selftests/x86/sigreturn: Use CX, not AX, as the scratch register

RAX is handled specially in ESPFIX64.  Use CX as our scratch
register so that, if something goes wrong with RAX handling, we'll
notice.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/9ceeb24ea56925586c330dc46306f757ddea9fb5.1473717910.git.luto@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

tools/testing/selftests/x86/sigreturn.c

@@ -106,7 +106,7 @@ asm (".pushsection .text\n\t"
      ".type int3, @function\n\t"
      ".align 4096\n\t"
      "int3:\n\t"
-     "mov %ss,%eax\n\t"
+     "mov %ss,%ecx\n\t"
      "int3\n\t"
      ".size int3, . - int3\n\t"
      ".align 4096, 0xcc\n\t"
@@ -306,7 +306,7 @@ static volatile sig_atomic_t sig_corrupt_final_ss;
 #ifdef __x86_64__
 # define REG_IP REG_RIP
 # define REG_SP REG_RSP
-# define REG_AX REG_RAX
+# define REG_CX REG_RCX
 
 struct selectors {
 	unsigned short cs, gs, fs, ss;
@@ -326,7 +326,7 @@ static unsigned short *csptr(ucontext_t *ctx)
 #else
 # define REG_IP REG_EIP
 # define REG_SP REG_ESP
-# define REG_AX REG_EAX
+# define REG_CX REG_ECX
 
 static greg_t *ssptr(ucontext_t *ctx)
 {
@@ -457,10 +457,10 @@ static void sigusr1(int sig, siginfo_t *info, void *ctx_void)
 	ctx->uc_mcontext.gregs[REG_IP] =
 		sig_cs == code16_sel ? 0 : (unsigned long)&int3;
 	ctx->uc_mcontext.gregs[REG_SP] = (unsigned long)0x8badf00d5aadc0deULL;
-	ctx->uc_mcontext.gregs[REG_AX] = 0;
+	ctx->uc_mcontext.gregs[REG_CX] = 0;
 
 	memcpy(&requested_regs, &ctx->uc_mcontext.gregs, sizeof(gregset_t));
-	requested_regs[REG_AX] = *ssptr(ctx);	/* The asm code does this. */
+	requested_regs[REG_CX] = *ssptr(ctx);	/* The asm code does this. */
 
 	return;
 }
@@ -482,7 +482,7 @@ static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
 	unsigned short ss;
 	asm ("mov %%ss,%0" : "=r" (ss));
 
-	greg_t asm_ss = ctx->uc_mcontext.gregs[REG_AX];
+	greg_t asm_ss = ctx->uc_mcontext.gregs[REG_CX];
 	if (asm_ss != sig_ss && sig == SIGTRAP) {
 		/* Sanity check failure. */
 		printf("[FAIL]\tSIGTRAP: ss = %hx, frame ss = %hx, ax = %llx\n",
@@ -654,8 +654,8 @@ static int test_valid_sigreturn(int cs_bits, bool use_16bit_ss, int force_ss)
 #endif
 
 		/* Sanity check on the kernel */
-		if (i == REG_AX && requested_regs[i] != resulting_regs[i]) {
-			printf("[FAIL]\tAX (saved SP) mismatch: requested 0x%llx; got 0x%llx\n",
+		if (i == REG_CX && requested_regs[i] != resulting_regs[i]) {
+			printf("[FAIL]\tCX (saved SP) mismatch: requested 0x%llx; got 0x%llx\n",
 			       (unsigned long long)requested_regs[i],
 			       (unsigned long long)resulting_regs[i]);
 			nerrs++;