tools/testing/selftests/x86/single_step_syscall.c

   1 /*
   2  * single_step_syscall.c - single-steps various x86 syscalls
   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  * This is a very simple series of tests that makes system calls with
  15  * the TF flag set.  This exercises some nasty kernel code in the
  16  * SYSENTER case: SYSENTER does not clear TF, so SYSENTER with TF set
  17  * immediately issues #DB from CPL 0.  This requires special handling in
  18  * the kernel.
  19  */
  20
  21 #define _GNU_SOURCE
  22
  23 #include <sys/time.h>
  24 #include <time.h>
  25 #include <stdlib.h>
  26 #include <sys/syscall.h>
  27 #include <unistd.h>
  28 #include <stdio.h>
  29 #include <string.h>
  30 #include <inttypes.h>
  31 #include <sys/mman.h>
  32 #include <sys/signal.h>
  33 #include <sys/ucontext.h>
  34 #include <asm/ldt.h>
  35 #include <err.h>
  36 #include <setjmp.h>
  37 #include <stddef.h>
  38 #include <stdbool.h>
  39 #include <sys/ptrace.h>
  40 #include <sys/user.h>
  41
  42 static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
  43                        int flags)
  44 {
  45         struct sigaction sa;
  46         memset(&sa, 0, sizeof(sa));
  47         sa.sa_sigaction = handler;
  48         sa.sa_flags = SA_SIGINFO | flags;
  49         sigemptyset(&sa.sa_mask);
  50         if (sigaction(sig, &sa, 0))
  51                 err(1, "sigaction");
  52 }
  53
  54 static volatile sig_atomic_t sig_traps;
  55
  56 #ifdef __x86_64__
  57 # define REG_IP REG_RIP
  58 # define WIDTH "q"
  59 #else
  60 # define REG_IP REG_EIP
  61 # define WIDTH "l"
  62 #endif
  63
  64 static unsigned long get_eflags(void)
  65 {
  66         unsigned long eflags;
  67         asm volatile ("pushf" WIDTH "\n\tpop" WIDTH " %0" : "=rm" (eflags));
  68         return eflags;
  69 }
  70
  71 static void set_eflags(unsigned long eflags)
  72 {
  73         asm volatile ("push" WIDTH " %0\n\tpopf" WIDTH
  74                       : : "rm" (eflags) : "flags");
  75 }
  76
  77 #define X86_EFLAGS_TF (1UL << 8)
  78
  79 static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
  80 {
  81         ucontext_t *ctx = (ucontext_t*)ctx_void;
  82
  83         if (get_eflags() & X86_EFLAGS_TF) {
  84                 set_eflags(get_eflags() & ~X86_EFLAGS_TF);
  85                 printf("[WARN]\tSIGTRAP handler had TF set\n");
  86                 _exit(1);
  87         }
  88
  89         sig_traps++;
  90
  91         if (sig_traps == 10000 || sig_traps == 10001) {
  92                 printf("[WARN]\tHit %d SIGTRAPs with si_addr 0x%lx, ip 0x%lx\n",
  93                        (int)sig_traps,
  94                        (unsigned long)info->si_addr,
  95                        (unsigned long)ctx->uc_mcontext.gregs[REG_IP]);
  96         }
  97 }
  98
  99 static void check_result(void)
 100 {
 101         unsigned long new_eflags = get_eflags();
 102         set_eflags(new_eflags & ~X86_EFLAGS_TF);
 103
 104         if (!sig_traps) {
 105                 printf("[FAIL]\tNo SIGTRAP\n");
 106                 exit(1);
 107         }
 108
 109         if (!(new_eflags & X86_EFLAGS_TF)) {
 110                 printf("[FAIL]\tTF was cleared\n");
 111                 exit(1);
 112         }
 113
 114         printf("[OK]\tSurvived with TF set and %d traps\n", (int)sig_traps);
 115         sig_traps = 0;
 116 }
 117
 118 int main()
 119 {
 120         int tmp;
 121
 122         sethandler(SIGTRAP, sigtrap, 0);
 123
 124         printf("[RUN]\tSet TF and check nop\n");
 125         set_eflags(get_eflags() | X86_EFLAGS_TF);
 126         asm volatile ("nop");
 127         check_result();
 128
 129 #ifdef __x86_64__
 130         printf("[RUN]\tSet TF and check syscall-less opportunistic sysret\n");
 131         set_eflags(get_eflags() | X86_EFLAGS_TF);
 132         extern unsigned char post_nop[];
 133         asm volatile ("pushf" WIDTH "\n\t"
 134                       "pop" WIDTH " %%r11\n\t"
 135                       "nop\n\t"
 136                       "post_nop:"
 137                       : : "c" (post_nop) : "r11");
 138         check_result();
 139 #endif
 140
 141         printf("[RUN]\tSet TF and check int80\n");
 142         set_eflags(get_eflags() | X86_EFLAGS_TF);
 143         asm volatile ("int $0x80" : "=a" (tmp) : "a" (SYS_getpid));
 144         check_result();
 145
 146         /*
 147          * This test is particularly interesting if fast syscalls use
 148          * SYSENTER: it triggers a nasty design flaw in SYSENTER.
 149          * Specifically, SYSENTER does not clear TF, so either SYSENTER
 150          * or the next instruction traps at CPL0.  (Of course, Intel
 151          * mostly forgot to document exactly what happens here.)  So we
 152          * get a CPL0 fault with usergs (on 64-bit kernels) and possibly
 153          * no stack.  The only sane way the kernel can possibly handle
 154          * it is to clear TF on return from the #DB handler, but this
 155          * happens way too early to set TF in the saved pt_regs, so the
 156          * kernel has to do something clever to avoid losing track of
 157          * the TF bit.
 158          *
 159          * Needless to say, we've had bugs in this area.
 160          */
 161         syscall(SYS_getpid);  /* Force symbol binding without TF set. */
 162         printf("[RUN]\tSet TF and check a fast syscall\n");
 163         set_eflags(get_eflags() | X86_EFLAGS_TF);
 164         syscall(SYS_getpid);
 165         check_result();
 166
 167         /* Now make sure that another fast syscall doesn't set TF again. */
 168         printf("[RUN]\tFast syscall with TF cleared\n");
 169         fflush(stdout);  /* Force a syscall */
 170         if (get_eflags() & X86_EFLAGS_TF) {
 171                 printf("[FAIL]\tTF is now set\n");
 172                 exit(1);
 173         }
 174         if (sig_traps) {
 175                 printf("[FAIL]\tGot SIGTRAP\n");
 176                 exit(1);
 177         }
 178         printf("[OK]\tNothing unexpected happened\n");
 179
 180         return 0;
 181 }