[v3,2/3] selftests/x86/mxcsr: Test the MXCSR state write via ptrace

diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
index 0388c4d60af0..621c47960be3 100644
--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -13,7 +13,7 @@  CAN_BUILD_WITH_NOPIE := $(shell ./check_cc.sh "$(CC)" trivial_program.c -no-pie)
 TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt test_mremap_vdso \
 			check_initial_reg_state sigreturn iopl ioperm \
 			test_vsyscall mov_ss_trap \
-			syscall_arg_fault fsgsbase_restore sigaltstack
+			syscall_arg_fault fsgsbase_restore sigaltstack mxcsr
 TARGETS_C_32BIT_ONLY := entry_from_vm86 test_syscall_vdso unwind_vdso \
 			test_FCMOV test_FCOMI test_FISTTP \
 			vdso_restorer
diff --git a/tools/testing/selftests/x86/mxcsr.c b/tools/testing/selftests/x86/mxcsr.c
new file mode 100644
index 000000000000..7c318c48b4be
--- /dev/null
+++ b/tools/testing/selftests/x86/mxcsr.c
@@ -0,0 +1,200 @@ 
+// SPDX-License-Identifier: GPL-2.0
+
+#define _GNU_SOURCE
+#include <err.h>
+#include <elf.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+#include <x86intrin.h>
+
+#include <sys/ptrace.h>
+#include <sys/syscall.h>
+#include <sys/wait.h>
+#include <sys/uio.h>
+
+#include "../kselftest.h" /* For __cpuid_count() */
+
+#define LEGACY_STATE_SIZE	24
+#define MXCSR_SIZE		8
+#define STSTATE_SIZE		8*16
+#define XMM_SIZE		16*16
+#define PADDING_SIZE		96
+#define XSAVE_HDR_SIZE		64
+
+struct xsave_buffer {
+	uint8_t		legacy_state[LEGACY_STATE_SIZE];
+	uint8_t		mxcsr[MXCSR_SIZE];
+	uint8_t		st_state[STSTATE_SIZE];
+	uint8_t		xmm_state[XMM_SIZE];
+	uint8_t		padding[PADDING_SIZE];
+	uint8_t		header[XSAVE_HDR_SIZE];
+	uint8_t		extended[0];
+};
+
+#ifdef __x86_64__
+#define REX_PREFIX	"0x48, "
+#else
+#define REX_PREFIX
+#endif
+
+#define XSAVE		".byte " REX_PREFIX "0x0f,0xae,0x27"
+#define XRSTOR		".byte " REX_PREFIX "0x0f,0xae,0x2f"
+
+static inline uint64_t xgetbv(uint32_t index)
+{
+	uint32_t eax, edx;
+
+	asm volatile("xgetbv"
+		     : "=a" (eax), "=d" (edx)
+		     : "c" (index));
+	return eax + ((uint64_t)edx << 32);
+}
+
+static inline void xsave(struct xsave_buffer *xbuf, uint64_t rfbm)
+{
+	uint32_t rfbm_lo = rfbm;
+	uint32_t rfbm_hi = rfbm >> 32;
+
+	asm volatile(XSAVE :: "D" (xbuf), "a" (rfbm_lo), "d" (rfbm_hi) : "memory");
+}
+
+static inline void xrstor(struct xsave_buffer *xbuf, uint64_t rfbm)
+{
+	uint32_t rfbm_lo = rfbm;
+	uint32_t rfbm_hi = rfbm >> 32;
+
+	asm volatile(XRSTOR :: "D" (xbuf), "a" (rfbm_lo), "d" (rfbm_hi));
+}
+
+static inline void clear_xstate_header(struct xsave_buffer *xbuf)
+{
+	memset(&xbuf->header, 0, sizeof(xbuf->header));
+}
+
+static inline uint32_t get_mxcsr(struct xsave_buffer *xbuf)
+{
+	return *((uint32_t *)xbuf->mxcsr);
+}
+
+static inline void set_mxcsr(struct xsave_buffer *xbuf, uint32_t val)
+{
+	*((uint32_t *)xbuf->mxcsr) = val;
+}
+
+#define XFEATURE_MASK_SSE		0x2
+#define XFEATURE_MASK_YMM		0x4
+
+#define CPUID_LEAF1_ECX_XSAVE_MASK	(1 << 26)
+#define CPUID_LEAF1_ECX_OSXSAVE_MASK	(1 << 27)
+#define CPUID_LEAF_XSTATE		0xd
+#define CPUID_SUBLEAF_XSTATE_USER	0x0
+#define CPUID_SUBLEAF_XSTATE_EXT	0x1
+
+static bool xsave_availability(void)
+{
+	uint32_t eax, ebx, ecx, edx;
+
+	__cpuid_count(1, 0, eax, ebx, ecx, edx);
+	if (!(ecx & CPUID_LEAF1_ECX_XSAVE_MASK))
+		return false;
+	if (!(ecx & CPUID_LEAF1_ECX_OSXSAVE_MASK))
+		return false;
+	return true;
+}
+
+static uint32_t get_xbuf_size(void)
+{
+	uint32_t eax, ebx, ecx, edx;
+
+	__cpuid_count(CPUID_LEAF_XSTATE, CPUID_SUBLEAF_XSTATE_USER,
+		      eax, ebx, ecx, edx);
+	return ebx;
+}
+
+static void ptrace_get(pid_t pid, struct iovec *iov)
+{
+	memset(iov->iov_base, 0, iov->iov_len);
+
+	if (ptrace(PTRACE_GETREGSET, pid, (uint32_t)NT_X86_XSTATE, iov))
+		err(1, "TRACE_GETREGSET");
+}
+
+static void ptrace_set(pid_t pid, struct iovec *iov)
+{
+	if (ptrace(PTRACE_SETREGSET, pid, (uint32_t)NT_X86_XSTATE, iov))
+		err(1, "TRACE_SETREGSET");
+}
+
+int main(void)
+{
+	struct xsave_buffer *xbuf;
+	uint32_t xbuf_size;
+	struct iovec iov;
+	uint32_t mxcsr;
+	pid_t child;
+	int status;
+
+	if (!xsave_availability())
+		printf("[SKIP]\tSkip as XSAVE not available.\n");
+
+	xbuf_size = get_xbuf_size();
+	if (!xbuf_size)
+		printf("[SKIP]\tSkip as XSAVE not available.\n");
+
+	if (!(xgetbv(0) & (XFEATURE_MASK_SSE | XFEATURE_MASK_YMM)))
+		printf("[SKIP]\tSkip as SSE state not available.\n");
+
+	xbuf = aligned_alloc(64, xbuf_size);
+	if (!xbuf)
+		err(1, "aligned_alloc()");
+
+	iov.iov_base = xbuf;
+	iov.iov_len = xbuf_size;
+
+	child = fork();
+	if (child < 0) {
+		err(1, "fork()");
+	} else if (!child) {
+		if (ptrace(PTRACE_TRACEME, 0, NULL, NULL))
+			err(1, "PTRACE_TRACEME");
+
+		raise(SIGTRAP);
+		_exit(0);
+	}
+
+	wait(&status);
+
+	if (WSTOPSIG(status) != SIGTRAP)
+		err(1, "raise(SIGTRAP)");
+
+	printf("[RUN]\tTest the MXCSR state write via ptrace().\n");
+
+	/* Set a benign value */
+	set_mxcsr(xbuf, 0xabc);
+	/* The MXCSR state should be loaded regardless of XSTATE_BV */
+	clear_xstate_header(xbuf);
+
+	/* Write the MXCSR state both locally and remotely. */
+	xrstor(xbuf, XFEATURE_MASK_SSE);
+	ptrace_set(child, &iov);
+
+	/* Read the MXCSR state back for both */
+	xsave(xbuf, XFEATURE_MASK_SSE);
+	mxcsr = get_mxcsr(xbuf);
+	ptrace_get(child, &iov);
+
+	/* Cross-check with each other */
+	if (mxcsr == get_mxcsr(xbuf))
+		printf("[OK]\tThe written state was read back correctly.\n");
+	else
+		printf("[FAIL]\tThe write (or read) was incorrect.\n");
+
+	ptrace(PTRACE_DETACH, child, NULL, NULL);
+	wait(&status);
+	if (!WIFEXITED(status) || WEXITSTATUS(status))
+		err(1, "PTRACE_DETACH");
+
+	free(xbuf);
+}