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

[Alexander Mikhalitsyn <alexander@mihalicyn.com>]

Am Mo., 9. Feb. 2026 um 20:06 Uhr schrieb 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.
>
> Signed-off-by: Andrei Vagin <avagin@google.com>

Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@futurfusion.io>

> ---
>  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;
> +}
> --
> 2.53.0.239.g8d8fc8a987-goog
>