scst_main: Terminate trace output for an empty CDB

The saved Cppcheck report warns about the formatter output:

  scst/src/scst_main.c:637: warning: Uninitialized variable: buf
  [uninitvar]

scst_dump_cdb does not write anything when cdb_len is zero, but
scst_trace_cmds prints its stack buffer as a string. Unknown opcode
groups can produce an empty CDB, so termination must not depend on
valid command contents.

Initialize nonempty output buffers and leave zero-capacity buffers
untouched. Add KUnit coverage for empty, ordinary, truncated, one-byte
and zero-size output. Include the new private test header in generated
kernel patches.

scripts/kernel/generate-kernel-patch

@@ -315,6 +315,7 @@ scst/src/scst_event.c scst/src/scst_event_priv.h scst/src/scst_no_dlm.c \
 scst/src/scst_test.h \
 scst/src/scst_event_test.h \
 scst/src/scst_dlm_test.h \
+scst/src/scst_main_test.h \
 scst/src/scst_copy_mgr_test.h"
 scst_05_targ="scst/src/scst_targ.c scst/src/scst_local_cmd.c \
 scst/src/scst_local_cmd.h \

scst/src/kunit/lib/suites/scst_kunit_lib.c

@@ -8,6 +8,7 @@
 #endif
 
 #include "../../../scst_lib_test.h"
+#include "../../../scst_main_test.h"
 #include "../../../scst_dlm.h"
 #include "../../../scst_dlm_test.h"
 
@@ -198,7 +199,45 @@ static void scst_dlm_directory_error_preserves_membership(struct kunit *test)
 }
 #endif
 
+static void scst_cdb_trace_bounds(struct kunit *test)
+{
+	static const struct {
+		unsigned int cdb_len;
+		size_t capacity;
+		const char *expected;
+	} cases[] = {
+		{ 0, 16, "" },
+		{ 3, 16, "00 ab ff" },
+		{ 3, 5, "00 a" },
+		{ 3, 1, "" },
+	};
+	u8 cdb[] = { 0x00, 0xab, 0xff };
+	struct scst_cmd *cmd;
+	char buf[18];
+	unsigned int i;
+
+	cmd = kunit_kzalloc(test, sizeof(*cmd), GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, cmd);
+	cmd->cdb = cdb;
+	for (i = 0; i < ARRAY_SIZE(cases); i++) {
+		memset(buf, 'x', sizeof(buf));
+		cmd->cdb_len = cases[i].cdb_len;
+		KUNIT_EXPECT_PTR_EQ(test, scst_dump_cdb(buf + 1, cases[i].capacity, cmd),
+				    buf + 1);
+		KUNIT_EXPECT_EQ(test, buf[0], 'x');
+		KUNIT_EXPECT_EQ(test, buf[cases[i].capacity + 1], 'x');
+		/* memcmp bounds the check even if the formatter misses the NUL. */
+		KUNIT_EXPECT_EQ(test, memcmp(buf + 1, cases[i].expected,
+					     strlen(cases[i].expected) + 1), 0);
+	}
+
+	memset(buf, 'x', sizeof(buf));
+	KUNIT_EXPECT_PTR_EQ(test, scst_dump_cdb(buf + 1, 0, cmd), buf + 1);
+	KUNIT_EXPECT_EQ(test, buf[1], 'x');
+}
+
 static struct kunit_case scst_lib_cases[] = {
+	KUNIT_CASE(scst_cdb_trace_bounds),
 #if (defined(CONFIG_DLM) || defined(CONFIG_DLM_MODULE)) && !defined(CONFIG_SCST_NO_DLM)
 	KUNIT_CASE(scst_dlm_directory_error_preserves_membership),
 #endif

scst/src/scst_main.c

@@ -39,7 +39,7 @@
 #include "scst_priv.h"
 #include "scst_mem.h"
 #include "scst_pres.h"
-#include "scst_test.h"
+#include "scst_main_test.h"
 
 #undef DEFAULT_SYMBOL_NAMESPACE
 #define DEFAULT_SYMBOL_NAMESPACE	SCST_NAMESPACE
@@ -626,11 +626,17 @@ char *scst_get_cmd_state_name(char *name, int len, unsigned int state)
 	return name;
 }
 
-static char *scst_dump_cdb(char *buf, int buf_len, struct scst_cmd *cmd)
+SCST_KUNIT_VISIBLE
+char *scst_dump_cdb(char *buf, size_t buf_len, const struct scst_cmd *cmd)
 {
-	char *p = buf, *end = buf + buf_len;
+	char *p = buf, *end;
 	int i;
 
+	if (!buf_len)
+		return buf;
+	*buf = '\0';
+	end = buf + buf_len;
+
 	for (i = 0; i < cmd->cdb_len && p < end; i++)
 		p += scnprintf(p, end - p, "%s%02x", i ? " " : "", cmd->cdb[i]);
 

scst/src/scst_main_test.h

@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef SCST_MAIN_TEST_H
+#define SCST_MAIN_TEST_H
+
+#include "scst_test.h"
+
+#ifdef SCST_KUNIT_ENABLED
+#include <linux/types.h>
+
+struct scst_cmd;
+
+char *scst_dump_cdb(char *buf, size_t buf_len, const struct scst_cmd *cmd);
+#endif
+
+#endif /* SCST_MAIN_TEST_H */