[DTrace-devel] [PATCH v2] parser: do not bounds-check arrays of size 0 or 1

[Nick Alcock]

This is both a pretty useless degenerate check to carry out, and breaks when
-fstrict-flex-arrays is used (whereupon trailing old-style flexible arrays
gain bounds of 0).  It is almost certainly wrong to use trailing zero-size
arrays and -fstrict-flex-arrays in conjunction, but since C doesn't
bounds-check this is commonplace.  In particular the Linux kernel has turned
this on *before* transitioning away from such arrays, rather than
afterwards.

Fixes system translators (in particular the pr_pgid and pr_sid members of
psinfo_t) in conjunction with Linux 6.5+.

Signed-off-by: Nick Alcock <nick.alcock at oracle.com>
---
 libdtrace/dt_parser.c                         | 24 ++++++++++---------
 .../unittest/arrays/tst.ctf-dynsized-bounds.d | 20 ++++++++++++++++
 2 files changed, 33 insertions(+), 11 deletions(-)
 create mode 100644 test/unittest/arrays/tst.ctf-dynsized-bounds.d

diff --git a/libdtrace/dt_parser.c b/libdtrace/dt_parser.c
index beb94676cd10c..f6addc78e32ba 100644
--- a/libdtrace/dt_parser.c
+++ b/libdtrace/dt_parser.c
@@ -3580,23 +3580,25 @@ dt_cook_op2(dt_node_t *dnp, uint_t idflags)
 
 		/*
 		 * Array bounds-checking.  (Non-associative arrays only.)
+		 *
+		 * Checking for arrays of size 0 and 1 is skipped: these
+		 * degenerate cases are often used for dynamically-sized arrays
+		 * at the ends of structures.
 		 */
 		artype = ctf_type_resolve(lp->dn_ctfp, lp->dn_type);
 		arkind = ctf_type_kind(lp->dn_ctfp, artype);
 
 		if (arkind == CTF_K_ARRAY &&
 		    !(lp->dn_kind == DT_NODE_VAR &&
-			lp->dn_ident->di_kind == DT_IDENT_ARRAY)) {
-			ctf_array_info(lp->dn_ctfp, artype, &r);
-
-			if (rp->dn_kind == DT_NODE_INT &&
-			    ctf_array_info(lp->dn_ctfp, type, &r) == 0 &&
-			    rp->dn_value >= r.ctr_nelems)
-				xyerror(D_ARR_BOUNDS, "index outside "
-				    "array bounds: %llu, max is %i\n",
-				    (long long unsigned)rp->dn_value,
-				    r.ctr_nelems);
-		}
+			lp->dn_ident->di_kind == DT_IDENT_ARRAY) &&
+		    rp->dn_kind == DT_NODE_INT &&
+		    ctf_array_info(lp->dn_ctfp, type, &r) == 0 &&
+		    r.ctr_nelems > 1 &&
+		    rp->dn_value >= r.ctr_nelems)
+			xyerror(D_ARR_BOUNDS, "index outside "
+				"array bounds: %llu, max is %i\n",
+				(long long unsigned)rp->dn_value,
+				r.ctr_nelems);
 
 		dt_node_type_assign(dnp, ctfp, type);
 		dt_node_attr_assign(dnp, dt_attr_min(lp->dn_attr, rp->dn_attr));
diff --git a/test/unittest/arrays/tst.ctf-dynsized-bounds.d b/test/unittest/arrays/tst.ctf-dynsized-bounds.d
new file mode 100644
index 0000000000000..9479a1e14c9b6
--- /dev/null
+++ b/test/unittest/arrays/tst.ctf-dynsized-bounds.d
@@ -0,0 +1,20 @@
+/*
+ * Oracle Linux DTrace.
+ * Copyright (c) 2018, 2022, Oracle and/or its affiliates. All rights reserved.
+ * Licensed under the Universal Permissive License v 1.0 as shown at
+ * http://oss.oracle.com/licenses/upl.
+ */
+
+/*
+ * ASSERTION:
+ * 	Array accesses work for CTF-declared arrays of dynamic size.
+ * 	pr_pgid is one such.
+ *
+ * SECTION: Pointers and Arrays/Array Declarations and Storage
+ */
+
+BEGIN
+{
+	trace(curpsinfo->pr_pgid);
+	exit(0);
+}
-- 
2.42.0.271.g85384428f1