selftests/powerpc/dexcr: Add chdexcr utility

Adds a utility to exercise the prctl DEXCR inheritance in the shell.
Supports setting and clearing each aspect.
Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
[mpe: Use correct SPDX license, use execvp() for usability, print errors]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240417112325.728010-9-bgray@linux.ibm.com

tools/testing/selftests/powerpc/dexcr/.gitignore

 dexcr_test
 hashchk_test
+chdexcr
 lsdexcr

tools/testing/selftests/powerpc/dexcr/Makefile

 TEST_GEN_PROGS := dexcr_test hashchk_test
-TEST_GEN_FILES := lsdexcr
+TEST_GEN_FILES := lsdexcr chdexcr
 
 include ../../lib.mk
 include ../flags.mk

tools/testing/selftests/powerpc/dexcr/chdexcr.c

+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include <errno.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/prctl.h>
+
+#include "dexcr.h"
+#include "utils.h"
+
+static void die(const char *msg)
+{
+	printf("%s\n", msg);
+	exit(1);
+}
+
+static void help(void)
+{
+	printf("Invoke a provided program with a custom DEXCR on-exec reset value\n"
+	       "\n"
+	       "usage: chdexcr [CHDEXCR OPTIONS] -- PROGRAM [ARGS...]\n"
+	       "\n"
+	       "Each configurable DEXCR aspect is exposed as an option.\n"
+	       "\n"
+	       "The normal option sets the aspect in the DEXCR. The --no- variant\n"
+	       "clears that aspect. For example, --ibrtpd sets the IBRTPD aspect bit,\n"
+	       "so indirect branch predicition will be disabled in the provided program.\n"
+	       "Conversely, --no-ibrtpd clears the aspect bit, so indirect branch\n"
+	       "prediction may occur.\n"
+	       "\n"
+	       "CHDEXCR OPTIONS:\n");
+
+	for (int i = 0; i < ARRAY_SIZE(aspects); i++) {
+		const struct dexcr_aspect *aspect = &aspects[i];
+
+		if (aspect->prctl == -1)
+			continue;
+
+		printf("  --%-6s / --no-%-6s : %s\n", aspect->opt, aspect->opt, aspect->desc);
+	}
+}
+
+static const struct dexcr_aspect *opt_to_aspect(const char *opt)
+{
+	for (int i = 0; i < ARRAY_SIZE(aspects); i++)
+		if (aspects[i].prctl != -1 && !strcmp(aspects[i].opt, opt))
+			return &aspects[i];
+
+	return NULL;
+}
+
+static int apply_option(const char *option)
+{
+	const struct dexcr_aspect *aspect;
+	const char *opt = NULL;
+	const char *set_prefix = "--";
+	const char *clear_prefix = "--no-";
+	unsigned long ctrl = 0;
+	int err;
+
+	if (!strcmp(option, "-h") || !strcmp(option, "--help")) {
+		help();
+		exit(0);
+	}
+
+	/* Strip out --(no-) prefix and determine ctrl value */
+	if (!strncmp(option, clear_prefix, strlen(clear_prefix))) {
+		opt = &option[strlen(clear_prefix)];
+		ctrl |= PR_PPC_DEXCR_CTRL_CLEAR_ONEXEC;
+	} else if (!strncmp(option, set_prefix, strlen(set_prefix))) {
+		opt = &option[strlen(set_prefix)];
+		ctrl |= PR_PPC_DEXCR_CTRL_SET_ONEXEC;
+	}
+
+	if (!opt || !*opt)
+		return 1;
+
+	aspect = opt_to_aspect(opt);
+	if (!aspect)
+		die("unknown aspect");
+
+	err = pr_set_dexcr(aspect->prctl, ctrl);
+	if (err)
+		die("failed to apply option");
+
+	return 0;
+}
+
+int main(int argc, char *const argv[])
+{
+	int i;
+
+	if (!dexcr_exists())
+		die("DEXCR not detected on this hardware");
+
+	for (i = 1; i < argc; i++)
+		if (apply_option(argv[i]))
+			break;
+
+	if (i < argc && !strcmp(argv[i], "--"))
+		i++;
+
+	if (i >= argc)
+		die("missing command");
+
+	execvp(argv[i], &argv[i]);
+	perror("execve");
+
+	return errno;
+}

tools/testing/selftests/powerpc/dexcr/dexcr.h

@@ -9,6 +9,7 @@
 #define _SELFTESTS_POWERPC_DEXCR_DEXCR_H
 
 #include <stdbool.h>
+#include <sys/prctl.h>
 #include <sys/types.h>
 
 #include "reg.h"
@@ -26,6 +27,52 @@
 #define PPC_RAW_HASHCHK(b, i, a) \
 	str(.long (0x7C0005E4 | PPC_RAW_HASH_ARGS(b, i, a));)
 
+struct dexcr_aspect {
+	const char *name;	/* Short display name */
+	const char *opt;	/* Option name for chdexcr */
+	const char *desc;	/* Expanded aspect meaning */
+	unsigned int index;	/* Aspect bit index in DEXCR */
+	unsigned long prctl;	/* 'which' value for get/set prctl */
+};
+
+static const struct dexcr_aspect aspects[] = {
+	{
+		.name = "SBHE",
+		.opt = "sbhe",
+		.desc = "Speculative branch hint enable",
+		.index = 0,
+		.prctl = PR_PPC_DEXCR_SBHE,
+	},
+	{
+		.name = "IBRTPD",
+		.opt = "ibrtpd",
+		.desc = "Indirect branch recurrent target prediction disable",
+		.index = 3,
+		.prctl = PR_PPC_DEXCR_IBRTPD,
+	},
+	{
+		.name = "SRAPD",
+		.opt = "srapd",
+		.desc = "Subroutine return address prediction disable",
+		.index = 4,
+		.prctl = PR_PPC_DEXCR_SRAPD,
+	},
+	{
+		.name = "NPHIE",
+		.opt = "nphie",
+		.desc = "Non-privileged hash instruction enable",
+		.index = 5,
+		.prctl = PR_PPC_DEXCR_NPHIE,
+	},
+	{
+		.name = "PHIE",
+		.opt = "phie",
+		.desc = "Privileged hash instruction enable",
+		.index = 6,
+		.prctl = -1,
+	},
+};
+
 bool dexcr_exists(void);
 
 bool pr_dexcr_aspect_supported(unsigned long which);

tools/testing/selftests/powerpc/dexcr/lsdexcr.c

@@ -12,52 +12,6 @@ static unsigned int dexcr;
 static unsigned int hdexcr;
 static unsigned int effective;
 
-struct dexcr_aspect {
-	const char *name;
-	const char *desc;
-	unsigned int index;
-	unsigned long prctl;
-	const char *sysctl;
-};
-
-static const struct dexcr_aspect aspects[] = {
-	{
-		.name = "SBHE",
-		.desc = "Speculative branch hint enable",
-		.index = 0,
-		.prctl = PR_PPC_DEXCR_SBHE,
-		.sysctl = "speculative_branch_hint_enable",
-	},
-	{
-		.name = "IBRTPD",
-		.desc = "Indirect branch recurrent target prediction disable",
-		.index = 3,
-		.prctl = PR_PPC_DEXCR_IBRTPD,
-		.sysctl = "indirect_branch_recurrent_target_prediction_disable",
-	},
-	{
-		.name = "SRAPD",
-		.desc = "Subroutine return address prediction disable",
-		.index = 4,
-		.prctl = PR_PPC_DEXCR_SRAPD,
-		.sysctl = "subroutine_return_address_prediction_disable",
-	},
-	{
-		.name = "NPHIE",
-		.desc = "Non-privileged hash instruction enable",
-		.index = 5,
-		.prctl = PR_PPC_DEXCR_NPHIE,
-		.sysctl = "nonprivileged_hash_instruction_enable",
-	},
-	{
-		.name = "PHIE",
-		.desc = "Privileged hash instruction enable",
-		.index = 6,
-		.prctl = -1,
-		.sysctl = NULL,
-	},
-};
-
 static void print_list(const char *list[], size_t len)
 {
 	for (size_t i = 0; i < len; i++) {
@@ -117,89 +71,57 @@ static void print_aspect(const struct dexcr_aspect *aspect)
 
 static void print_aspect_config(const struct dexcr_aspect *aspect)
 {
-	char sysctl_path[128] = "/proc/sys/kernel/dexcr/";
-	const char *reason = "unknown";
+	const char *reason = NULL;
 	const char *reason_hyp = NULL;
-	const char *reason_sysctl = "no sysctl";
 	const char *reason_prctl = "no prctl";
 	bool actual = effective & DEXCR_PR_BIT(aspect->index);
-	bool expected = false;
-
-	long sysctl_ctrl = 0;
-	int prctl_ctrl = 0;
-	int err;
-
-	if (aspect->prctl >= 0) {
-		prctl_ctrl = pr_get_dexcr(aspect->prctl);
-		if (prctl_ctrl < 0)
-			reason_prctl = "(failed to read prctl)";
-		else {
-			if (prctl_ctrl & PR_PPC_DEXCR_CTRL_SET) {
+	bool expected = actual;  /* Assume it's fine if we don't expect a specific set/clear value */
+
+	if (actual)
+		reason = "set by unknown";
+	else
+		reason = "cleared by unknown";
+
+	if (aspect->prctl != -1) {
+		int ctrl = pr_get_dexcr(aspect->prctl);
+
+		if (ctrl < 0) {
+			reason_prctl = "failed to read prctl";
+		} else {
+			if (ctrl & PR_PPC_DEXCR_CTRL_SET) {
 				reason_prctl = "set by prctl";
 				expected = true;
-			} else if (prctl_ctrl & PR_PPC_DEXCR_CTRL_CLEAR) {
+			} else if (ctrl & PR_PPC_DEXCR_CTRL_CLEAR) {
 				reason_prctl = "cleared by prctl";
 				expected = false;
-			} else
+			} else {
 				reason_prctl = "unknown prctl";
+			}
 
 			reason = reason_prctl;
 		}
 	}
 
-	if (aspect->sysctl) {
-		strcat(sysctl_path, aspect->sysctl);
-		err = read_long(sysctl_path, &sysctl_ctrl, 10);
-		if (err)
-			reason_sysctl = "(failed to read sysctl)";
-		else {
-			switch (sysctl_ctrl) {
-			case 0:
-				reason_sysctl = "cleared by sysctl";
-				reason = reason_sysctl;
-				expected = false;
-				break;
-			case 1:
-				reason_sysctl = "set by sysctl";
-				reason = reason_sysctl;
-				expected = true;
-				break;
-			case 2:
-				reason_sysctl = "not modified by sysctl";
-				break;
-			case 3:
-				reason_sysctl = "cleared by sysctl (permanent)";
-				reason = reason_sysctl;
-				expected = false;
-				break;
-			case 4:
-				reason_sysctl = "set by sysctl (permanent)";
-				reason = reason_sysctl;
-				expected = true;
-				break;
-			default:
-				reason_sysctl = "unknown sysctl";
-				break;
-			}
-		}
-	}
-
-
 	if (hdexcr & DEXCR_PR_BIT(aspect->index)) {
 		reason_hyp = "set by hypervisor";
 		reason = reason_hyp;
 		expected = true;
-	} else
+	} else {
 		reason_hyp = "not modified by hypervisor";
+	}
 
-	printf("%12s (%d): %-28s (%s, %s, %s)\n",
+	printf("%12s (%d): %-28s (%s, %s)\n",
 	       aspect->name,
 	       aspect->index,
 	       reason,
 	       reason_hyp,
-	       reason_sysctl,
 	       reason_prctl);
 
+	/*
+	 * The checks are not atomic, so this can technically trigger if the
+	 * hypervisor makes a change while we are checking each source. It's
+	 * far more likely to be a bug if we see this though.
+	 */
 	if (actual != expected)
 		printf("                : ! actual %s does not match config\n", aspect->name);
 }