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

@Apteryks added an MIT license. Happy to dual-license it under the GPL too if you need.

I prefer GPL myself, as I like free software to remain free, but MIT being compatible with GPL, if that's your preference, it's OK! Thank you.

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.