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1 /*
2  * syscall_nt.c - checks syscalls with NT set
3  * Copyright (c) 2014-2015 Andrew Lutomirski
4  *
5  * This program is free software; you can redistribute it and/or modify
6  * it under the terms and conditions of the GNU General Public License,
7  * version 2, as published by the Free Software Foundation.
8  *
9  * This program is distributed in the hope it will be useful, but
10  * WITHOUT ANY WARRANTY; without even the implied warranty of
11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
12  * General Public License for more details.
13  *
14  * Some obscure user-space code requires the ability to make system calls
15  * with FLAGS.NT set.  Make sure it works.
16  */
17 
18 #include <stdio.h>
19 #include <unistd.h>
20 #include <string.h>
21 #include <signal.h>
22 #include <err.h>
23 #include <sys/syscall.h>
24 #include <asm/processor-flags.h>
25 
26 #ifdef __x86_64__
27 # define WIDTH "q"
28 #else
29 # define WIDTH "l"
30 #endif
31 
32 static unsigned int nerrs;
33 
get_eflags(void)34 static unsigned long get_eflags(void)
35 {
36 	unsigned long eflags;
37 	asm volatile ("pushf" WIDTH "\n\tpop" WIDTH " %0" : "=rm" (eflags));
38 	return eflags;
39 }
40 
set_eflags(unsigned long eflags)41 static void set_eflags(unsigned long eflags)
42 {
43 	asm volatile ("push" WIDTH " %0\n\tpopf" WIDTH
44 		      : : "rm" (eflags) : "flags");
45 }
46 
sethandler(int sig,void (* handler)(int,siginfo_t *,void *),int flags)47 static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
48 		       int flags)
49 {
50 	struct sigaction sa;
51 	memset(&sa, 0, sizeof(sa));
52 	sa.sa_sigaction = handler;
53 	sa.sa_flags = SA_SIGINFO | flags;
54 	sigemptyset(&sa.sa_mask);
55 	if (sigaction(sig, &sa, 0))
56 		err(1, "sigaction");
57 }
58 
sigtrap(int sig,siginfo_t * si,void * ctx_void)59 static void sigtrap(int sig, siginfo_t *si, void *ctx_void)
60 {
61 }
62 
do_it(unsigned long extraflags)63 static void do_it(unsigned long extraflags)
64 {
65 	unsigned long flags;
66 
67 	set_eflags(get_eflags() | extraflags);
68 	syscall(SYS_getpid);
69 	flags = get_eflags();
70 	if ((flags & extraflags) == extraflags) {
71 		printf("[OK]\tThe syscall worked and flags are still set\n");
72 	} else {
73 		printf("[FAIL]\tThe syscall worked but flags were cleared (flags = 0x%lx but expected 0x%lx set)\n",
74 		       flags, extraflags);
75 		nerrs++;
76 	}
77 }
78 
main(void)79 int main(void)
80 {
81 	printf("[RUN]\tSet NT and issue a syscall\n");
82 	do_it(X86_EFLAGS_NT);
83 
84 	/*
85 	 * Now try it again with TF set -- TF forces returns via IRET in all
86 	 * cases except non-ptregs-using 64-bit full fast path syscalls.
87 	 */
88 
89 	sethandler(SIGTRAP, sigtrap, 0);
90 
91 	printf("[RUN]\tSet NT|TF and issue a syscall\n");
92 	do_it(X86_EFLAGS_NT | X86_EFLAGS_TF);
93 
94 	return nerrs == 0 ? 0 : 1;
95 }
96