/*
 * Copyright © 2018 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 /proc/$KERNEL_THREAD/fd/ is empty.

#undef NDEBUG
#include <sys/syscall.h>
#include <assert.h>
#include <dirent.h>
#include <limits.h>
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

#include "proc.h"

#define PF_KHTREAD 0x00200000

/*
 * Test for kernel threadness atomically with openat().
 *
 * Return /proc/$PID/fd descriptor if process is kernel thread.
 * Return -1 if a process is userspace process.
 */
static int kernel_thread_fd(unsigned int pid)
{
        unsigned int flags = 0;
        char buf[4096];
        int dir_fd, fd;
        ssize_t rv;

        snprintf(buf, sizeof(buf), "/proc/%u", pid);
        dir_fd = open(buf, O_RDONLY|O_DIRECTORY);
        if (dir_fd == -1)
                return -1;

        /*
         * Believe it or not, struct task_struct::flags is directly exposed
         * to userspace!
         */
        fd = openat(dir_fd, "stat", O_RDONLY);
        if (fd == -1) {
                close(dir_fd);
                return -1;
        }
        rv = read(fd, buf, sizeof(buf));
        close(fd);
        if (0 < rv && rv <= sizeof(buf)) {
                unsigned long long flags_ull;
                char *p, *end;
                int i;

                assert(buf[rv - 1] == '\n');
                buf[rv - 1] = '\0';

                /* Search backwards: ->comm can contain whitespace and ')'. */
                for (i = 0; i < 43; i++) {
                        p = strrchr(buf, ' ');
                        assert(p);
                        *p = '\0';
                }

                p = strrchr(buf, ' ');
                assert(p);

                flags_ull = xstrtoull(p + 1, &end);
                assert(*end == '\0');
                assert(flags_ull == (unsigned int)flags_ull);

                flags = flags_ull;
        }

        fd = -1;
        if (flags & PF_KHTREAD) {
                fd = openat(dir_fd, "fd", O_RDONLY|O_DIRECTORY);
        }
        close(dir_fd);
        return fd;
}

static void test_readdir(int fd)
{
        DIR *d;
        struct dirent *de;

        d = fdopendir(fd);
        assert(d);

        de = xreaddir(d);
        assert(streq(de->d_name, "."));
        assert(de->d_type == DT_DIR);

        de = xreaddir(d);
        assert(streq(de->d_name, ".."));
        assert(de->d_type == DT_DIR);

        de = xreaddir(d);
        assert(!de);
}

static inline int sys_statx(int dirfd, const char *pathname, int flags,
                            unsigned int mask, void *stx)
{
        return syscall(SYS_statx, dirfd, pathname, flags, mask, stx);
}

static void test_lookup_fail(int fd, const char *pathname)
{
        char stx[256] __attribute__((aligned(8)));
        int rv;

        rv = sys_statx(fd, pathname, AT_SYMLINK_NOFOLLOW, 0, (void *)stx);
        assert(rv == -1 && errno == ENOENT);
}

static void test_lookup(int fd)
{
        char buf[64];
        unsigned int u;
        int i;

        for (i = INT_MIN; i < INT_MIN + 1024; i++) {
                snprintf(buf, sizeof(buf), "%d", i);
                test_lookup_fail(fd, buf);
        }
        for (i = -1024; i < 1024; i++) {
                snprintf(buf, sizeof(buf), "%d", i);
                test_lookup_fail(fd, buf);
        }
        for (u = INT_MAX - 1024; u < (unsigned int)INT_MAX + 1024; u++) {
                snprintf(buf, sizeof(buf), "%u", u);
                test_lookup_fail(fd, buf);
        }
        for (u = UINT_MAX - 1024; u != 0; u++) {
                snprintf(buf, sizeof(buf), "%u", u);
                test_lookup_fail(fd, buf);
        }
}

int main(void)
{
        unsigned int pid;
        int fd;

        /*
         * In theory this will loop indefinitely if kernel threads are exiled
         * from /proc.
         *
         * Start with kthreadd.
         */
        pid = 2;
        while ((fd = kernel_thread_fd(pid)) == -1 && pid < 1024) {
                pid++;
        }
        /* EACCES if run as non-root. */
        if (pid >= 1024)
                return 1;

        test_readdir(fd);
        test_lookup(fd);

        return 0;
}