selftests/x86: Support Atom for syscall_arg_fault test

Atom-based CPUs trigger stack fault when invoke 32-bit SYSENTER instruction
with invalid register values. So we also need SIGBUS handling in this case.

Following is assembly when the fault exception happens.

(gdb) disassemble $eip
Dump of assembler code for function __kernel_vsyscall:
   0xf7fd8fe0 <+0>:     push   %ecx
   0xf7fd8fe1 <+1>:     push   %edx
   0xf7fd8fe2 <+2>:     push   %ebp
   0xf7fd8fe3 <+3>:     mov    %esp,%ebp
   0xf7fd8fe5 <+5>:     sysenter
   0xf7fd8fe7 <+7>:     int    $0x80
=> 0xf7fd8fe9 <+9>:     pop    %ebp
   0xf7fd8fea <+10>:    pop    %edx
   0xf7fd8feb <+11>:    pop    %ecx
   0xf7fd8fec <+12>:    ret
End of assembler dump.

According to Intel SDM, this could also be a Stack Segment Fault(#SS, 12),
except a normal Page Fault(#PF, 14). Especially, in section 6.9 of Vol.3A,
both stack and page faults are within the 10th(lowest priority) class, and
as it said, "exceptions within each class are implementation-dependent and
may vary from processor to processor". It's expected for processors like
Intel Atom to trigger stack fault(SIGBUS), while we get page fault(SIGSEGV)
from common Core processors.

Signed-off-by: Tong Bo <bo.tong@intel.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/x86/syscall_arg_fault.c

@@ -43,7 +43,7 @@ static sigjmp_buf jmpbuf;
 
 static volatile sig_atomic_t n_errs;
 
-static void sigsegv(int sig, siginfo_t *info, void *ctx_void)
+static void sigsegv_or_sigbus(int sig, siginfo_t *info, void *ctx_void)
 {
 	ucontext_t *ctx = (ucontext_t*)ctx_void;
 
@@ -73,7 +73,13 @@ int main()
 	if (sigaltstack(&stack, NULL) != 0)
 		err(1, "sigaltstack");
 
-	sethandler(SIGSEGV, sigsegv, SA_ONSTACK);
+	sethandler(SIGSEGV, sigsegv_or_sigbus, SA_ONSTACK);
+	/*
+	 * The actual exception can vary.  On Atom CPUs, we get #SS
+	 * instead of #PF when the vDSO fails to access the stack when
+	 * ESP is too close to 2^32, and #SS causes SIGBUS.
+	 */
+	sethandler(SIGBUS, sigsegv_or_sigbus, SA_ONSTACK);
 	sethandler(SIGILL, sigill, SA_ONSTACK);
 
 	/*