RFE: add transaction support to the libseccomp API

doc/man/man3/seccomp_transaction_start.3

@@ -0,0 +1,134 @@
+.TH "seccomp_transaction_start" 3 "21 September 2023" "[email protected]" "libseccomp Documentation"
+.\" //////////////////////////////////////////////////////////////////////////
+.SH NAME
+.\" //////////////////////////////////////////////////////////////////////////
+seccomp_transaction_start, seccomp_transaction_commit, seccomp_transaction_reject \- Manage seccomp filter transactions
+.\" //////////////////////////////////////////////////////////////////////////
+.SH SYNOPSIS
+.\" //////////////////////////////////////////////////////////////////////////
+.nf
+.B #include <seccomp.h>
+.sp
+.B typedef void * scmp_filter_ctx;
+.sp
+.BI "int seccomp_transaction_start(scmp_filter_ctx " ctx ");
+.BI "int seccomp_transaction_commit(scmp_filter_ctx " ctx ");
+.BI "void seccomp_transaction_reject(scmp_filter_ctx " ctx ");
+.sp
+Link with \fI\-lseccomp\fP.
+.fi
+.\" //////////////////////////////////////////////////////////////////////////
+.SH DESCRIPTION
+.\" //////////////////////////////////////////////////////////////////////////
+.P
+The
+.BR seccomp_transaction_start ()
+function starts a new seccomp filter
+transaction that the caller can use to perform any number of filter
+modifications which can then be committed to the filter using
+.BR seccomp_transaction_commit ()
+or rejected using
+.BR seccomp_transaction_reject ().
+It is important to note that transactions only affect the seccomp filter state
+while it is being managed by libseccomp; seccomp filters which have been loaded
+into the kernel can not be modified, only new seccomp filters can be added on
+top of the existing loaded filter stack.
+.P
+Finishing, or committing, a transaction is optional, although it is encouraged.
+At any point in time, regardless of the transaction state, the seccomp filter
+is determined by all of the libseccomp operations performed on the filter up to
+that point.  Committing a transaction simply flushes the transaction rollback
+marker of the current transaction making the filter changes permanent;
+rejecting a transaction rolls the filter state back to immediately before the
+transaction was started.
+.P
+Transactions can be nested arbitrarily deep with the
+.BR seccomp_transaction_commit ()
+and
+.BR seccomp_transaction_reject ()
+functions always operating on the deepest, or more recently started transaction.
+A nested set of filter modifications, even if committed, is still subject to
+rejection by shallower, or older transactions that have yet to be committed or
+rejected.
+.\" //////////////////////////////////////////////////////////////////////////
+.SH RETURN VALUE
+.\" //////////////////////////////////////////////////////////////////////////
+The
+.BR seccomp_transaction_start ()
+and
+.BR seccomp_transaction_commit ()
+functions return zero on success or one of the following error codes on
+failure:
+.TP
+.B -ENOMEM
+The library was unable to allocate enough memory.
+.\" //////////////////////////////////////////////////////////////////////////
+.SH EXAMPLES
+.\" //////////////////////////////////////////////////////////////////////////
+.nf
+#include <seccomp.h>
+
+int libseccomp_generate(scmp_filter_ctx *ctx)
+{
+	int rc;
+
+	rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(close), 0);
+	if (rc)
+		return rc;
+	rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(exit_group), 0);
+	if (rc)
+		return rc;
+	rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(exit), 0);
+	if (rc)
+		return rc;
+
+	return 0;
+}
+
+int main(int argc, char *argv[])
+{
+	int rc = \-1;
+	scmp_filter_ctx ctx;
+
+	ctx = seccomp_init(SCMP_ACT_KILL);
+	if (ctx == NULL)
+		goto out;
+
+	rc = seccomp_transaction_start(ctx)
+	if (rc)
+		goto out;
+	rc = libseccomp_generate(ctx);
+	if (rc == 0) {
+		rc = seccomp_transaction_commit(ctx);
+		if (rc)
+			goto out;
+	} else
+		seccomp_transaction_reject(ctx);
+
+	/* ... */
+
+out:
+	seccomp_release(ctx);
+	return \-rc;
+}
+.fi
+.\" //////////////////////////////////////////////////////////////////////////
+.SH NOTES
+.\" //////////////////////////////////////////////////////////////////////////
+.P
+While the seccomp filter can be generated independent of the kernel, kernel
+support is required to load and enforce the seccomp filter generated by
+libseccomp.
+.P
+The libseccomp project site, with more information and the source code
+repository, can be found at https://github.com/seccomp/libseccomp.  This tool,
+as well as the libseccomp library, is currently under development, please
+report any bugs at the project site or directly to the author.
+.\" //////////////////////////////////////////////////////////////////////////
+.SH AUTHOR
+.\" //////////////////////////////////////////////////////////////////////////
+Paul Moore <[email protected]>
+.\" //////////////////////////////////////////////////////////////////////////
+.SH SEE ALSO
+.\" //////////////////////////////////////////////////////////////////////////
+.BR seccomp_init (3),

include/seccomp.h.in

@@ -849,6 +849,35 @@ int seccomp_export_bpf(const scmp_filter_ctx ctx, int fd);
  */
 int seccomp_export_bpf_mem(const scmp_filter_ctx ctx, void *buf, size_t *len);
 
+/**
+ * Start a filter transaction
+ * @param ctx the filter context
+ *
+ * This function starts a filter transaction for modifying the seccomp filter.
+ * Returns zero on success, negative values on failure.
+ *
+ */
+int seccomp_transaction_start(const scmp_filter_ctx ctx);
+
+/**
+ * Reject the current filter transaction
+ * @param ctx the filter context
+ *
+ * This function rejects the current seccomp filter transaction.
+ *
+ */
+void seccomp_transaction_reject(const scmp_filter_ctx ctx);
+
+/**
+ * Commit the current filter transaction
+ * @param ctx the filter context
+ *
+ * This function commits the current seccomp filter transaction.  Returns zero
+ * on success, negative values on failure.
+ *
+ */
+int seccomp_transaction_commit(const scmp_filter_ctx ctx);
+
 /**
  * Precompute the seccomp filter for future use
  * @param ctx the filter context

src/api.c

@@ -793,6 +793,45 @@ API int seccomp_export_bpf_mem(const scmp_filter_ctx ctx, void *buf,
 	return rc;
 }
 
+/* NOTE - function header comment in include/seccomp.h */
+API int seccomp_transaction_start(const scmp_filter_ctx ctx)
+{
+	int rc;
+	struct db_filter_col *col;
+
+	if (_ctx_valid(ctx))
+		return _rc_filter(-EINVAL);
+	col = (struct db_filter_col *)ctx;
+
+	rc = db_col_transaction_start(col, true);
+	return _rc_filter(rc);
+}
+
+/* NOTE - function header comment in include/seccomp.h */
+API void seccomp_transaction_reject(const scmp_filter_ctx ctx)
+{
+	struct db_filter_col *col;
+
+	if (_ctx_valid(ctx))
+		return;
+	col = (struct db_filter_col *)ctx;
+
+	db_col_transaction_abort(col, true);
+}
+
+/* NOTE - function header comment in include/seccomp.h */
+API int seccomp_transaction_commit(const scmp_filter_ctx ctx)
+{
+	struct db_filter_col *col;
+
+	if (_ctx_valid(ctx))
+		return _rc_filter(-EINVAL);
+	col = (struct db_filter_col *)ctx;
+
+	db_col_transaction_commit(col, true);
+	return _rc_filter(0);
+}
+
 /* NOTE - function header comment in include/seccomp.h */
 API int seccomp_precompute(const scmp_filter_ctx ctx)
 {

src/db.c

@@ -2369,7 +2369,7 @@ int db_col_rule_add(struct db_filter_col *col,
 	}
 
 	/* create a checkpoint */
-	rc = db_col_transaction_start(col);
+	rc = db_col_transaction_start(col, false);
 	if (rc != 0)
 		goto add_return;
 
@@ -2396,9 +2396,9 @@ int db_col_rule_add(struct db_filter_col *col,
 
 	/* commit the transaction or abort */
 	if (rc == 0)
-		db_col_transaction_commit(col);
+		db_col_transaction_commit(col, false);
 	else
-		db_col_transaction_abort(col);
+		db_col_transaction_abort(col, false);
 
 add_return:
 	/* update the misc state */
@@ -2415,12 +2415,13 @@ int db_col_rule_add(struct db_filter_col *col,
 /**
  * Start a new seccomp filter transaction
  * @param col the filter collection
+ * @param user true if initiated by a user
  *
  * This function starts a new seccomp filter transaction for the given filter
  * collection.  Returns zero on success, negative values on failure.
  *
  */
-int db_col_transaction_start(struct db_filter_col *col)
+int db_col_transaction_start(struct db_filter_col *col, bool user)
 {
 	int rc;
 	unsigned int iter;
@@ -2439,6 +2440,7 @@ int db_col_transaction_start(struct db_filter_col *col)
 		 *       transaction is current/correct */
 
 		col->snapshots->shadow = false;
+		col->snapshots->user = user;
 		return 0;
 	}
 
@@ -2486,6 +2488,8 @@ int db_col_transaction_start(struct db_filter_col *col)
 		} while (rule_o != filter_o->rules);
 	}
 
+	snap->user = user;
+
 	/* add the snapshot to the list */
 	snap->next = col->snapshots;
 	col->snapshots = snap;
@@ -2502,23 +2506,35 @@ int db_col_transaction_start(struct db_filter_col *col)
 /**
  * Abort the top most seccomp filter transaction
  * @param col the filter collection
+ * @param user true if initiated by a user
  *
  * This function aborts the most recent seccomp filter transaction.
  *
  */
-void db_col_transaction_abort(struct db_filter_col *col)
+void db_col_transaction_abort(struct db_filter_col *col, bool user)
 {
 	int iter;
 	unsigned int filter_cnt;
 	struct db_filter **filters;
 	struct db_filter_snap *snap;
 
-	if (col->snapshots == NULL)
+	snap = col->snapshots;
+	if (snap == NULL)
+		return;
+
+	/* replace the current filter with the last snapshot, skipping shadow
+	 * snapshots are they are duplicates of the current snapshot */
+	if (snap->shadow) {
+		struct db_filter_snap *tmp = snap;
+		snap = snap->next;
+		_db_snap_release(tmp);
+	}
+
+	if (snap->user != user)
 		return;
 
-	/* replace the current filter with the last snapshot */
-	snap = col->snapshots;
 	col->snapshots = snap->next;
+
 	filter_cnt = col->filter_cnt;
 	filters = col->filters;
 	col->filter_cnt = snap->filter_cnt;
@@ -2537,13 +2553,14 @@ void db_col_transaction_abort(struct db_filter_col *col)
 /**
  * Commit the top most seccomp filter transaction
  * @param col the filter collection
+ * @param user true if initiated by a user
  *
  * This function commits the most recent seccomp filter transaction and
  * attempts to create a shadow transaction that is a duplicate of the current
  * filter to speed up future transactions.
  *
  */
-void db_col_transaction_commit(struct db_filter_col *col)
+void db_col_transaction_commit(struct db_filter_col *col, bool user)
 {
 	int rc;
 	unsigned int iter;
@@ -2559,12 +2576,16 @@ void db_col_transaction_commit(struct db_filter_col *col)
 	if (snap->shadow) {
 		/* leave the shadow intact, but drop the next snapshot */
 		if (snap->next) {
-			snap->next = snap->next->next;
-			_db_snap_release(snap->next);
+			struct db_filter_snap *tmp = snap->next;
+			snap->next = tmp->next;
+			_db_snap_release(tmp);
 		}
 		return;
 	}
 
+	if (snap->user != user)
+		return;
+
 	/* adjust the number of filters if needed */
 	if (col->filter_cnt > snap->filter_cnt) {
 		unsigned int tmp_i;
@@ -2593,8 +2614,8 @@ void db_col_transaction_commit(struct db_filter_col *col)
 		 *       at the cost of a not reaping all the memory possible */
 
 		do {
-			_db_release(snap->filters[snap->filter_cnt--]);
-		} while (snap->filter_cnt > col->filter_cnt);
+			_db_release(snap->filters[--snap->filter_cnt]);
+		} while (col->filter_cnt < snap->filter_cnt);
 	}
 
 	/* loop through each filter and update the rules on the snapshot */

src/db.h

@@ -145,6 +145,7 @@ struct db_filter_snap {
 	struct db_filter **filters;
 	unsigned int filter_cnt;
 	bool shadow;
+	bool user;
 
 	struct db_filter_snap *next;
 };
@@ -215,9 +216,9 @@ int db_col_rule_add(struct db_filter_col *col,
 int db_col_syscall_priority(struct db_filter_col *col,
 			    int syscall, uint8_t priority);
 
-int db_col_transaction_start(struct db_filter_col *col);
-void db_col_transaction_abort(struct db_filter_col *col);
-void db_col_transaction_commit(struct db_filter_col *col);
+int db_col_transaction_start(struct db_filter_col *col, bool user);
+void db_col_transaction_abort(struct db_filter_col *col, bool user);
+void db_col_transaction_commit(struct db_filter_col *col, bool user);
 
 int db_col_precompute(struct db_filter_col *col);
 void db_col_precompute_reset(struct db_filter_col *col);

src/python/libseccomp.pxd

@@ -173,6 +173,10 @@ cdef extern from "seccomp.h":
     int seccomp_export_bpf_mem(const scmp_filter_ctx ctx, void *buf,
                                size_t *len)
 
+    int seccomp_transaction_start(const scmp_filter_ctx ctx)
+    void seccomp_transaction_reject(const scmp_filter_ctx ctx)
+    int seccomp_transaction_commit(const scmp_filter_ctx ctx)
+
     int seccomp_precompute(const scmp_filter_ctx ctx)
 
 # kate: syntax python;