Command-line tool to call renameat2(2)

renameat2.c

/*
 * Copyright (c) 2023 Russell Harmon
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */
#define _GNU_SOURCE
#include <stdlib.h>
#include <fcntl.h>
#include <stdio.h>
#include <getopt.h>
#include <stddef.h>
#include <stdbool.h>
#include <stdarg.h>
#include <errno.h>
#include <unistd.h>
#include <sys/syscall.h>

#ifndef RENAME_NOREPLACE
#define RENAME_NOREPLACE (1 << 0) /* Don't overwrite target */ // from include/uapi/linux/fs.h
#endif

#ifndef RENAME_EXCHANGE
#define RENAME_EXCHANGE (1 << 1) /* Exchange source and dest */ // from include/uapi/linux/fs.h
#endif

#ifndef SYS_renameat2
#if defined(__x86_64__)
#define SYS_renameat2 314 // from arch/x86/syscalls/syscall_64.tbl
#elif defined(__i386__)
#define SYS_renameat2 353 // from arch/x86/syscalls/syscall_32.tbl
#else
#error Architecture unsupported
#endif
#endif // ifndef SYS_renameat2

static void fatal_fprintf(FILE *out, const char * restrict fmt, ...) {
    va_list ap;

    va_start(ap, fmt);
    int ret = vfprintf(out, fmt, ap);
    int saved_errno = errno;
    va_end(ap);

    if (ret < 0) {
        errno = saved_errno;
        perror("vfprintf");
        exit(EXIT_FAILURE);
    }
}

static void print_usage(FILE *out, char *progname) {
    fatal_fprintf(out, "Usage: %s [options] SOURCE DEST\n", progname);
    fatal_fprintf(out, "Call the renameat2(2) system call.\n");
    fatal_fprintf(out, "\n");
    fatal_fprintf(out, " -h, --help      This help message\n");
    fatal_fprintf(out, " -e, --exchange  Atomically exchange SOURCE and DEST\n");
    fatal_fprintf(out, " -n, --noreplace Don't overwrite DEST if it already exists\n");
}

int main(int argc, char *argv[]) {
    int flags = 0;

    while (true) {
        static const struct option long_options[] = {
            {"exchange", no_argument, NULL, 'e'},
            {"noreplace", no_argument, NULL, 'n'},
            {"help", no_argument, NULL, 'h'},
            {NULL, 0, NULL, 0}
        };

        int c = getopt_long(argc, argv, "enh", long_options, NULL);
        if (c == -1) {
            break;
        }

        switch (c) {
            case 'n':
                flags |= RENAME_NOREPLACE;
                break;
            case 'e':
                flags |= RENAME_EXCHANGE;
                break;
            case 'h':
                print_usage(stdout, argv[0]);
                exit(EXIT_SUCCESS);
            case '?':
                print_usage(stderr, argv[0]);
                exit(EXIT_FAILURE);
            default:
                fprintf(stderr, "?? getopt returned character code 0%o ??\n", c);
                exit(EXIT_FAILURE);
        }
    }

    if (argc - optind != 2) {
        print_usage(stderr, argv[0]);
        exit(EXIT_FAILURE);
    }
    char *source = argv[optind], *dest = argv[optind + 1];

    if (syscall(SYS_renameat2, AT_FDCWD, source, AT_FDCWD, dest, flags) != 0) {
        perror("renameat2");
        exit(EXIT_FAILURE);
    }
}

renameat2.x86_64

It's possible you can get it to work at boot with a custom initrd that sets up /sys in the way you want, but I suspect you'll have a lot of trouble getting it working after boot. However you could also potentially try and figure out if anything has /sys open (lsof -r /sys) and kill whatever has open fds into /sys and try again.

Thinking about it, lots of programs could be monitoring events in sysfs including systemd, udev, etc. But a clean swap could just carry all open fd_s with it, but obviously not without upsetting this system call. So I'm looking at plan "b": to start with making lsusb take a new command line option: --sysfs-root=<alternate_to_slash_sys> . So far can only fool lsusb -t because the rest of lsusb uses libusb which is carrying 30 years of crud making it hard to decipher. As soon as I write a manage, I'll push clone_pseudo_fs . It can clone /sys , /dev and /proc (when /proc/kmsg is excluded).

@doug-gilbert I think mount --move should do what you need, but you'll need to unshare mounts first (mount --make-rprivate /); and hope there's no other mount namespace around using it (containers etc...)

@eatnumber1 thanks for the license from me as well, also using this!

@doug-gilbert I think mount --move should do what you need, but you'll need to unshare mounts first (mount --make-rprivate /); and hope there's no other mount namespace around using it (containers etc...)

mount --move /sys /tmp/ssys/
mount: /tmp/ssys: bad option; moving a mount residing under a shared mount is unsupported.
       dmesg(1) may have more information after failed mount system call.

Tried to move /tmp and got the same error message. Nothing was added to /var/log/syslog when those mounts were run (on Debian 12.0). Looks like another dead-end . Pushed https://github.com/doug-gilbert/clone_pseudo_fs but its README is wrong. Basically works with some rough edges.

Try reading (or actually running commands) until the end:

you'll need to unshare mounts first (mount --make-rprivate /); and hope there's no other mount namespace around using it (containers etc...)

(if you want to do that just for some specific command, unshare -m is a great way to manipulate mount points locally without disturbing anyone as well; you could even just umount the old /sys or mount something else over it and nobody would care)

Anyway, it's off topic for renameat; will stop spamming everyone on this topic.

Good news, util-linux starting with this 2023-12-14 commit has an exch command. It will take a while for it to trickle down though.