Re: [PATCH 10/10] selftests/x86: Check restoring FPU state with larger xstate_size

[Alexander Mikhalitsyn]

Am Mo., 15. Juni 2026 um 21:42 Uhr schrieb Andrei Vagin <avagin@xxxxxxxxxx>:
>
> Add two new test cases to sigframe_fpu_portability.c:
>
> The first test case (test_larger_xstate_size) verifies that the kernel
> can restore FPU state from a signal frame that has xstate_size larger
> than the current task's fpstate->user_size, but the buffer doesn't
> contain states of any unsupported features. This test case emulates a
> case when a process is migrated from a newer cpu to an older cpu, but
> the process doesn't use any unsupported features.
>
> The second test case (test_unsupported_xfeatures) verifies that the
> kernel correctly rejects restoring FPU state from a signal frame if it
> contains states of any unsupported features.
>
> Signed-off-by: Andrei Vagin <avagin@xxxxxxxxxx>

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

> ---
>  .../selftests/x86/sigframe_fpu_portability.c  | 129 +++++++++++++++++-
>  1 file changed, 128 insertions(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/x86/sigframe_fpu_portability.c b/tools/testing/selftests/x86/sigframe_fpu_portability.c
> index 462219905303..b349efcf05c3 100644
> --- a/tools/testing/selftests/x86/sigframe_fpu_portability.c
> +++ b/tools/testing/selftests/x86/sigframe_fpu_portability.c
> @@ -27,6 +27,14 @@
>   * - test_insufficient_xstate_size:
>   *   Verifies that the kernel rejects a frame if xstate_size is too small for
>   *   the features enabled in xfeatures.
> + *
> + * - test_larger_xstate_size:
> + *   Verifies that the kernel restores state from a frame with xstate_size
> + *   larger than the current task's size, if no unsupported features are active.
> + *
> + * - test_unsupported_xfeatures:
> + *   Verifies that the kernel rejects a frame if it contains unsupported
> + *   features in the xsave header.
>   */
>
>  #define SIGFRAME_XSTATE_HDR_OFFSET     512
> @@ -203,15 +211,134 @@ static void test_insufficient_xstate_size(void)
>         clearhandler(SIGSEGV);
>  }
>
> +static char fpu_buffer[8192] __attribute__((aligned(64)));
> +#define UNSUPPORTED_XFEATURE (1ULL<<62)
> +
> +static void __handle_larger_xstate_size(int sig, siginfo_t *si, void *ucp, bool mod_xhdr)
> +{
> +       ucontext_t *uc = ucp;
> +       void *fp = uc->uc_mcontext.fpregs;
> +       struct _fpx_sw_bytes *sw = get_fpx_sw_bytes(fp);
> +       size_t copy_size;
> +       uint64_t *ymmh_p, xfeatures;
> +       struct xsave_buffer *xbuf;
> +
> +       if (sw->magic1 != FP_XSTATE_MAGIC1) {
> +               sig_print("magic1 is not valid\n");
> +               return;
> +       }
> +
> +       copy_size = sw->xstate_size;
> +       if (copy_size > sizeof(fpu_buffer)) {
> +               sig_print("fpu_buffer is too small\n");
> +               return;
> +       }
> +
> +       memset(fpu_buffer, 0, sizeof(fpu_buffer));
> +       memcpy(fpu_buffer, fp, copy_size);
> +
> +       xbuf = (struct xsave_buffer *)fpu_buffer;
> +       sw = get_fpx_sw_bytes(fpu_buffer);
> +
> +       sw->xstate_size += 64;
> +       sw->extended_size += 64;
> +       xfeatures = get_fpx_sw_bytes_features(xbuf);
> +       set_fpx_sw_bytes_features(fpu_buffer, xfeatures | UNSUPPORTED_XFEATURE);
> +
> +       *(uint32_t *)((char *)fpu_buffer + sw->xstate_size) = FP_XSTATE_MAGIC2;
> +
> +       if (mod_xhdr) {
> +               xfeatures = get_xstatebv(xbuf);
> +               set_xstatebv(xbuf, xfeatures | UNSUPPORTED_XFEATURE);
> +       }
> +
> +       ymmh_p = (uint64_t *)(fpu_buffer + ymm_offset);
> +       ymmh_p[0] = TEST_YMMH_VAL;
> +       ymmh_p[1] = TEST_YMMH_VAL + 1;
> +
> +       /* Update fpregs to point to the new buffer */
> +       uc->uc_mcontext.fpregs = (fpregset_t)fpu_buffer;
> +}
> +
> +static void handle_larger_xstate_size(int sig, siginfo_t *si, void *ucp)
> +{
> +       __handle_larger_xstate_size(sig, si, ucp, false);
> +}
> +
> +static void test_larger_xstate_size(void)
> +{
> +       uint64_t v[4] = {0, 0, 0, 0};
> +
> +       sig_err_buf[0] = 0;
> +       sethandler(SIGUSR1, handle_larger_xstate_size, 0);
> +
> +       v[0] = 0x1111111111111111ULL;
> +       v[1] = 0x2222222222222222ULL;
> +       v[2] = 0x3333333333333333ULL;
> +       v[3] = 0x4444444444444444ULL;
> +       write_ymm0(v);
> +
> +       raise(SIGUSR1);
> +       v[0] = v[1] = v[2] = v[3] = 0;
> +       read_ymm0(v);
> +
> +       if (sig_err_buf[0])
> +               ksft_test_result_fail("%s\n", sig_err_buf);
> +       else if (v[2] == TEST_YMMH_VAL && v[3] == TEST_YMMH_VAL + 1)
> +               ksft_test_result_pass("YMM state restored correctly with larger xstate_size\n");
> +       else
> +               ksft_test_result_fail(
> +                               "Got upper bits: 0x%lx 0x%lx (expected %lx %lx)\n",
> +                              v[2], v[3], TEST_YMMH_VAL, TEST_YMMH_VAL + 1);
> +
> +       clearhandler(SIGUSR1);
> +}
> +
> +static void handle_unsupported_xfeatures(int sig, siginfo_t *si, void *ucp)
> +{
> +       __handle_larger_xstate_size(sig, si, ucp, true);
> +}
> +
> +static void test_unsupported_xfeatures(void)
> +{
> +       uint64_t v[4] = {0, 0, 0, 0};
> +
> +       sig_err_buf[0] = 0;
> +
> +       sethandler(SIGUSR1, handle_unsupported_xfeatures, 0);
> +       sethandler(SIGSEGV, handle_segv, 0);
> +
> +       v[0] = 0x1111111111111111ULL;
> +       v[1] = 0x2222222222222222ULL;
> +       v[2] = 0x3333333333333333ULL;
> +       v[3] = 0x4444444444444444ULL;
> +       write_ymm0(v);
> +
> +       if (sigsetjmp(segv_jmpbuf, 1) == 0) {
> +               raise(SIGUSR1);
> +               sig_print("raise(SIGUSR1) returned (expected SIGSEGV)\n");
> +       }
> +
> +       clearhandler(SIGUSR1);
> +       clearhandler(SIGSEGV);
> +
> +       if (sig_err_buf[0])
> +               ksft_test_result_fail("%s\n", sig_err_buf);
> +       else
> +               ksft_test_result_pass("Unsupported feature in xsave header triggered SIGSEGV\n");
> +}
> +
>  int main(void)
>  {
>         ksft_print_header();
> -       ksft_set_plan(2);
> +       ksft_set_plan(4);
>
>         check_avx_support();
>
>         test_shrunk_xstate_size();
>         test_insufficient_xstate_size();
> +       test_larger_xstate_size();
> +       test_unsupported_xfeatures();
>
>         ksft_finished();
>         return 0;
> --
> 2.54.0.1189.g8c84645362-goog
>
>