Re: [PATCH 7/7] selftests/x86: Add tests for signal frame FPU portability

[Chang S. Bae]

On 9/7/2026 9:34 PM, Andrei Vagin wrote:
> Add a new selftest tools/testing/selftests/x86/sigframe_fpu_portability.c
> to verify signal frame portability and consistency when the xstate size
> is shrunk:
>
> - test_valid_shrunk_xstate_size: Verifies that the kernel correctly
>    restores the xstate context from a signal frame where xstate_size has
>    been manually shrunk to only cover active features, as long as the
>    FP_XSTATE_MAGIC2 marker is correctly placed. This simulates migrating
>    a process created on a host with fewer xstate features to a host with
>    more features.
>
> - test_invalid_shrunk_xstate_size: Verifies that the kernel rejects
>    (via SIGSEGV) a signal frame where xstate_size is smaller than required
>    by the enabled features in the xfeatures mask.
>
> Reviewed-by: Alexander Mikhalitsyn <alexander@xxxxxxxxxxxxx>
> Signed-off-by: Andrei Vagin <avagin@xxxxxxxxxx>

...


> +static void sig_print(const char *msg)
> +{
> +	int left = SIGNAL_BUF_LEN - strlen(sig_err_buf) - 1;
> +
> +	strncat(sig_err_buf, msg, left);
> +}
> +
> +

Nit: extra empty line


> +static void __handle_shrunk_xstate_size(int sig, siginfo_t *si, void *ucp, bool valid_size)
> +{
> +	ucontext_t *uc = ucp;
> +	void *fp = uc->uc_mcontext.fpregs;
> +	struct _fpx_sw_bytes *sw;
> +	struct xsave_buffer *xbuf;
> +	uint64_t xfeatures, *ymmh_p;
> +
> +	if (!fp) {
> +		sig_print("fpregs is NULL\n");
> +		return;
> +	}
> +
> +	sw = get_fpx_sw_bytes(fp);
> +	if (sw->magic1 != FP_XSTATE_MAGIC1) {
> +		sig_print("magic1 is not valid\n");
> +		return;
> +	}
> +
> +	xbuf = (struct xsave_buffer *)fp;
> +
> +	/*
> +	 * Both test cases shrink the frame to contain only AVX (FP + SSE + YMM).
> +	 * If valid_size is true, set xstate_size to match the enabled features.
> +	 * If valid_size is false, set xstate_size too small (SSE only), which
> +	 * the kernel must reject.
> +	 */
> +	if (valid_size)
> +		sw->xstate_size = xstate_size_ymm;
> +	else
> +		sw->xstate_size = XSTATE_SSE_ONLY_SIZE;
Yeah, I think this note explains the distinction between the two test 
cases and the point of testing:

  Reviewed-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>

Thanks,
Chang