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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 
     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 
     41 static void set_eflags(unsigned long eflags)
     42 {
     43 	asm volatile ("push" WIDTH " %0\n\tpopf" WIDTH
     44 		      : : "rm" (eflags) : "flags");
     45 }
     46 
     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 
     59 static void sigtrap(int sig, siginfo_t *si, void *ctx_void)
     60 {
     61 }
     62 
     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 
     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