lkdtm updates for v5.17-rc1

- Fix printk() usage during recursion (Ard Biesheuvel)
 - Fix rodata section to actually have contents (Christophe Leroy)
 - Add notes about lkdtm_kernel_info usage (Kees Cook)
 - Avoid stack-entropy selftest when LKDTM is disabled (Misono Tomohiro)
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Merge tag 'lkdtm-v5.17-rc1' of https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux into char-misc-next

Kees writes:

lkdtm updates for v5.17-rc1

- Fix printk() usage during recursion (Ard Biesheuvel)
- Fix rodata section to actually have contents (Christophe Leroy)
- Add notes about lkdtm_kernel_info usage (Kees Cook)
- Avoid stack-entropy selftest when LKDTM is disabled (Misono Tomohiro)

* tag 'lkdtm-v5.17-rc1' of https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  selftest/lkdtm: Skip stack-entropy test if lkdtm is not available
  lkdtm: Fix content of section containing lkdtm_rodata_do_nothing()
  lkdtm: avoid printk() in recursive_loop()
  lkdtm: Note that lkdtm_kernel_info should be removed in the future

drivers/misc/lkdtm/Makefile

@@ -20,7 +20,7 @@ CFLAGS_REMOVE_rodata.o		+= $(CC_FLAGS_LTO)
 
 OBJCOPYFLAGS :=
 OBJCOPYFLAGS_rodata_objcopy.o	:= \
-			--rename-section .noinstr.text=.rodata,alloc,readonly,load
+			--rename-section .noinstr.text=.rodata,alloc,readonly,load,contents
 targets += rodata.o rodata_objcopy.o
 $(obj)/rodata_objcopy.o: $(obj)/rodata.o FORCE
 	$(call if_changed,objcopy)

drivers/misc/lkdtm/bugs.c

@@ -41,20 +41,22 @@ static DEFINE_SPINLOCK(lock_me_up);
  * Make sure compiler does not optimize this function or stack frame away:
  * - function marked noinline
  * - stack variables are marked volatile
- * - stack variables are written (memset()) and read (pr_info())
- * - function has external effects (pr_info())
- * */
+ * - stack variables are written (memset()) and read (buf[..] passed as arg)
+ * - function may have external effects (memzero_explicit())
+ * - no tail recursion possible
+ */
 static int noinline recursive_loop(int remaining)
 {
 	volatile char buf[REC_STACK_SIZE];
+	volatile int ret;
 
 	memset((void *)buf, remaining & 0xFF, sizeof(buf));
-	pr_info("loop %d/%d ...\n", (int)buf[remaining % sizeof(buf)],
-		recur_count);
 	if (!remaining)
-		return 0;
+		ret = 0;
 	else
-		return recursive_loop(remaining - 1);
+		ret = recursive_loop((int)buf[remaining % sizeof(buf)] - 1);
+	memzero_explicit((void *)buf, sizeof(buf));
+	return ret;
 }
 
 /* If the depth is negative, use the default, otherwise keep parameter. */

drivers/misc/lkdtm/core.c

@@ -212,7 +212,11 @@ module_param(cpoint_count, int, 0644);
 MODULE_PARM_DESC(cpoint_count, " Crash Point Count, number of times the "\
 				"crash point is to be hit to trigger action");
 
-/* For test debug reporting. */
+/*
+ * For test debug reporting when CI systems provide terse summaries.
+ * TODO: Remove this once reasonable reporting exists in most CI systems:
+ * https://lore.kernel.org/lkml/CAHk-=wiFvfkoFixTapvvyPMN9pq5G-+Dys2eSyBa1vzDGAO5+A@mail.gmail.com
+ */
 char *lkdtm_kernel_info;
 
 /* Return the crashtype number or NULL if the name is invalid */

tools/testing/selftests/lkdtm/stack-entropy.sh

@@ -4,13 +4,27 @@
 # Measure kernel stack entropy by sampling via LKDTM's REPORT_STACK test.
 set -e
 samples="${1:-1000}"
+TRIGGER=/sys/kernel/debug/provoke-crash/DIRECT
+KSELFTEST_SKIP_TEST=4
+
+# Verify we have LKDTM available in the kernel.
+if [ ! -r $TRIGGER ] ; then
+	/sbin/modprobe -q lkdtm || true
+	if [ ! -r $TRIGGER ] ; then
+		echo "Cannot find $TRIGGER (missing CONFIG_LKDTM?)"
+	else
+		echo "Cannot write $TRIGGER (need to run as root?)"
+	fi
+	# Skip this test
+	exit $KSELFTEST_SKIP_TEST
+fi
 
 # Capture dmesg continuously since it may fill up depending on sample size.
 log=$(mktemp -t stack-entropy-XXXXXX)
 dmesg --follow >"$log" & pid=$!
 report=-1
 for i in $(seq 1 $samples); do
-        echo "REPORT_STACK" >/sys/kernel/debug/provoke-crash/DIRECT
+        echo "REPORT_STACK" > $TRIGGER
 	if [ -t 1 ]; then
 		percent=$(( 100 * $i / $samples ))
 		if [ "$percent" -ne "$report" ]; then