Closing all open file descriptors

To avoid blindly closing file descriptors in the range of possible file descriptors, this is sometimes implemented (on Linux) by listing open file descriptors in /proc/self/fd/ and calling close(2) on each one. close_range() can take care of this without requiring /proc and within a single system call, which provides significant performance benefits.

Closing file descriptors before exec

File descriptors can be closed safely using

/* we don't want anything past stderr here */
close_range(3, ~0U, CLOSE_RANGE_UNSHARE);
execve(....);

CLOSE_RANGE_UNSHARE is conceptually equivalent to

unshare(CLONE_FILES);
close_range(first, last, 0);

but can be more efficient: if the unshared range extends past the current maximum number of file descriptors allocated in the caller's file descriptor table (the common case when last is ~0U), the kernel will unshare a new file descriptor table for the caller up to first, copying as few file descriptors as possible. This avoids subsequent close(2) calls entirely; the whole operation is complete once the table is unshared.

Closing files on exec

This is particularly useful in cases where multiple pre-exec setup steps risk conflicting with each other. For example, setting up a seccomp(2) profile can conflict with a close_range() call: if the file descriptors are closed before the seccomp(2) profile is set up, the profile setup can't use them itself, or control their closure; if the file descriptors are closed afterwards, the seccomp profile can't block the close_range() call or any fallbacks. Using CLOSE_RANGE_CLOEXEC avoids this: the descriptors can be marked before the seccomp(2) profile is set up, and the profile can control access to close_range() without affecting the calling process.

Program source

#define _GNU_SOURCE
#include <dirent.h>
#include <fcntl.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
/* Show the contents of the symbolic links in /proc/self/fd */
static void
show_fds(void)
{
    DIR            *dirp;
    char           path[PATH_MAX], target[PATH_MAX];
    ssize_t        len;
    struct dirent  *dp;
    dirp = opendir("/proc/self/fd");
    if (dirp  == NULL) {
        perror("opendir");
        exit(EXIT_FAILURE);
    }
    for (;;) {
        dp = readdir(dirp);
        if (dp == NULL)
            break;
        if (dp->d_type == DT_LNK) {
            snprintf(path, sizeof(path), "/proc/self/fd/%s",
                     dp->d_name);
            len = readlink(path, target, sizeof(target));
            printf("%s ==> %.*s\n", path, (int) len, target);
        }
    }
    closedir(dirp);
}
int
main(int argc, char *argv[])
{
    int  fd;
    for (size_t j = 1; j < argc; j++) {
        fd = open(argv[j], O_RDONLY);
        if (fd == -1) {
            perror(argv[j]);
            exit(EXIT_FAILURE);
        }
        printf("%s opened as FD %d\n", argv[j], fd);
    }
    show_fds();
    printf("========= About to call close_range() =======\n");
    if (close_range(3, ~0U, 0) == -1) {
        perror("close_range");
        exit(EXIT_FAILURE);
    }
    show_fds();
    exit(EXIT_FAILURE);
}