[PATCH 4/4] selftests/exec: add test for HWCAP inheritance

[Andrei Vagin <avagin@google.com>]

Verify that HWCAPs are correctly inherited/preserved across execve() when
modified via prctl(PR_SET_MM_AUXV).

The test performs the following steps:
* reads the current AUXV using prctl(PR_GET_AUXV);
* finds an HWCAP entry and toggles its most significant bit;
* replaces the AUXV of the current process with the modified one using
  prctl(PR_SET_MM, PR_SET_MM_AUXV);
* executes itself to verify that the new program sees the modified HWCAP
  value.

Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@futurfusion.io>
Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Andrei Vagin <avagin@google.com>
---
 tools/testing/selftests/exec/.gitignore      |   1 +
 tools/testing/selftests/exec/Makefile        |   1 +
 tools/testing/selftests/exec/hwcap_inherit.c | 105 +++++++++++++++++++
 3 files changed, 107 insertions(+)
 create mode 100644 tools/testing/selftests/exec/hwcap_inherit.c

diff --git a/tools/testing/selftests/exec/.gitignore b/tools/testing/selftests/exec/.gitignore
index 7f3d1ae762ec..2ff245fd0ba6 100644
--- a/tools/testing/selftests/exec/.gitignore
+++ b/tools/testing/selftests/exec/.gitignore
@@ -19,3 +19,4 @@ null-argv
 xxxxxxxx*
 pipe
 S_I*.test
+hwcap_inherit
\ No newline at end of file
diff --git a/tools/testing/selftests/exec/Makefile b/tools/testing/selftests/exec/Makefile
index 45a3cfc435cf..e73005965e05 100644
--- a/tools/testing/selftests/exec/Makefile
+++ b/tools/testing/selftests/exec/Makefile
@@ -20,6 +20,7 @@ TEST_FILES := Makefile
 TEST_GEN_PROGS += recursion-depth
 TEST_GEN_PROGS += null-argv
 TEST_GEN_PROGS += check-exec
+TEST_GEN_PROGS += hwcap_inherit
 
 EXTRA_CLEAN := $(OUTPUT)/subdir.moved $(OUTPUT)/execveat.moved $(OUTPUT)/xxxxx*	\
 	       $(OUTPUT)/S_I*.test
diff --git a/tools/testing/selftests/exec/hwcap_inherit.c b/tools/testing/selftests/exec/hwcap_inherit.c
new file mode 100644
index 000000000000..1b43b2dbb1d0
--- /dev/null
+++ b/tools/testing/selftests/exec/hwcap_inherit.c
@@ -0,0 +1,105 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#define _GNU_SOURCE
+#include <sys/auxv.h>
+#include <sys/prctl.h>
+#include <sys/wait.h>
+#include <linux/prctl.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include <errno.h>
+#include <elf.h>
+#include <linux/auxvec.h>
+
+#include "../kselftest.h"
+
+static int find_msb(unsigned long v)
+{
+	return sizeof(v)*8 - __builtin_clzl(v) - 1;
+}
+
+int main(int argc, char *argv[])
+{
+	unsigned long auxv[1024], hwcap, new_hwcap, hwcap_idx;
+	int size, hwcap_type = 0, hwcap_feature, count, status;
+	char hwcap_str[32], hwcap_type_str[32];
+	pid_t pid;
+
+	if (argc > 1 && strcmp(argv[1], "verify") == 0) {
+		unsigned long type = strtoul(argv[2], NULL, 16);
+		unsigned long expected = strtoul(argv[3], NULL, 16);
+		unsigned long hwcap = getauxval(type);
+
+		if (hwcap != expected) {
+			ksft_print_msg("HWCAP mismatch: type %lx, expected %lx, got %lx\n",
+					type, expected, hwcap);
+			return 1;
+		}
+		ksft_print_msg("HWCAP matched: %lx\n", hwcap);
+		return 0;
+	}
+
+	ksft_print_header();
+	ksft_set_plan(1);
+
+	size = prctl(PR_GET_AUXV, auxv, sizeof(auxv), 0, 0);
+	if (size == -1)
+		ksft_exit_fail_perror("prctl(PR_GET_AUXV)");
+
+	count = size / sizeof(unsigned long);
+
+	/* Find the "latest" feature and try to mask it out. */
+	for (int i = 0; i < count - 1; i += 2) {
+		hwcap = auxv[i + 1];
+		if (hwcap == 0)
+			continue;
+		switch (auxv[i]) {
+		case AT_HWCAP4:
+		case AT_HWCAP3:
+		case AT_HWCAP2:
+		case AT_HWCAP:
+			hwcap_type = auxv[i];
+			hwcap_feature = find_msb(hwcap);
+			hwcap_idx = i + 1;
+			break;
+		default:
+			continue;
+		}
+	}
+	if (hwcap_type == 0)
+		ksft_exit_skip("No features found, skipping test\n");
+	hwcap = auxv[hwcap_idx];
+	new_hwcap = hwcap ^ (1UL << hwcap_feature);
+	auxv[hwcap_idx] = new_hwcap;
+
+	if (prctl(PR_SET_MM, PR_SET_MM_AUXV, auxv, size, 0) < 0) {
+		if (errno == EPERM)
+			ksft_exit_skip("prctl(PR_SET_MM_AUXV) requires CAP_SYS_RESOURCE\n");
+		ksft_exit_fail_perror("prctl(PR_SET_MM_AUXV)");
+	}
+
+	pid = fork();
+	if (pid < 0)
+		ksft_exit_fail_perror("fork");
+	if (pid == 0) {
+		char *new_argv[] = { argv[0], "verify", hwcap_type_str, hwcap_str, NULL };
+
+		snprintf(hwcap_str, sizeof(hwcap_str), "%lx", new_hwcap);
+		snprintf(hwcap_type_str, sizeof(hwcap_type_str), "%x", hwcap_type);
+
+		execv(argv[0], new_argv);
+		perror("execv");
+		exit(1);
+	}
+
+	if (waitpid(pid, &status, 0) == -1)
+		ksft_exit_fail_perror("waitpid");
+	if (status != 0)
+		ksft_exit_fail_msg("HWCAP inheritance failed (status %d)\n", status);
+
+	ksft_test_result_pass("HWCAP inheritance succeeded\n");
+	ksft_exit_pass();
+	return 0;
+}
-- 
2.53.0.983.g0bb29b3bc5-goog