/*
 * single_step_syscall.c - single-steps various x86 syscalls
 * Copyright (c) 2014-2015 Andrew Lutomirski
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms and conditions of the GNU General Public License,
 * version 2, as published by the Free Software Foundation.
 *
 * This program is distributed in the hope it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * 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;
        sigemptyset(&sa.sa_mask);
        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"
#else
# define REG_IP REG_EIP
# define WIDTH "l"
#endif

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");
                _exit(1);
        }

        sig_traps++;

        if (sig_traps == 10000 || sig_traps == 10001) {
                printf("[WARN]\tHit %d SIGTRAPs with si_addr 0x%lx, ip 0x%lx\n",
                       (int)sig_traps,
                       (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");
                exit(1);
        }

        if (!(new_eflags & X86_EFLAGS_TF)) {
                printf("[FAIL]\tTF was cleared\n");
                exit(1);
        }

        printf("[OK]\tSurvived with TF set and %d traps\n", (int)sig_traps);
        sig_traps = 0;
}

int main()
{
        int tmp;

        sethandler(SIGTRAP, sigtrap, 0);

        printf("[RUN]\tSet TF and check nop\n");
        set_eflags(get_eflags() | X86_EFLAGS_TF);
        asm volatile ("nop");
        check_result();

#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"
                      "nop\n\t"
                      "post_nop:"
                      : : "c" (post_nop) : "r11");
        check_result();
#endif

        printf("[RUN]\tSet TF and check int80\n");
        set_eflags(get_eflags() | X86_EFLAGS_TF);
        asm volatile ("int $0x80" : "=a" (tmp) : "a" (SYS_getpid));
        check_result();

        /*
         * 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);
        syscall(SYS_getpid);
        check_result();

        /* 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");
                exit(1);
        }
        if (sig_traps) {
                printf("[FAIL]\tGot SIGTRAP\n");
                exit(1);
        }
        printf("[OK]\tNothing unexpected happened\n");

        return 0;
}