/*
 * Copyright (c) 2023 Alexey Dobriyan <adobriyan@gmail.com>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */
/*
 * Test that userspace stack is NX. Requires linking with -Wl,-z,noexecstack
 * because I don't want to bother with PT_GNU_STACK detection.
 *
 * Fill the stack with INT3's and then try to execute some of them:
 * SIGSEGV -- good, SIGTRAP -- bad.
 *
 * Regular stack is completely overwritten before testing.
 * Test doesn't exit SIGSEGV handler after first fault at INT3.
 */
#undef _GNU_SOURCE
#define _GNU_SOURCE
#undef NDEBUG
#include <assert.h>
#include <signal.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/resource.h>
#include <unistd.h>

#define PAGE_SIZE 4096

/*
 * This is memset(rsp, 0xcc, -1); but down.
 * It will SIGSEGV when bottom of the stack is reached.
 * Byte-size access is important! (see rdi tweak in the signal handler).
 */
void make_stack1(void);
asm(
".pushsection .text\n"
".globl make_stack1\n"
".align 16\n"
"make_stack1:\n"
        "mov $0xcc, %al\n"
#if defined __amd64__
        "mov %rsp, %rdi\n"
        "mov $-1, %rcx\n"
#elif defined __i386__
        "mov %esp, %edi\n"
        "mov $-1, %ecx\n"
#else
#error
#endif
        "std\n"
        "rep stosb\n"
        /* unreachable */
        "hlt\n"
".type make_stack1,@function\n"
".size make_stack1,.-make_stack1\n"
".popsection\n"
);

/*
 * memset(p, 0xcc, -1);
 * It will SIGSEGV when top of the stack is reached.
 */
void make_stack2(uint64_t p);
asm(
".pushsection .text\n"
".globl make_stack2\n"
".align 16\n"
"make_stack2:\n"
        "mov $0xcc, %al\n"
#if defined __amd64__
        "mov $-1, %rcx\n"
#elif defined __i386__
        "mov $-1, %ecx\n"
#else
#error
#endif
        "cld\n"
        "rep stosb\n"
        /* unreachable */
        "hlt\n"
".type make_stack2,@function\n"
".size make_stack2,.-make_stack2\n"
".popsection\n"
);

static volatile int test_state = 0;
static volatile unsigned long stack_min_addr;

#if defined __amd64__
#define RDI     REG_RDI
#define RIP     REG_RIP
#define RIP_STRING "rip"
#elif defined __i386__
#define RDI     REG_EDI
#define RIP     REG_EIP
#define RIP_STRING "eip"
#else
#error
#endif

static void sigsegv(int _, siginfo_t *__, void *uc_)
{
        /*
         * Some Linux versions didn't clear DF before entering signal
         * handler. make_stack1() doesn't have a chance to clear DF
         * either so we clear it by hand here.
         */
        asm volatile ("cld" ::: "memory");

        ucontext_t *uc = uc_;

        if (test_state == 0) {
                /* Stack is faulted and cleared from RSP to the lowest address. */
                stack_min_addr = ++uc->uc_mcontext.gregs[RDI];
                if (1) {
                        printf("stack min %lx\n", stack_min_addr);
                }
                uc->uc_mcontext.gregs[RIP] = (uintptr_t)&make_stack2;
                test_state = 1;
        } else if (test_state == 1) {
                /* Stack has been cleared from top to bottom. */
                unsigned long stack_max_addr = uc->uc_mcontext.gregs[RDI];
                if (1) {
                        printf("stack max %lx\n", stack_max_addr);
                }
                /* Start faulting pages on stack and see what happens. */
                uc->uc_mcontext.gregs[RIP] = stack_max_addr - PAGE_SIZE;
                test_state = 2;
        } else if (test_state == 2) {
                /* Stack page is NX -- good, test next page. */
                uc->uc_mcontext.gregs[RIP] -= PAGE_SIZE;
                if (uc->uc_mcontext.gregs[RIP] == stack_min_addr) {
                        /* One more SIGSEGV and test ends. */
                        test_state = 3;
                }
        } else {
                printf("PASS\tAll stack pages are NX\n");
                _exit(EXIT_SUCCESS);
        }
}

static void sigtrap(int _, siginfo_t *__, void *uc_)
{
        const ucontext_t *uc = uc_;
        unsigned long rip = uc->uc_mcontext.gregs[RIP];
        printf("FAIL\texecutable page on the stack: " RIP_STRING " %lx\n", rip);
        _exit(EXIT_FAILURE);
}

int main(void)
{
        {
                struct sigaction act = {};
                sigemptyset(&act.sa_mask);
                act.sa_flags = SA_SIGINFO;
                act.sa_sigaction = &sigsegv;
                int rv = sigaction(SIGSEGV, &act, NULL);
                assert(rv == 0);
        }
        {
                struct sigaction act = {};
                sigemptyset(&act.sa_mask);
                act.sa_flags = SA_SIGINFO;
                act.sa_sigaction = &sigtrap;
                int rv = sigaction(SIGTRAP, &act, NULL);
                assert(rv == 0);
        }
        {
                struct rlimit rlim;
                int rv = getrlimit(RLIMIT_STACK, &rlim);
                assert(rv == 0);
                /* Cap stack at time-honored 8 MiB value. */
                rlim.rlim_max = rlim.rlim_cur;
                if (rlim.rlim_max > 8 * 1024 * 1024) {
                        rlim.rlim_max = 8 * 1024 * 1024;
                }
                rv = setrlimit(RLIMIT_STACK, &rlim);
                assert(rv == 0);
        }
        {
                /*
                 * We don't know now much stack SIGSEGV handler uses.
                 * Bump this by 1 page every time someone complains,
                 * or rewrite it in assembly.
                 */
                const size_t len = SIGSTKSZ;
                void *p = mmap(NULL, len, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
                assert(p != MAP_FAILED);
                stack_t ss = {};
                ss.ss_sp = p;
                ss.ss_size = len;
                int rv = sigaltstack(&ss, NULL);
                assert(rv == 0);
        }
        make_stack1();
        /*
         * Unreachable, but if _this_ INT3 is ever reached, it's a bug somewhere.
         * Fold it into main SIGTRAP pathway.
         */
        __builtin_trap();
}