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// SPDX-License-Identifier: GPL-2.0-only
* single_step_syscall.c - single-steps various x86 syscalls
* Copyright (c) 2014-2015 Andrew Lutomirski
* This is a very simple series of tests that makes system calls with
* the TF flag set. This exercises some nasty kernel code in the
* SYSENTER case: SYSENTER does not clear TF, so SYSENTER with TF set
* immediately issues #DB from CPL 0. This requires special handling in
* the kernel.
#define _GNU_SOURCE
#include <sys/time.h>
#include <time.h>
#include <stdlib.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <inttypes.h>
#include <sys/mman.h>
#include <sys/signal.h>
#include <sys/ucontext.h>
#include <asm/ldt.h>
#include <err.h>
#include <setjmp.h>
#include <stddef.h>
#include <stdbool.h>
#include <sys/ptrace.h>
#include <sys/user.h>
static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
int flags)
struct sigaction sa;
memset(&sa, 0, sizeof(sa));
sa.sa_sigaction = handler;
sa.sa_flags = SA_SIGINFO | flags;
if (sigaction(sig, &sa, 0))
err(1, "sigaction");
static volatile sig_atomic_t sig_traps;
#ifdef __x86_64__
# define REG_IP REG_RIP
# define WIDTH "q"
# define INT80_CLOBBERS "r8", "r9", "r10", "r11"
# define REG_IP REG_EIP
# define WIDTH "l"
# define INT80_CLOBBERS
static unsigned long get_eflags(void)
unsigned long eflags;
asm volatile ("pushf" WIDTH "\n\tpop" WIDTH " %0" : "=rm" (eflags));
return eflags;
static void set_eflags(unsigned long eflags)
asm volatile ("push" WIDTH " %0\n\tpopf" WIDTH
: : "rm" (eflags) : "flags");
#define X86_EFLAGS_TF (1UL << 8)
static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
ucontext_t *ctx = (ucontext_t*)ctx_void;
if (get_eflags() & X86_EFLAGS_TF) {
set_eflags(get_eflags() & ~X86_EFLAGS_TF);
printf("[WARN]\tSIGTRAP handler had TF set\n");
if (sig_traps == 10000 || sig_traps == 10001) {
printf("[WARN]\tHit %d SIGTRAPs with si_addr 0x%lx, ip 0x%lx\n",
(unsigned long)info->si_addr,
(unsigned long)ctx->uc_mcontext.gregs[REG_IP]);
static void check_result(void)
unsigned long new_eflags = get_eflags();
set_eflags(new_eflags & ~X86_EFLAGS_TF);
if (!sig_traps) {
printf("[FAIL]\tNo SIGTRAP\n");
if (!(new_eflags & X86_EFLAGS_TF)) {
printf("[FAIL]\tTF was cleared\n");
printf("[OK]\tSurvived with TF set and %d traps\n", (int)sig_traps);
sig_traps = 0;
int main()
#ifdef CAN_BUILD_32
int tmp;
sethandler(SIGTRAP, sigtrap, 0);
printf("[RUN]\tSet TF and check nop\n");
set_eflags(get_eflags() | X86_EFLAGS_TF);
asm volatile ("nop");
#ifdef __x86_64__
printf("[RUN]\tSet TF and check syscall-less opportunistic sysret\n");
set_eflags(get_eflags() | X86_EFLAGS_TF);
extern unsigned char post_nop[];
asm volatile ("pushf" WIDTH "\n\t"
"pop" WIDTH " %%r11\n\t"
: : "c" (post_nop) : "r11");
#ifdef CAN_BUILD_32
printf("[RUN]\tSet TF and check int80\n");
set_eflags(get_eflags() | X86_EFLAGS_TF);
asm volatile ("int $0x80" : "=a" (tmp) : "a" (SYS_getpid)
* This test is particularly interesting if fast syscalls use
* SYSENTER: it triggers a nasty design flaw in SYSENTER.
* Specifically, SYSENTER does not clear TF, so either SYSENTER
* or the next instruction traps at CPL0. (Of course, Intel
* mostly forgot to document exactly what happens here.) So we
* get a CPL0 fault with usergs (on 64-bit kernels) and possibly
* no stack. The only sane way the kernel can possibly handle
* it is to clear TF on return from the #DB handler, but this
* happens way too early to set TF in the saved pt_regs, so the
* kernel has to do something clever to avoid losing track of
* the TF bit.
* Needless to say, we've had bugs in this area.
syscall(SYS_getpid); /* Force symbol binding without TF set. */
printf("[RUN]\tSet TF and check a fast syscall\n");
set_eflags(get_eflags() | X86_EFLAGS_TF);
/* Now make sure that another fast syscall doesn't set TF again. */
printf("[RUN]\tFast syscall with TF cleared\n");
fflush(stdout); /* Force a syscall */
if (get_eflags() & X86_EFLAGS_TF) {
printf("[FAIL]\tTF is now set\n");
if (sig_traps) {
printf("[FAIL]\tGot SIGTRAP\n");
printf("[OK]\tNothing unexpected happened\n");
return 0;