On 21/05/2019 13.34, Christian Brauner wrote:

The performance is striking. For good measure, comparing the following
simple close_all_fds() userspace implementation that is essentially just
glibc's version in [6]:

static int close_all_fds(void)
{
        DIR *dir;
        struct dirent *direntp;

        dir = opendir("/proc/self/fd");
        if (!dir)
                return -1;

        while ((direntp = readdir(dir))) {
                int fd;
                if (strcmp(direntp->d_name, ".") == 0)
                        continue;
                if (strcmp(direntp->d_name, "..") == 0)
                        continue;
                fd = atoi(direntp->d_name);
                if (fd == 0 || fd == 1 || fd == 2)
                        continue;
                close(fd);
        }

        closedir(dir); /* cannot fail */
        return 0;
}

Before anybody copy-pastes this, please note that it lacks a check for
fd == dirfd(dir). If all of /proc/self/fd is returned in the first
getdents() syscall one won't notice, but...

Rasmus