ffe1e3 (patch)
tree01c7571c7c762ceee70638549a99834fdd7c411b /www/appfunc.html
parentInitial commit. (diff)
downloadsqlite3-63847496f14c813a5d80efd5b7de0f1294ffe1e3.tar.xz
sqlite3-63847496f14c813a5d80efd5b7de0f1294ffe1e3.zip
Adding upstream version 3.45.1.upstream/3.45.1
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
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+<div class=fancy>
+<div class=nosearch>
+<div class="fancy_title">
+Application-Defined SQL Functions
+</div>
+<div class="fancy_toc">
+<a onclick="toggle_toc()">
+<span class="fancy_toc_mark" id="toc_mk">&#x25ba;</span>
+Table Of Contents
+</a>
+<div id="toc_sub"><div class="fancy-toc1"><a href="#executive_summary">1. Executive Summary</a></div>
+<div class="fancy-toc1"><a href="#defining_new_sql_functions">2. Defining New SQL Functions</a></div>
+<div class="fancy-toc2"><a href="#common_parameters">2.1. Common Parameters</a></div>
+<div class="fancy-toc2"><a href="#multiple_calls_to_sqlite3_create_function_for_the_same_function">2.2. Multiple Calls To sqlite3_create_function() For The Same Function</a></div>
+<div class="fancy-toc2"><a href="#callbacks">2.3. Callbacks</a></div>
+<div class="fancy-toc3"><a href="#the_scalar_function_callback">2.3.1. The Scalar Function Callback</a></div>
+<div class="fancy-toc3"><a href="#the_aggregate_function_callbacks">2.3.2. The Aggregate Function Callbacks</a></div>
+<div class="fancy-toc3"><a href="#the_window_function_callbacks">2.3.3. The Window Function Callbacks</a></div>
+<div class="fancy-toc3"><a href="#examples">2.3.4. Examples</a></div>
+<div class="fancy-toc1"><a href="#security_implications">3. Security Implications</a></div>
+</div>
+</div>
+<script>
+function toggle_toc(){
+var sub = document.getElementById("toc_sub")
+var mk = document.getElementById("toc_mk")
+if( sub.style.display!="block" ){
+sub.style.display = "block";
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+sub.style.display = "none";
+mk.innerHTML = "&#x25ba;";
+}
+}
+</script>
+</div>
+
+
+
+
+
+<h1 id="executive_summary"><span>1. </span>Executive Summary</h1>
+
+<p>Applications that use SQLite can define custom SQL functions that call
+back into application code to compute their results. The custom SQL
+function implementations can be embedded in the application code itself,
+or can be <a href="loadext.html">loadable extensions</a>.
+
+</p><p>Application-defined or custom SQL functions are created using the
+<a href="c3ref/create_function.html">sqlite3_create_function()</a> family of interfaces.
+Custom SQL functions can be scalar functions, aggregate functions,
+or <a href="windowfunctions.html">window functions</a>.
+Custom SQL functions can have any number of arguments from 0 up to
+<a href="limits.html#max_function_arg">SQLITE_MAX_FUNCTION_ARG</a>.
+The <a href="c3ref/create_function.html">sqlite3_create_function()</a> interface specifies callbacks that are
+invoked to carry out the processing for the new SQL function.
+
+</p><p>SQLite also supports custom <a href="vtab.html#tabfunc2">table-valued functions</a>, but they are
+implemented by a different mechanism that is not covered in this document.
+
+</p><h1 id="defining_new_sql_functions"><span>2. </span>Defining New SQL Functions</h1>
+
+<p>
+The <a href="c3ref/create_function.html">sqlite3_create_function()</a> family of interfaces is used to create
+new custom SQL functions. Each member of this family is a wrapper around
+a common core. All family members accomplish the same thing; they merely
+have different calling signatures.
+
+</p><ul>
+<li><p><b><a href="c3ref/create_function.html">sqlite3_create_function()</a></b> &rarr;
+The original version of sqlite3_create_function() allows the application
+to create a single new SQL function that can be either a scalar or an
+aggregate. The name of the function is specified using UTF8.
+
+</p></li><li><p><b><a href="c3ref/create_function.html">sqlite3_create_function16()</a></b> &rarr;
+This variant works exactly like the sqlite3_create_function() original
+except that the name of the function itself is specified as a UTF16
+string rather than as a UTF8 string.
+
+</p></li><li><p><b><a href="c3ref/create_function.html">sqlite3_create_function_v2()</a></b> &rarr;
+This variant works like the original sqlite3_create_function() except
+that it includes an additional parameter that is a pointer to a
+destructor for the <a href="c3ref/user_data.html">sqlite3_user_data()</a> pointer that is passed in
+as the 5th argument to all of the sqlite3_create_function() variants.
+That destructor function (if it is non-NULL) is called when the
+custom function is deleted - usually when the database connection is
+closing.
+
+</p></li><li><p><b><a href="c3ref/create_function.html">sqlite3_create_window_function()</a></b> &rarr;
+This variant works like the original sqlite3_create_function() except
+that it accepts a different set of callback pointers - the callback
+pointers used by <a href="windowfunctions.html">window function</a> definitions.
+</p></li></ul>
+
+<h2 id="common_parameters"><span>2.1. </span>Common Parameters</h2>
+
+<p>Many of the parameters passed to the <a href="c3ref/create_function.html">sqlite3_create_function()</a>
+family of interfaces are common across the entire family.
+
+</p><ol>
+<li><p><b>db</b> &rarr;
+The 1st parameter is always a pointer to the <a href="c3ref/sqlite3.html">database connection</a>
+on which the custom SQL function will work. Custom SQL functions are
+created separately for each database connection. There is no short-hand
+mechanism for creating SQL functions that work across all database
+connections.
+
+</p></li><li><p><b>zFunctionName</b> &rarr;
+The 2nd parameter is the name of the SQL function that is being
+created. The name is usually in UTF8, except that the name should
+be in UTF16 in the native byte order for <a href="c3ref/create_function.html">sqlite3_create_function16()</a>.
+</p><p>
+The maximum length of a SQL function name is 255 bytes of UTF8.
+Any attempt to create a function with a longer name will result in
+an <a href="rescode.html#misuse">SQLITE_MISUSE</a> error.
+</p>
+The SQL function creation interfaces may be called multiple
+times with the same function name.
+If two calls have the same function number but a different number of
+arguments, for example, then two variants of the SQL function will
+be registered, each taking a different number of arguments.
+
+</li><li><p><b>nArg</b> &rarr;
+The 3rd parameter is always the number of arguments that the function
+accepts. The value must be an integer between -1 and
+<a href="limits.html#max_function_arg">SQLITE_MAX_FUNCTION_ARG</a> (default value: 127). A value of -1 means
+that the SQL function is a variadic function that can take any number
+of arguments between 0 and <a href="limits.html#max_function_arg">SQLITE_MAX_FUNCTION_ARG</a>.
+
+</p></li><li><p><b>eTextRep</b> &rarr;
+The 4th parameter is a 32-bit integer flag whose bits convey various
+properties about the new function. The original purpose of this
+parameter was to specify the preferred text encoding for the function,
+using one of the following constants:
+</p><ul>
+<li> <a href="c3ref/c_any.html">SQLITE_UTF8</a>
+</li><li> <a href="c3ref/c_any.html">SQLITE_UTF16BE</a>
+</li><li> <a href="c3ref/c_any.html">SQLITE_UTF16LE</a>
+</li></ul>
+All custom SQL functions will accept text in any encoding. Encoding
+conversions will happen automatically. The preferred encoding merely
+specifies the encoding for which the function implementation is optimized.
+It is possible to specify multiple functions with the same name and the
+same number of arguments, but different preferred encodings and different
+callbacks used to implement the function, and SQLite will chose the
+set of callbacks for which the input encodings most closely match the
+preferred encoding.
+<p>
+The 4th parameter as more recently be extended with additional flag bits
+to convey additional information about the function. The additional
+bits include:
+</p><ul>
+<li> <a href="c3ref/c_deterministic.html#sqlitedeterministic">SQLITE_DETERMINISTIC</a>
+</li><li> <a href="c3ref/c_deterministic.html#sqlitedirectonly">SQLITE_DIRECTONLY</a>
+</li><li> <a href="c3ref/c_deterministic.html#sqliteinnocuous">SQLITE_INNOCUOUS</a>
+</li><li> <a href="c3ref/c_deterministic.html#sqlitesubtype">SQLITE_SUBTYPE</a>
+</li></ul>
+<p>
+Additional bits may be added in future versions of SQLite.
+
+</p></li><li><p><b>pApp</b> &rarr;
+The 5th parameter is an arbitrary pointer that is passed through
+into the callback routines. SQLite itself does nothing with this
+pointer, except to make it available to the callbacks, and to pass
+it into the destructor when the function is unregistered.
+</p></li></ol>
+
+<h2 id="multiple_calls_to_sqlite3_create_function_for_the_same_function"><span>2.2. </span>Multiple Calls To sqlite3_create_function() For The Same Function</h2>
+
+<p>
+It is common for an application to invoke sqlite3_create_function() multiple
+times for the same SQL function. For example, if an SQL function can take
+either 2 or 3 arguments, then sqlite3_create_function() would be invoked
+once for the 2-argument version and a second time for the 3-argument version.
+The underlying implementation (the callbacks) can be different for both
+variants.
+
+</p><p>
+An application can also register multiple SQL functions with the same name
+and same number of arguments, but a different preferred text encoding.
+In that case, SQLite will invoke the function using the callbacks for
+the version whose preferred text encoding most closely matches the database
+text encoding. In this way, multiple implementations of the same function
+can be provided that are optimized for UTF8 or UTF16.
+
+</p><p>
+If multiple calls to sqlite3_create_function() specify the same function name,
+and the same number of arguments, and the same preferred text encoding, then
+the callbacks and other parameters of the second call overwrite the first,
+and the destructor callback from the first call (if it exists) is invoked.
+
+
+</p><h2 id="callbacks"><span>2.3. </span>Callbacks</h2>
+
+<p>
+SQLite evaluates an SQL function by invoking callback routines.
+
+</p><h3 id="the_scalar_function_callback"><span>2.3.1. </span>The Scalar Function Callback</h3>
+
+<p>Scalar SQL functions are implemented by a single callback in the
+<b>xFunc</b> parameter to sqlite3_create_function().
+The following code demonstrations the implementation of a "noop(X)"
+scalar SQL function that merely returns its argument:
+
+</p><div class="codeblock"><pre>static void noopfunc(
+ sqlite3_context *context,
+ int argc,
+ sqlite3_value **argv
+){
+ assert( argc==1 );
+ sqlite3_result_value(context, argv&#91;0&#93;);
+}
+</pre></div>
+
+<p>
+The 1st parameter, <b>context</b>, is a pointer to an opaque object
+that describes the content from which the SQL function was invoked. This
+context point becomes the first parameter to many other routines that
+the function implement might to invoke, including:
+
+<div class='columns' style='columns: 15em auto;'>
+<ul style='padding-top:0;'>
+<li><a href='c3ref/aggregate_context.html'>sqlite3_aggregate_context</a></li>
+<li><a href='c3ref/context_db_handle.html'>sqlite3_context_db_handle</a></li>
+<li><a href='c3ref/get_auxdata.html'>sqlite3_get_auxdata</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_blob</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_blob64</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_double</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_error</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_error16</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_error_code</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_error_nomem</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_error_toobig</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_int</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_int64</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_null</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_pointer</a></li>
+<li><a href='c3ref/result_subtype.html'>sqlite3_result_subtype</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_text</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_text16</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_text16be</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_text16le</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_text64</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_value</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_zeroblob</a></li>
+<li><a href='c3ref/result_blob.html'>sqlite3_result_zeroblob64</a></li>
+<li><a href='c3ref/get_auxdata.html'>sqlite3_set_auxdata</a></li>
+<li><a href='c3ref/user_data.html'>sqlite3_user_data</a></li>
+</ul>
+</div>
+
+
+</p><p>The <a href="c3ref/result_blob.html">sqlite3_result() family of functions</a> are
+used to specify the result of the scalar SQL function. One or more of
+these should be invoked by the callback to set the function return value.
+If none of these routines are invoked for a specific callback, then the
+return value will be NULL.
+
+</p><p>The <a href="c3ref/user_data.html">sqlite3_user_data()</a> routine returns a copy of the <b>pArg</b>
+pointer that was given to <a href="c3ref/create_function.html">sqlite3_create_function()</a> when the SQL
+function was created.
+
+</p><p>The <a href="c3ref/context_db_handle.html">sqlite3_context_db_handle()</a> routine returns a pointer to the
+<a href="c3ref/sqlite3.html">database connection</a> object.
+
+</p><p>The <a href="c3ref/aggregate_context.html">sqlite3_aggregate_context()</a> routine is used only in the
+implementations of aggregate and window functions. Scalar functions
+may not use <a href="c3ref/aggregate_context.html">sqlite3_aggregate_context()</a>. The <a href="c3ref/aggregate_context.html">sqlite3_aggregate_context()</a>
+function is included in the interface list only for completeness.
+
+</p><p>
+The 2nd and 3rd arguments to the scalar SQL function implemenetation,
+<b>argc</b> and <b>argv</b>, are
+the number of arguments to the SQL function itself and the values for
+each argument of the SQL function.
+Argument values can be of any datatype and are thus stored in
+instances of the <a href="c3ref/value.html">sqlite3_value</a> object.
+Specific C-language values can be extracted from this object using
+the <a href="c3ref/value_blob.html">sqlite3_value() family of interfaces</a>.
+
+</p><h3 id="the_aggregate_function_callbacks"><span>2.3.2. </span>The Aggregate Function Callbacks</h3>
+
+<p>Aggregate SQL functions are implemented by using two callback
+functions, <b>xStep</b> and <b>xFinal</b>. The xStep() function
+is called for each row of the aggregate and the xFinal() function
+is invoked to compute the final answer at the end.
+The following (slightly simplified) version of the built-in
+count() function illustrates:
+
+</p><div class="codeblock"><pre>typedef struct CountCtx CountCtx;
+struct CountCtx {
+ i64 n;
+};
+static void countStep(sqlite3_context *context, int argc, sqlite3_value **argv){
+ CountCtx *p;
+ p = sqlite3_aggregate_context(context, sizeof(*p));
+ if( (argc==0 || SQLITE_NULL!=sqlite3_value_type(argv&#91;0&#93;)) && p ){
+ p->n++;
+ }
+}
+static void countFinalize(sqlite3_context *context){
+ CountCtx *p;
+ p = sqlite3_aggregate_context(context, 0);
+ sqlite3_result_int64(context, p ? p->n : 0);
+}
+</pre></div>
+
+<p>Recall that there are two versions of the count() aggregate.
+With zero arguments, count() returns a count of the number of rows.
+With one argument, count() returns the number of times that the
+argument was non-NULL.
+
+</p><p>The countStep() callback is invoked once for each row in the aggregate.
+As you can see, the count is incremented if either there are no arguments,
+or if the one argument is not NULL.
+
+</p><p>The step function for an aggregate should always begin with a call
+to the <a href="c3ref/aggregate_context.html">sqlite3_aggregate_context()</a> routine to fetch the persistent
+state of the aggregate function. On the first invocation of the step()
+function, the aggregate context is initialized to a block of memory
+that is N bytes in size, where N is the second parameter to
+sqlite3_aggregate_context() and that memory is zeroed. On all subsequent
+calls to the step() function, the same block of memory is returned.
+Except, sqlite3_aggregate_context() might return NULL in the case of
+an out-of-memory error, so aggregate functions should be prepared to
+deal with that case.
+
+</p><p>After all rows are processed the countFinalize() routine is called
+exactly once. This routine computes the final result and invokes
+one of the <a href="c3ref/result_blob.html">sqlite3_result()</a> family of functions
+to set the final result. The aggregate context will be freed automatically
+by SQLite, though the xFinalize() routine must clean up any substructure
+associated with the aggregate context before it returns. If the xStep()
+method is called one or more times, then SQLite guarantees thta the
+xFinal() method will be called at once, even if the query aborts.
+
+</p><h3 id="the_window_function_callbacks"><span>2.3.3. </span>The Window Function Callbacks</h3>
+
+<p><a href="windowfunctions.html">Window functions</a> use the same xStep() and xFinal() callbacks that
+aggregate functions use, plus two others: <b>xValue</b> and <b>xInverse</b>.
+See the documentation on
+<a href="windowfunctions.html#udfwinfunc">application-defined window functions</a> for further details.
+
+</p><h3 id="examples"><span>2.3.4. </span>Examples</h3>
+
+<p>There are dozens and dozens of SQL function implementations scattered
+throughout the SQLite source code that can be used as example applications.
+The built-in SQL functions use the same interface as application-defined
+SQL functions, so built-in functions can be used as examples too.
+Search for "sqlite3_context" in the SQLite source code to find examples.
+
+<a name="sec"></a>
+
+</p><h1 id="security_implications"><span>3. </span>Security Implications</h1>
+
+<p>
+Application-defined SQL functions can become security vulnerabilities if
+not carefully managed. Suppose, for example, an application defines
+a new "system(X)" SQL function that runs its argument X as a command and
+returns the integer result code. Perhaps the implementation is like this:
+
+</p><div class="codeblock"><pre>static void systemFunc(
+ sqlite3_context *context,
+ int argc,
+ sqlite3_value **argv
+){
+ const char *zCmd = (const char*)sqlite3_value_text(argv&#91;0&#93;);
+ if( zCmd!=0 ){
+ int rc = system(zCmd);
+ sqlite3_result_int(context, rc);
+ }
+}
+</pre></div>
+
+<p>
+This is a function with powerful side-effects. Most programmers would
+be naturally cautious about using it, but probably would not see the
+harm in merely having it available. But there is great risk in merely
+defining such a function, even if the application itself never invokes
+it!
+
+</p><p>
+Suppose the application normally does a query against table TAB1
+when it starts up. If an attacker can gain access to the database
+file and modify the schema like this:
+
+</p><div class="codeblock"><pre>ALTER TABLE tab1 RENAME TO tab1_real;
+CREATE VIEW tab1 AS SELECT * FROM tab1 WHERE system('rm -rf *') IS NOT NULL;
+</pre></div>
+
+<p>
+Then, when the application attempts to open the database, register the
+system() function, then run an innocent query against the "tab1" table,
+it instead deletes all the files in its working directory. Yikes!
+
+</p><p>
+To prevent this kind of mischief, applications that create their own
+custom SQL functions should take one or more of the following safety
+precautions. The more precautions taken the better:
+
+</p><ol>
+<li><p>
+Invoke <a href="c3ref/db_config.html">sqlite3_db_config</a>(db,<a href="c3ref/c_dbconfig_defensive.html#sqlitedbconfigtrustedschema">SQLITE_DBCONFIG_TRUSTED_SCHEMA</a>,0,0)
+on each <a href="c3ref/sqlite3.html">database connection</a> as soon as it is opened.
+This prevents application-defined functions from being used in places
+where an attacker might be able to surreptiously invoke them by modifying
+a database schema:
+</p><ul>
+<li> In VIEWs.
+</li><li> In TRIGGERSs.
+</li><li> In CHECK constraints of a table definition.
+</li><li> In DEFAULT constraints of a table definition.
+</li><li> In the definitions of generated columns.
+</li><li> In the expression part of an index on an expression.
+</li><li> In the WHERE clause of a partial index.
+</li></ul>
+<p>
+To put it another way, this setting requires that application-defined
+functions only be run directly by top-level SQL invoked from the application
+itself, not as a consequence of doing some other innocent-looking query.
+
+</p></li><li><p>
+Use the <a href="pragma.html#pragma_trusted_schema">PRAGMA trusted_schema=OFF</a> SQL statement to disable trusted
+schema. This has the same effect as the previous bullet, but does not
+require the use of C-code and hence can be performed in programs written
+in another programming language and that do not have access SQLite
+C-language APIs.
+
+</p></li><li><p>
+Compile SQLite using the <a href="compile.html#trusted_schema">-DSQLITE_TRUSTED_SCHEMA=0</a> compile-time option.
+This make SQLite distrust application-defined functions inside of
+the schema by default.
+
+</p></li><li><p>
+If any application-defined SQL functions have potentially dangerous
+side-effects, or if they could potentially leak sensitive information
+to an attacker if misused, then tag those functions using the
+<a href="c3ref/c_deterministic.html#sqlitedirectonly">SQLITE_DIRECTONLY</a> option on the "enc" parameter. This means
+that the function can never be run from schema-code even if the
+trusted-schema option is on.
+
+</p></li><li><p>
+Never tag an application-defined SQL function with <a href="c3ref/c_deterministic.html#sqliteinnocuous">SQLITE_INNOCUOUS</a>
+unless you really need to and you have checked the implementation closely
+and are certain that it can do no harm even if it falls under the
+control of an attacker.
+</p></li></ol>
+<p align="center"><small><i>This page last modified on <a href="https://sqlite.org/docsrc/honeypot" id="mtimelink" data-href="https://sqlite.org/docsrc/finfo/pages/appfunc.in?m=da7fbd0b40">2023-02-27 02:07:35</a> UTC </small></i></p>
+