[DTrace-devel] [PATCH 4/4] Pad strings in the output buffer with NUL bytes after terminating byte

[eugene.loh at oracle.com]

From: Eugene Loh <eugene.loh at oracle.com>

The consumer checks if there are non-NUL bytes after the terminating
NUL byte to decide whether to print the contents of the output buffer
as a string or as raw bytes.  So, for strings, make sure that the
string is padded with NUL bytes.

A robust test that shows the problem can be hard to devise.  Some tests
encounter the problem.  This patch adds another test that might show
the problem.

Signed-off-by: Eugene Loh <eugene.loh at oracle.com>
---
 libdtrace/dt_cg.c                      | 46 ++++++++++++++++++--------
 test/unittest/error/tst.trace_string.d | 26 +++++++++++++++
 2 files changed, 59 insertions(+), 13 deletions(-)
 create mode 100644 test/unittest/error/tst.trace_string.d

diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
index 9b3592b9c..6dcf4cd3d 100644
--- a/libdtrace/dt_cg.c
+++ b/libdtrace/dt_cg.c
@@ -1596,6 +1596,22 @@ dt_cg_check_ptr_arg(dt_irlist_t *dlp, dt_regset_t *drp, dt_node_t *dnp,
 
 static void dt_cg_setx(dt_irlist_t *dlp, int reg, uint64_t x);
 
+/*
+ * Store a pointer to the 'memory block of zeros' in reg.
+ */
+static void
+dt_cg_zerosptr(int reg, dt_irlist_t *dlp, dt_regset_t *drp)
+{
+	dtrace_hdl_t	*dtp = yypcb->pcb_hdl;
+	dt_ident_t	*zero_off = dt_dlib_get_var(dtp, "ZERO_OFF");
+
+	dt_cg_access_dctx(reg, dlp, drp, DCTX_STRTAB);
+	emite(dlp, BPF_ALU64_IMM(BPF_ADD, reg, -1), zero_off);
+}
+
+/*
+ * Store a value to the output buffer.
+ */
 static int
 dt_cg_store_val(dt_pcb_t *pcb, dt_node_t *dnp, dtrace_actkind_t kind,
 		dt_pfargv_t *pfp, int arg)
@@ -1676,6 +1692,7 @@ dt_cg_store_val(dt_pcb_t *pcb, dt_node_t *dnp, dtrace_actkind_t kind,
 
 		goto ok;
 	} else if (dt_node_is_string(dnp)) {
+		uint_t	lbl_ok = dt_irlist_label(dlp);
 		size_t	strsize = dtp->dt_options[DTRACEOPT_STRSIZE];
 
 		if (!not_null)
@@ -1702,6 +1719,22 @@ dt_cg_store_val(dt_pcb_t *pcb, dt_node_t *dnp, dtrace_actkind_t kind,
 		dt_regset_xalloc(drp, BPF_REG_0);
 		emit(dlp, BPF_CALL_HELPER(BPF_FUNC_probe_read_str));
 		dt_regset_free_args(drp);
+
+		/*
+		 * Pad the rest with zeroes, if necessary.
+		 */
+		emit(dlp,  BPF_BRANCH_IMM(BPF_JGE, BPF_REG_0, strsize + 1, lbl_ok));
+		if (dt_regset_xalloc_args(drp) == -1)
+			longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
+		emit(dlp,  BPF_MOV_REG(BPF_REG_1, BPF_REG_9));
+		emit(dlp,  BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, off));
+		emit(dlp,  BPF_ALU64_REG(BPF_ADD, BPF_REG_1, BPF_REG_0));
+		emit(dlp,  BPF_MOV_IMM(BPF_REG_2, strsize + 1));
+		emit(dlp,  BPF_ALU64_REG(BPF_SUB, BPF_REG_2, BPF_REG_0));
+		dt_cg_zerosptr(BPF_REG_3, dlp, drp);
+		emit(dlp,  BPF_CALL_HELPER(dtp->dt_bpfhelper[BPF_FUNC_probe_read_kernel]));
+		dt_regset_free_args(drp);
+		emitl(dlp, lbl_ok, BPF_NOP());
 		dt_regset_free(drp, BPF_REG_0);
 		TRACE_REGSET("store_val(): End   ");
 
@@ -3042,19 +3075,6 @@ dt_cg_pop_stack(int reg, dt_irlist_t *dlp, dt_regset_t *drp)
 	dt_regset_free(drp, treg);
 }
 
-/*
- * Store a pointer to the 'memory block of zeros' in reg.
- */
-static void
-dt_cg_zerosptr(int reg, dt_irlist_t *dlp, dt_regset_t *drp)
-{
-	dtrace_hdl_t	*dtp = yypcb->pcb_hdl;
-	dt_ident_t	*zero_off = dt_dlib_get_var(dtp, "ZERO_OFF");
-
-	dt_cg_access_dctx(reg, dlp, drp, DCTX_STRTAB);
-	emite(dlp, BPF_ALU64_IMM(BPF_ADD, reg, -1), zero_off);
-}
-
 /*
  * Generate code to promote signed scalars (size < 64 bits) to native register
  * size (64 bits).
diff --git a/test/unittest/error/tst.trace_string.d b/test/unittest/error/tst.trace_string.d
new file mode 100644
index 000000000..4b06aef88
--- /dev/null
+++ b/test/unittest/error/tst.trace_string.d
@@ -0,0 +1,26 @@
+/*
+ * Oracle Linux DTrace.
+ * Copyright (c) 2025, 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: Test ERROR probe firing.
+ *
+ * SECTION: dtrace Provider
+ */
+
+#pragma D option quiet
+
+ERROR
+{
+	trace("Error fired");
+	exit(0);
+}
+
+BEGIN
+{
+	*(char *)NULL;
+	exit(1);
+}
-- 
2.43.5