2 * single_step_syscall.c - single-steps various x86 syscalls
3 * Copyright (c) 2014-2015 Andrew Lutomirski
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.
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.
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
26 #include <sys/syscall.h>
32 #include <sys/signal.h>
33 #include <sys/ucontext.h>
39 #include <sys/ptrace.h>
42 static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
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))
54 static volatile sig_atomic_t sig_traps;
57 # define REG_IP REG_RIP
60 # define REG_IP REG_EIP
64 static unsigned long get_eflags(void)
67 asm volatile ("pushf" WIDTH "\n\tpop" WIDTH " %0" : "=rm" (eflags));
71 static void set_eflags(unsigned long eflags)
73 asm volatile ("push" WIDTH " %0\n\tpopf" WIDTH
74 : : "rm" (eflags) : "flags");
77 #define X86_EFLAGS_TF (1UL << 8)
79 static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
81 ucontext_t *ctx = (ucontext_t*)ctx_void;
83 if (get_eflags() & X86_EFLAGS_TF) {
84 set_eflags(get_eflags() & ~X86_EFLAGS_TF);
85 printf("[WARN]\tSIGTRAP handler had TF set\n");
91 if (sig_traps == 10000 || sig_traps == 10001) {
92 printf("[WARN]\tHit %d SIGTRAPs with si_addr 0x%lx, ip 0x%lx\n",
94 (unsigned long)info->si_addr,
95 (unsigned long)ctx->uc_mcontext.gregs[REG_IP]);
99 static void check_result(void)
101 unsigned long new_eflags = get_eflags();
102 set_eflags(new_eflags & ~X86_EFLAGS_TF);
105 printf("[FAIL]\tNo SIGTRAP\n");
109 if (!(new_eflags & X86_EFLAGS_TF)) {
110 printf("[FAIL]\tTF was cleared\n");
114 printf("[OK]\tSurvived with TF set and %d traps\n", (int)sig_traps);
122 sethandler(SIGTRAP, sigtrap, 0);
124 printf("[RUN]\tSet TF and check nop\n");
125 set_eflags(get_eflags() | X86_EFLAGS_TF);
126 asm volatile ("nop");
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"
137 : : "c" (post_nop) : "r11");
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));
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
159 * Needless to say, we've had bugs in this area.
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);
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");
175 printf("[FAIL]\tGot SIGTRAP\n");
178 printf("[OK]\tNothing unexpected happened\n");