root/tools/testing/selftests/x86/single_step_syscall.c

DEFINITIONS

1. sethandler
2. get_eflags
3. set_eflags
4. sigtrap
5. check_result
6. main

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * single_step_syscall.c - single-steps various x86 syscalls
   4  * Copyright (c) 2014-2015 Andrew Lutomirski
   5  *
   6  * This is a very simple series of tests that makes system calls with
   7  * the TF flag set.  This exercises some nasty kernel code in the
   8  * SYSENTER case: SYSENTER does not clear TF, so SYSENTER with TF set
   9  * immediately issues #DB from CPL 0.  This requires special handling in
  10  * the kernel.
  11  */
  12 
  13 #define _GNU_SOURCE
  14 
  15 #include <sys/time.h>
  16 #include <time.h>
  17 #include <stdlib.h>
  18 #include <sys/syscall.h>
  19 #include <unistd.h>
  20 #include <stdio.h>
  21 #include <string.h>
  22 #include <inttypes.h>
  23 #include <sys/mman.h>
  24 #include <sys/signal.h>
  25 #include <sys/ucontext.h>
  26 #include <asm/ldt.h>
  27 #include <err.h>
  28 #include <setjmp.h>
  29 #include <stddef.h>
  30 #include <stdbool.h>
  31 #include <sys/ptrace.h>
  32 #include <sys/user.h>
  33 
  34 static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
  35                        int flags)
  36 {
  37         struct sigaction sa;
  38         memset(&sa, 0, sizeof(sa));
  39         sa.sa_sigaction = handler;
  40         sa.sa_flags = SA_SIGINFO | flags;
  41         sigemptyset(&sa.sa_mask);
  42         if (sigaction(sig, &sa, 0))
  43                 err(1, "sigaction");
  44 }
  45 
  46 static volatile sig_atomic_t sig_traps;
  47 
  48 #ifdef __x86_64__
  49 # define REG_IP REG_RIP
  50 # define WIDTH "q"
  51 # define INT80_CLOBBERS "r8", "r9", "r10", "r11"
  52 #else
  53 # define REG_IP REG_EIP
  54 # define WIDTH "l"
  55 # define INT80_CLOBBERS
  56 #endif
  57 
  58 static unsigned long get_eflags(void)
  59 {
  60         unsigned long eflags;
  61         asm volatile ("pushf" WIDTH "\n\tpop" WIDTH " %0" : "=rm" (eflags));
  62         return eflags;
  63 }
  64 
  65 static void set_eflags(unsigned long eflags)
  66 {
  67         asm volatile ("push" WIDTH " %0\n\tpopf" WIDTH
  68                       : : "rm" (eflags) : "flags");
  69 }
  70 
  71 #define X86_EFLAGS_TF (1UL << 8)
  72 
  73 static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
  74 {
  75         ucontext_t *ctx = (ucontext_t*)ctx_void;
  76 
  77         if (get_eflags() & X86_EFLAGS_TF) {
  78                 set_eflags(get_eflags() & ~X86_EFLAGS_TF);
  79                 printf("[WARN]\tSIGTRAP handler had TF set\n");
  80                 _exit(1);
  81         }
  82 
  83         sig_traps++;
  84 
  85         if (sig_traps == 10000 || sig_traps == 10001) {
  86                 printf("[WARN]\tHit %d SIGTRAPs with si_addr 0x%lx, ip 0x%lx\n",
  87                        (int)sig_traps,
  88                        (unsigned long)info->si_addr,
  89                        (unsigned long)ctx->uc_mcontext.gregs[REG_IP]);
  90         }
  91 }
  92 
  93 static void check_result(void)
  94 {
  95         unsigned long new_eflags = get_eflags();
  96         set_eflags(new_eflags & ~X86_EFLAGS_TF);
  97 
  98         if (!sig_traps) {
  99                 printf("[FAIL]\tNo SIGTRAP\n");
 100                 exit(1);
 101         }
 102 
 103         if (!(new_eflags & X86_EFLAGS_TF)) {
 104                 printf("[FAIL]\tTF was cleared\n");
 105                 exit(1);
 106         }
 107 
 108         printf("[OK]\tSurvived with TF set and %d traps\n", (int)sig_traps);
 109         sig_traps = 0;
 110 }
 111 
 112 int main()
 113 {
 114 #ifdef CAN_BUILD_32
 115         int tmp;
 116 #endif
 117 
 118         sethandler(SIGTRAP, sigtrap, 0);
 119 
 120         printf("[RUN]\tSet TF and check nop\n");
 121         set_eflags(get_eflags() | X86_EFLAGS_TF);
 122         asm volatile ("nop");
 123         check_result();
 124 
 125 #ifdef __x86_64__
 126         printf("[RUN]\tSet TF and check syscall-less opportunistic sysret\n");
 127         set_eflags(get_eflags() | X86_EFLAGS_TF);
 128         extern unsigned char post_nop[];
 129         asm volatile ("pushf" WIDTH "\n\t"
 130                       "pop" WIDTH " %%r11\n\t"
 131                       "nop\n\t"
 132                       "post_nop:"
 133                       : : "c" (post_nop) : "r11");
 134         check_result();
 135 #endif
 136 #ifdef CAN_BUILD_32
 137         printf("[RUN]\tSet TF and check int80\n");
 138         set_eflags(get_eflags() | X86_EFLAGS_TF);
 139         asm volatile ("int $0x80" : "=a" (tmp) : "a" (SYS_getpid)
 140                         : INT80_CLOBBERS);
 141         check_result();
 142 #endif
 143 
 144         /*
 145          * This test is particularly interesting if fast syscalls use
 146          * SYSENTER: it triggers a nasty design flaw in SYSENTER.
 147          * Specifically, SYSENTER does not clear TF, so either SYSENTER
 148          * or the next instruction traps at CPL0.  (Of course, Intel
 149          * mostly forgot to document exactly what happens here.)  So we
 150          * get a CPL0 fault with usergs (on 64-bit kernels) and possibly
 151          * no stack.  The only sane way the kernel can possibly handle
 152          * it is to clear TF on return from the #DB handler, but this
 153          * happens way too early to set TF in the saved pt_regs, so the
 154          * kernel has to do something clever to avoid losing track of
 155          * the TF bit.
 156          *
 157          * Needless to say, we've had bugs in this area.
 158          */
 159         syscall(SYS_getpid);  /* Force symbol binding without TF set. */
 160         printf("[RUN]\tSet TF and check a fast syscall\n");
 161         set_eflags(get_eflags() | X86_EFLAGS_TF);
 162         syscall(SYS_getpid);
 163         check_result();
 164 
 165         /* Now make sure that another fast syscall doesn't set TF again. */
 166         printf("[RUN]\tFast syscall with TF cleared\n");
 167         fflush(stdout);  /* Force a syscall */
 168         if (get_eflags() & X86_EFLAGS_TF) {
 169                 printf("[FAIL]\tTF is now set\n");
 170                 exit(1);
 171         }
 172         if (sig_traps) {
 173                 printf("[FAIL]\tGot SIGTRAP\n");
 174                 exit(1);
 175         }
 176         printf("[OK]\tNothing unexpected happened\n");
 177 
 178         return 0;
 179 }