Re: [PATCH 11/13] example/service_ll: create a sample systemd service fuse server

Darrick J. Wong
2026-04-14T23:56:11Z

On Thu, Apr 09, 2026 at 03:23:24PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
>
> Create a simple fuse server that can be run as a systemd service.
> I plan to create some more single-file fuse server examples, so most of
> the boilerplate code goes in a separate file.
>
> Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
> ---
> example/single_file.h | 149 ++++++++++
> lib/util.h | 35 ++
> example/meson.build | 14 +
> example/service_ll.c | 308 ++++++++++++++++++++
> example/service_ll.socket.in | 15 +
> example/service_ll@.service | 102 +++++++
> example/single_file.c | 646 ++++++++++++++++++++++++++++++++++++++++++
> meson.build | 1
> 8 files changed, 1270 insertions(+)
> create mode 100644 example/single_file.h
> create mode 100644 example/service_ll.c
> create mode 100644 example/service_ll.socket.in
> create mode 100644 example/service_ll@.service
> create mode 100644 example/single_file.c
>
>
> diff --git a/example/single_file.h b/example/single_file.h
> new file mode 100644
> index 00000000000000..af91ea8196a408
> --- /dev/null
> +++ b/example/single_file.h
> @@ -0,0 +1,149 @@
> +/*
> + * FUSE: Filesystem in Userspace
> + * Copyright (C) 2026 Oracle.
> + *
> + * This program can be distributed under the terms of the GNU GPLv2.
> + * See the file GPL2.txt.
> + */
> +#ifndef FUSE_SINGLE_FILE_H_
> +#define FUSE_SINGLE_FILE_H_
> +
> +static inline uint64_t round_up(uint64_t b, unsigned int align)
> +{
> + unsigned int m;
> +
> + if (align == 0)
> + return b;
> + m = b % align;
> + if (m)
> + b += align - m;
> + return b;
> +}
> +
> +static inline uint64_t round_down(uint64_t b, unsigned int align)
> +{
> + unsigned int m;
> +
> + if (align == 0)
> + return b;
> + m = b % align;
> + return b - m;
> +}
> +
> +static inline uint64_t howmany(uint64_t b, unsigned int align)
> +{
> + unsigned int m;
> +
> + if (align == 0)
> + return b;
> + m = (b % align) ? 1 : 0;
> + return (b / align) + m;
> +}
> +
> +struct single_file {
> + int backing_fd;
> +
> + int64_t isize;
> + uint64_t blocks;
> +
> + mode_t mode;
> +
> + bool ro;
> + bool allow_dio;
> + bool sync;
> + bool require_bdev;
> +
> + unsigned int blocksize;
> +
> + struct timespec atime;
> + struct timespec mtime;
> +
> + pthread_mutex_t lock;
> +};
> +
> +extern struct single_file single_file;
> +
> +static inline uint64_t b_to_fsbt(uint64_t off)
> +{
> + return off / single_file.blocksize;
> +}
> +
> +static inline uint64_t b_to_fsb(uint64_t off)
> +{
> + return (off + single_file.blocksize - 1) / single_file.blocksize;
> +}
> +
> +static inline uint64_t fsb_to_b(uint64_t fsb)
> +{
> + return fsb * single_file.blocksize;
> +}
> +
> +enum single_file_opt_keys {
> + SINGLE_FILE_RO = 171717, /* how many options could we possibly have? */
> + SINGLE_FILE_RW,
> + SINGLE_FILE_REQUIRE_BDEV,
> + SINGLE_FILE_DIO,
> + SINGLE_FILE_NODIO,
> + SINGLE_FILE_SYNC,
> + SINGLE_FILE_NOSYNC,
> + SINGLE_FILE_SIZE,
> + SINGLE_FILE_BLOCKSIZE,
> +
> + SINGLE_FILE_NR_KEYS,
> +};
> +
> +#define SINGLE_FILE_OPT_KEYS \
> + FUSE_OPT_KEY("ro", SINGLE_FILE_RO), \
> + FUSE_OPT_KEY("rw", SINGLE_FILE_RW), \
> + FUSE_OPT_KEY("require_bdev", SINGLE_FILE_REQUIRE_BDEV), \
> + FUSE_OPT_KEY("dio", SINGLE_FILE_DIO), \
> + FUSE_OPT_KEY("nodio", SINGLE_FILE_NODIO), \
> + FUSE_OPT_KEY("sync", SINGLE_FILE_SYNC), \
> + FUSE_OPT_KEY("nosync", SINGLE_FILE_NOSYNC), \
> + FUSE_OPT_KEY("size=%s", SINGLE_FILE_SIZE), \
> + FUSE_OPT_KEY("blocksize=%s", SINGLE_FILE_BLOCKSIZE)
> +
> +int single_file_opt_proc(void *data, const char *arg, int key,
> + struct fuse_args *outargs);
> +
> +unsigned long long parse_num_blocks(const char *arg, int log_block_size);
> +
> +struct fuse_service;
> +int single_file_service_open(struct fuse_service *sf, const char *path);
> +
> +void single_file_check_read(off_t pos, size_t *count);
> +int single_file_check_write(off_t pos, size_t *count);
> +
> +int single_file_configure(const char *device, const char *filename);
> +int single_file_configure_simple(const char *filename);
> +void single_file_close(void);
> +
> +/* low-level fuse operation handlers */
> +
> +bool is_single_file_child(fuse_ino_t parent, const char *name);
> +bool is_single_file_ino(fuse_ino_t ino);
> +
> +void single_file_ll_readdir(fuse_req_t req, fuse_ino_t ino, size_t size,
> + off_t off, struct fuse_file_info *fi);
> +
> +void single_file_ll_statfs(fuse_req_t req, fuse_ino_t ino);
> +
> +void single_file_ll_statx(fuse_req_t req, fuse_ino_t ino, int flags, int mask,
> + struct fuse_file_info *fi);
> +
> +void single_file_ll_getattr(fuse_req_t req, fuse_ino_t ino,
> + struct fuse_file_info *fi);
> +void single_file_ll_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr,
> + int to_set, struct fuse_file_info *fi);
> +
> +void single_file_ll_lookup(fuse_req_t req, fuse_ino_t parent, const char *name);
> +void single_file_ll_open(fuse_req_t req, fuse_ino_t ino,
> + struct fuse_file_info *fi);
> +
> +void single_file_ll_fsync(fuse_req_t req, fuse_ino_t ino, int datasync,
> + struct fuse_file_info *fi);
> +
> +int reply_buf_limited(fuse_req_t req, const char *buf, size_t bufsize,
> + off_t off, size_t maxsize);
> +
> +#endif /* FUSE_SINGLE_FILE_H_ */
> diff --git a/lib/util.h b/lib/util.h
> index 107a2bfdd6105b..6ec6604fb74caf 100644
> --- a/lib/util.h
> +++ b/lib/util.h
> @@ -4,6 +4,9 @@
> #include <stdint.h>
> #include <stdbool.h>
>
> +#define max(x, y) ((x) > (y) ? (x) : (y))
> +#define min(x, y) ((x) < (y) ? (x) : (y))
> +
> #define ROUND_UP(val, round_to) (((val) + (round_to - 1)) & ~(round_to - 1))
>
> #define likely(x) __builtin_expect(!!(x), 1)
> @@ -46,4 +49,36 @@ static inline uint64_t fuse_higher_32_bits(uint64_t nr)
> #define fallthrough do {} while (0)
> #endif
>
> +static inline uint64_t round_up(uint64_t b, unsigned int align)
> +{
> + unsigned int m;
> +
> + if (align == 0)
> + return b;
> + m = b % align;
> + if (m)
> + b += align - m;
> + return b;
> +}
> +
> +static inline uint64_t round_down(uint64_t b, unsigned int align)
> +{
> + unsigned int m;
> +
> + if (align == 0)
> + return b;
> + m = b % align;
> + return b - m;
> +}
> +
> +static inline uint64_t howmany(uint64_t b, unsigned int align)
> +{
> + unsigned int m;
> +
> + if (align == 0)
> + return b;
> + m = (b % align) ? 1 : 0;
> + return (b / align) + m;
> +}
> +
> #endif /* FUSE_UTIL_H_ */
> diff --git a/example/meson.build b/example/meson.build
> index 76cf2d96db0349..e948f6ba74fdfa 100644
> --- a/example/meson.build
> +++ b/example/meson.build
> @@ -12,6 +12,15 @@ if not platform.endswith('bsd') and platform != 'dragonfly'
> examples += [ 'null' ]
> endif
>
> +single_file_examples = [ ]
> +
> +if platform.endswith('linux')
> + single_file_examples += [ 'service_ll' ]
> + configure_file(input: 'service_ll.socket.in',
> + output: 'service_ll.socket',
> + configuration: private_cfg)
> +endif
> +
> threaded_examples = [ 'notify_inval_inode',
> 'invalidate_path',
> 'notify_store_retrieve',
> @@ -25,6 +34,11 @@ foreach ex : examples
> install: false)
> endforeach
>
> +foreach ex : single_file_examples
> + executable(ex, [ex + '.c', 'single_file.c'],
> + dependencies: [ libfuse_dep ],
> + install: false)
> +endforeach
>
> foreach ex : threaded_examples
> executable(ex, ex + '.c',
> diff --git a/example/service_ll.c b/example/service_ll.c
> new file mode 100644
> index 00000000000000..da1d64f03be5f2
> --- /dev/null
> +++ b/example/service_ll.c
> @@ -0,0 +1,308 @@
> +/*
> + * FUSE: Filesystem in Userspace
> + * Copyright (C) 2026 Oracle.
> + *
> + * This program can be distributed under the terms of the GNU GPLv2.
> + * See the file GPL2.txt.
> + */
> +
> +/** @file
> + *
> + * minimal example filesystem using low-level API and systemd service api
> + *
> + * Compile with:
> + *
> + * gcc -Wall single_file.c service_ll.c `pkg-config fuse3 --cflags --libs` -o service_ll
> + *
> + * Note: If the pkg-config command fails due to the absence of the fuse3.pc
> + * file, you should configure the path to the fuse3.pc file in the
> + * PKG_CONFIG_PATH variable.
> + *
> + * Change the ExecStart line in service_ll@.service:
> + *
> + * ExecStart=/path/to/service_ll
> + *
> + * to point to the actual path of the service_ll binary.
> + *
> + * Finally, install the service_ll@.service and service_ll.socket files to the
> + * systemd service directory, usually /run/systemd/system.
> + *
> + * ## Source code ##
> + * \include service_ll.c
> + * \include service_ll.socket
> + * \include service_ll@.service
> + * \include single_file.c
> + * \include single_file.h
> + */
> +
> +#define FUSE_USE_VERSION FUSE_MAKE_VERSION(3, 19)
> +
> +#ifndef _GNU_SOURCE
> +#define _GNU_SOURCE
> +#endif
> +
> +#include <fuse_lowlevel.h>
> +#include <fuse_service.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <errno.h>
> +#include <fcntl.h>
> +#include <unistd.h>
> +#include <assert.h>
> +#include <pthread.h>
> +#include "single_file.h"
> +
> +struct service_ll {
> + struct fuse_session *se;
> + char *device;
> + struct fuse_service *service;
> +
> + /* really booleans */
> + int debug;
> +};
> +
> +static struct service_ll ll = { };
> +
> +static void service_ll_init(void *userdata, struct fuse_conn_info *conn)
> +{
> + (void)userdata;
> +
> + conn->time_gran = 1;
> +}
> +
> +static void service_ll_read(fuse_req_t req, fuse_ino_t ino, size_t count,
> + off_t pos, struct fuse_file_info *fi)
> +{
> + void *buf = NULL;
> + ssize_t got;
> + int ret;
> +
> + if (!is_single_file_ino(ino)) {
> + ret = EIO;
> + goto out_reply;
> + }
> +
> + if (ll.debug)
> + fprintf(stderr, "%s: pos 0x%llx count 0x%llx\n",
> + __func__,
> + (unsigned long long)pos,
> + (unsigned long long)count);
> +
> + if (!single_file.allow_dio && fi->direct_io) {
> + ret = ENOSYS;
> + goto out_reply;
> + }
> +
> + single_file_check_read(pos, &count);
> +
> + if (!count) {
> + fuse_reply_buf(req, buf, 0);
> + return;
> + }
> +
> + buf = malloc(count);
> + if (!buf) {
> + ret = ENOMEM;
> + goto out_reply;
> + }
> +
> + got = pread(single_file.backing_fd, buf, count, pos);
> + if (got < 0) {
> + ret = errno;
> + goto out_reply;
> + }
> +
> + fuse_reply_buf(req, buf, got);
> + goto out_buf;
> +
> +out_reply:
> + fuse_reply_err(req, ret);
> +out_buf:
> + free(buf);
> +}
> +
> +static void service_ll_write(fuse_req_t req, fuse_ino_t ino, const char *buf,
> + size_t count, off_t pos,
> + struct fuse_file_info *fi)
> +{
> + ssize_t got;
> + int ret;
> +
> + if (!is_single_file_ino(ino)) {
> + ret = EIO;
> + goto out_reply;
> + }
> +
> + if (ll.debug)
> + fprintf(stderr, "%s: pos 0x%llx count 0x%llx\n",
> + __func__,
> + (unsigned long long)pos,
> + (unsigned long long)count);
> +
> + if (!single_file.allow_dio && fi->direct_io) {
> + ret = ENOSYS;
> + goto out_reply;
> + }
> +
> + ret = -single_file_check_write(pos, &count);
> + if (ret)
> + goto out_reply;
> +
> + got = pwrite(single_file.backing_fd, buf, count, pos);
> + if (got < 0) {
> + ret = errno;
> + goto out_reply;
> + }
> +
> + if (single_file.sync) {
> + ret = fsync(single_file.backing_fd);
> + if (ret < 0) {
> + ret = errno;
> + goto out_reply;
> + }
> + }
> +
> + fuse_reply_write(req, got);
> + return;
> +
> +out_reply:
> + fuse_reply_err(req, ret);
> +}
> +
> +static const struct fuse_lowlevel_ops service_ll_oper = {
> + .lookup = single_file_ll_lookup,
> + .getattr = single_file_ll_getattr,
> + .setattr = single_file_ll_setattr,
> + .readdir = single_file_ll_readdir,
> + .open = single_file_ll_open,
> + .statfs = single_file_ll_statfs,
> + .statx = single_file_ll_statx,
> + .fsync = single_file_ll_fsync,
> +
> + .init = service_ll_init,
> + .read = service_ll_read,
> + .write = service_ll_write,
> +};
> +
> +#define SERVICE_LL_OPT(t, p, v) { t, offsetof(struct service_ll, p), v }
> +
> +static struct fuse_opt service_ll_opts[] = {
> + SERVICE_LL_OPT("debug", debug, 1),
> + SINGLE_FILE_OPT_KEYS,
> + FUSE_OPT_END
> +};
> +
> +static int service_ll_opt_proc(void *data, const char *arg, int key,
> + struct fuse_args *outargs)
> +{
> + int ret = single_file_opt_proc(data, arg, key, outargs);
> +
> + if (ret < 1)
> + return ret;
> +
> + switch (key) {
> + case FUSE_OPT_KEY_NONOPT:
> + if (!ll.device) {
> + ll.device = strdup(arg);
> + return 0;
> + }
> + return 1;
> + }
> +
> + return 1;
> +}
> +
> +int main(int argc, char *argv[])
> +{
> + struct fuse_args args = FUSE_ARGS_INIT(argc, argv);
> + struct fuse_cmdline_opts opts = { };
> + int ret = 1;
> +
> + if (fuse_service_accept(&ll.service))
> + goto err_args;
> +
> + if (!fuse_service_accepted(ll.service))
> + goto err_args;
> +
> + if (fuse_service_append_args(ll.service, &args))
> + goto err_service;
> +
> + if (fuse_opt_parse(&args, &ll, service_ll_opts, service_ll_opt_proc))
> + goto err_service;
> +
> + if (fuse_service_parse_cmdline_opts(&args, &opts))
> + goto err_service;
> +
> + if (opts.show_help) {
> + printf("usage: %s [options] <device> <mountpoint>\n\n", argv[0]);
> + fuse_cmdline_help();
> + fuse_lowlevel_help();
> + ret = 0;
> + goto err_service;
> + } else if (opts.show_version) {
> + printf("FUSE library version %s\n", fuse_pkgversion());
> + fuse_lowlevel_version();
> + ret = 0;
> + goto err_service;
> + }
> +
> + if (!opts.mountpoint || !ll.device) {
> + printf("usage: %s [options] <device> <mountpoint>\n", argv[0]);
> + printf(" %s --help\n", argv[0]);
> + goto err_service;
> + }
> +
> + if (single_file_service_open(ll.service, ll.device))
> + goto err_service;
> +
> + if (fuse_service_finish_file_requests(ll.service))
> + goto err_singlefile;
> +
> + if (single_file_configure(ll.device, NULL))
> + goto err_singlefile;
> +
> + ll.se = fuse_session_new(&args, &service_ll_oper,
> + sizeof(service_ll_oper), NULL);
> + if (ll.se == NULL)
> + goto err_singlefile;
> +
> + if (fuse_set_signal_handlers(ll.se))
> + goto err_session;
> +
> + if (fuse_service_session_mount(ll.service, ll.se, S_IFDIR, &opts))
> + goto err_signals;
> +
> + fuse_service_send_goodbye(ll.service, 0);
> + fuse_service_release(ll.service);
> +
> + fuse_daemonize(opts.foreground);

This is unnecessary -- fuse services are already daemons, being systemd
services. fuse_service_session_mount should just do the chdir and leave
everything alone; it already sets opts->foreground = 1.

> +
> + /* Block until ctrl+c or fusermount -u */
> + if (opts.singlethread) {
> + ret = fuse_session_loop(ll.se);
> + } else {
> + struct fuse_loop_config *config = fuse_loop_cfg_create();

NULL pointer check needed here.

> +
> + fuse_loop_cfg_set_clone_fd(config, opts.clone_fd);
> + fuse_loop_cfg_set_max_threads(config, opts.max_threads);
> + ret = fuse_session_loop_mt(ll.se, config);
> + fuse_loop_cfg_destroy(config);
> + }
> +
> + fuse_session_unmount(ll.se);

You can't unmount from within a service if you've already sent the
goodby packet, so the fallible calls (e.g. fuse_loop_cfg_create) should
happen before fuse_service_session_mount.

Also if a service is going to unmount, it should be calling
fuse_service_session_unmount(ll.service).

--D

> +err_signals:
> + fuse_remove_signal_handlers(ll.se);
> +err_session:
> + fuse_session_destroy(ll.se);
> +err_singlefile:
> + single_file_close();
> +err_service:
> + free(opts.mountpoint);
> + free(ll.device);
> + fuse_service_send_goodbye(ll.service, ret);
> + fuse_service_destroy(&ll.service);
> +err_args:
> + fuse_opt_free_args(&args);
> + return fuse_service_exit(ret);
> +}
> diff --git a/example/service_ll.socket.in b/example/service_ll.socket.in
> new file mode 100644
> index 00000000000000..c41c382878a0cd
> --- /dev/null
> +++ b/example/service_ll.socket.in
> @@ -0,0 +1,15 @@
> +# SPDX-License-Identifier: GPL-2.0-or-later
> +#
> +# Copyright (C) 2026 Oracle. All Rights Reserved.
> +# Author: Darrick J. Wong <djwong@kernel.org>
> +[Unit]
> +Description=Socket for service_ll Service
> +
> +[Socket]
> +ListenSequentialPacket=@FUSE_SERVICE_SOCKET_DIR_RAW@/service_ll
> +Accept=yes
> +SocketMode=@FUSE_SERVICE_SOCKET_PERMS@
> +RemoveOnStop=yes
> +
> +[Install]
> +WantedBy=sockets.target
> diff --git a/example/service_ll@.service b/example/service_ll@.service
> new file mode 100644
> index 00000000000000..016d839babe3cc
> --- /dev/null
> +++ b/example/service_ll@.service
> @@ -0,0 +1,102 @@
> +# SPDX-License-Identifier: GPL-2.0-or-later
> +#
> +# Copyright (C) 2026 Oracle. All Rights Reserved.
> +# Author: Darrick J. Wong <djwong@kernel.org>
> +[Unit]
> +Description=service_ll Sample Fuse Service
> +
> +# Don't leave failed units behind, systemd does not clean them up!
> +CollectMode=inactive-or-failed
> +
> +[Service]
> +Type=exec
> +ExecStart=/path/to/service_ll
> +
> +# Try to capture core dumps
> +LimitCORE=infinity
> +
> +SyslogIdentifier=%N
> +
> +# No realtime CPU scheduling
> +RestrictRealtime=true
> +
> +# Don't let us see anything in the regular system, and don't run as root
> +DynamicUser=true
> +ProtectSystem=strict
> +ProtectHome=true
> +PrivateTmp=true
> +PrivateDevices=true
> +PrivateUsers=true
> +
> +# No network access
> +PrivateNetwork=true
> +ProtectHostname=true
> +RestrictAddressFamilies=none
> +IPAddressDeny=any
> +
> +# Don't let the program mess with the kernel configuration at all
> +ProtectKernelLogs=true
> +ProtectKernelModules=true
> +ProtectKernelTunables=true
> +ProtectControlGroups=true
> +ProtectProc=invisible
> +RestrictNamespaces=true
> +RestrictFileSystems=
> +
> +# Hide everything in /proc, even /proc/mounts
> +ProcSubset=pid
> +
> +# Only allow the default personality Linux
> +LockPersonality=true
> +
> +# No writable memory pages
> +MemoryDenyWriteExecute=true
> +
> +# Don't let our mounts leak out to the host
> +PrivateMounts=true
> +
> +# Restrict system calls to the native arch and only enough to get things going
> +SystemCallArchitectures=native
> +SystemCallFilter=@system-service
> +SystemCallFilter=~@privileged
> +SystemCallFilter=~@resources
> +
> +SystemCallFilter=~@clock
> +SystemCallFilter=~@cpu-emulation
> +SystemCallFilter=~@debug
> +SystemCallFilter=~@module
> +SystemCallFilter=~@reboot
> +SystemCallFilter=~@swap
> +
> +SystemCallFilter=~@mount
> +
> +# libfuse io_uring wants to pin cores and memory
> +SystemCallFilter=mbind
> +SystemCallFilter=sched_setaffinity
> +
> +# Leave a breadcrumb if we get whacked by the system call filter
> +SystemCallErrorNumber=EL3RST
> +
> +# Log to the kernel dmesg, just like an in-kernel filesystem driver
> +StandardOutput=append:/dev/ttyprintk
> +StandardError=append:/dev/ttyprintk
> +
> +# Run with no capabilities at all
> +CapabilityBoundingSet=
> +AmbientCapabilities=
> +NoNewPrivileges=true
> +
> +# We don't create files
> +UMask=7777
> +
> +# No access to hardware /dev files at all
> +ProtectClock=true
> +DevicePolicy=closed
> +
> +# Don't mess with set[ug]id anything.
> +RestrictSUIDSGID=true
> +
> +# Don't let OOM kills of processes in this containment group kill the whole
> +# service, because we don't want filesystem drivers to go down.
> +OOMPolicy=continue
> +OOMScoreAdjust=-1000
> diff --git a/example/single_file.c b/example/single_file.c
> new file mode 100644
> index 00000000000000..b2dedf59de9552
> --- /dev/null
> +++ b/example/single_file.c
> @@ -0,0 +1,646 @@
> +/*
> + * FUSE: Filesystem in Userspace
> + * Copyright (C) 2026 Oracle.
> + *
> + * This program can be distributed under the terms of the GNU GPLv2.
> + * See the file GPL2.txt.
> + */
> +#define _GNU_SOURCE
> +#include <pthread.h>
> +#include <errno.h>
> +#include <stdlib.h>
> +#include <errno.h>
> +#include <stdio.h>
> +#include <string.h>
> +#include <unistd.h>
> +#include <sys/ioctl.h>
> +#include <sys/stat.h>
> +#ifdef __linux__
> +#include <linux/fs.h>
> +#include <linux/stat.h>
> +#endif
> +
> +#define FUSE_USE_VERSION (FUSE_MAKE_VERSION(3, 19))
> +
> +#include "fuse_lowlevel.h"
> +#include "fuse_service.h"
> +#include "single_file.h"
> +
> +#define min(x, y) ((x) < (y) ? (x) : (y))
> +
> +#if __has_attribute(__fallthrough__)
> +#define fallthrough __attribute__((__fallthrough__))
> +#else
> +#define fallthrough do {} while (0)
> +#endif
> +
> +struct dirbuf {
> + char *p;
> + size_t size;
> +};
> +
> +struct single_file_stat {
> + struct fuse_entry_param entry;
> +};
> +
> +#define SINGLE_FILE_INO (FUSE_ROOT_ID + 1)
> +
> +static const char *single_file_name = "single_file";
> +static bool single_file_name_set;
> +
> +struct single_file single_file = {
> + .backing_fd = -1,
> + .allow_dio = true,
> + .mode = S_IFREG | 0444,
> + .lock = PTHREAD_MUTEX_INITIALIZER,
> +};
> +
> +static void dirbuf_add(fuse_req_t req, struct dirbuf *b, const char *name,
> + fuse_ino_t ino)
> +{
> + struct stat stbuf;
> + size_t oldsize = b->size;
> +
> + b->size += fuse_add_direntry(req, NULL, 0, name, NULL, 0);
> + b->p = (char *) realloc(b->p, b->size);
> + memset(&stbuf, 0, sizeof(stbuf));
> + stbuf.st_ino = ino;
> + fuse_add_direntry(req, b->p + oldsize, b->size - oldsize, name, &stbuf,
> + b->size);
> +}
> +
> +int reply_buf_limited(fuse_req_t req, const char *buf, size_t bufsize,
> + off_t off, size_t maxsize)
> +{
> + if (off < bufsize)
> + return fuse_reply_buf(req, buf + off,
> + min(bufsize - off, maxsize));
> + else
> + return fuse_reply_buf(req, NULL, 0);
> +}
> +
> +bool is_single_file_child(fuse_ino_t parent, const char *name)
> +{
> + return parent == FUSE_ROOT_ID &&
> + strcmp(name, single_file_name) == 0;
> +}
> +
> +bool is_single_file_ino(fuse_ino_t ino)
> +{
> + return ino == SINGLE_FILE_INO;
> +}
> +
> +void single_file_ll_readdir(fuse_req_t req, fuse_ino_t ino, size_t size,
> + off_t off, struct fuse_file_info *fi)
> +{
> + struct dirbuf b;
> +
> + (void) fi;
> +
> + switch (ino) {
> + case FUSE_ROOT_ID:
> + break;
> + case SINGLE_FILE_INO:
> + fuse_reply_err(req, ENOTDIR);
> + return;
> + default:
> + fuse_reply_err(req, ENOENT);
> + return;
> + }
> +
> + memset(&b, 0, sizeof(b));
> + dirbuf_add(req, &b, ".", FUSE_ROOT_ID);
> + dirbuf_add(req, &b, "..", FUSE_ROOT_ID);
> + dirbuf_add(req, &b, single_file_name, SINGLE_FILE_INO);
> + reply_buf_limited(req, b.p, b.size, off, size);
> + free(b.p);
> +}
> +
> +static bool sf_stat(fuse_ino_t ino, struct single_file_stat *llstat)
> +{
> + struct fuse_entry_param *entry = &llstat->entry;
> + struct stat *stbuf = &entry->attr;
> +
> + if (ino == FUSE_ROOT_ID) {
> + stbuf->st_mode = S_IFDIR | 0755;
> + stbuf->st_nlink = 2;
> + } else if (ino == SINGLE_FILE_INO) {
> + stbuf->st_mode = single_file.mode;
> + stbuf->st_nlink = 1;
> + stbuf->st_size = single_file.isize;
> + stbuf->st_blksize = single_file.blocksize;
> + stbuf->st_blocks = howmany(single_file.isize, 512);
> + stbuf->st_atim = single_file.atime;
> + stbuf->st_mtim = single_file.mtime;
> + } else {
> + return false;
> + }
> + stbuf->st_ino = ino;
> +
> + entry->generation = ino + 1;
> + entry->attr_timeout = 0.0;
> + entry->entry_timeout = 0.0;
> + entry->ino = ino;
> +
> + return true;
> +}
> +
> +#if defined(STATX_BASIC_STATS)
> +static inline void sf_set_statx_attr(struct statx *stx,
> + uint64_t statx_flag, int set)
> +{
> + if (set)
> + stx->stx_attributes |= statx_flag;
> + stx->stx_attributes_mask |= statx_flag;
> +}
> +
> +static void sf_statx_directio(struct statx *stx)
> +{
> + struct statx devx;
> + int ret;
> +
> + ret = statx(single_file.backing_fd, "", AT_EMPTY_PATH, STATX_DIOALIGN,
> + &devx);
> + if (ret)
> + return;
> + if (!(devx.stx_mask & STATX_DIOALIGN))
> + return;
> +
> + stx->stx_mask |= STATX_DIOALIGN;
> + stx->stx_dio_mem_align = devx.stx_dio_mem_align;
> + stx->stx_dio_offset_align = devx.stx_dio_offset_align;
> +}
> +
> +static bool sf_statx(fuse_ino_t ino, int statx_mask, struct statx *stx)
> +{
> + (void)statx_mask;
> +
> + if (ino == FUSE_ROOT_ID) {
> + stx->stx_mode = S_IFDIR | 0755;
> + stx->stx_nlink = 2;
> + } else if (ino == SINGLE_FILE_INO) {
> + stx->stx_mode = single_file.mode;
> + stx->stx_nlink = 1;
> + stx->stx_size = single_file.isize;
> + stx->stx_blksize = single_file.blocksize;
> + stx->stx_blocks = howmany(single_file.isize, 512);
> + stx->stx_atime.tv_sec = single_file.atime.tv_sec;
> + stx->stx_atime.tv_nsec = single_file.atime.tv_nsec;
> + stx->stx_mtime.tv_sec = single_file.mtime.tv_sec;
> + stx->stx_mtime.tv_nsec = single_file.mtime.tv_nsec;
> + } else {
> + return false;
> + }
> + stx->stx_mask = STATX_BASIC_STATS;
> + stx->stx_ino = ino;
> +
> + sf_set_statx_attr(stx, STATX_ATTR_IMMUTABLE, single_file.ro);
> + sf_statx_directio(stx);
> +
> + return true;
> +}
> +
> +void single_file_ll_statx(fuse_req_t req, fuse_ino_t ino, int flags, int mask,
> + struct fuse_file_info *fi)
> +{
> + struct statx stx = { };
> + bool filled;
> +
> + (void)flags;
> + (void)fi;
> +
> + pthread_mutex_lock(&single_file.lock);
> + filled = sf_statx(ino, mask, &stx);
> + pthread_mutex_unlock(&single_file.lock);
> + if (!filled)
> + fuse_reply_err(req, ENOENT);
> + else
> + fuse_reply_statx(req, 0, &stx, 0.0);
> +}
> +#else
> +void single_file_ll_statx(fuse_req_t req, fuse_ino_t ino, int flags, int mask,
> + struct fuse_file_info *fi)
> +{
> + fuse_reply_err(req, ENOSYS);
> +}
> +#endif /* STATX_BASIC_STATS */
> +
> +void single_file_ll_statfs(fuse_req_t req, fuse_ino_t ino)
> +{
> + struct statvfs buf;
> +
> + (void)ino;
> +
> + pthread_mutex_lock(&single_file.lock);
> + buf.f_bsize = single_file.blocksize;
> + buf.f_frsize = 0;
> +
> + buf.f_blocks = single_file.blocks;
> + buf.f_bfree = 0;
> + buf.f_bavail = 0;
> + buf.f_files = 1;
> + buf.f_ffree = 0;
> + buf.f_favail = 0;
> + buf.f_fsid = 0x50C00L;
> + buf.f_flag = 0;
> + if (single_file.ro)
> + buf.f_flag |= ST_RDONLY;
> + buf.f_namemax = 255;
> + pthread_mutex_unlock(&single_file.lock);
> +
> + fuse_reply_statfs(req, &buf);
> +}
> +
> +void single_file_ll_getattr(fuse_req_t req, fuse_ino_t ino,
> + struct fuse_file_info *fi)
> +{
> + struct single_file_stat llstat;
> + bool filled;
> +
> + (void) fi;
> +
> + memset(&llstat, 0, sizeof(llstat));
> + pthread_mutex_lock(&single_file.lock);
> + filled = sf_stat(ino, &llstat);
> + pthread_mutex_unlock(&single_file.lock);
> + if (!filled)
> + fuse_reply_err(req, ENOENT);
> + else
> + fuse_reply_attr(req, &llstat.entry.attr,
> + llstat.entry.attr_timeout);
> +}
> +
> +static void get_now(struct timespec *now)
> +{
> +#ifdef CLOCK_REALTIME
> + if (!clock_gettime(CLOCK_REALTIME, now))
> + return;
> +#endif
> +
> + now->tv_sec = time(NULL);
> + now->tv_nsec = 0;
> +}
> +
> +void single_file_ll_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr,
> + int to_set, struct fuse_file_info *fi)
> +{
> + struct timespec now;
> +
> + if (ino != SINGLE_FILE_INO)
> + goto deny;
> + if (to_set & (FUSE_SET_ATTR_UID | FUSE_SET_ATTR_GID |
> + FUSE_SET_ATTR_SIZE))
> + goto deny;
> +
> + get_now(&now);
> +
> + pthread_mutex_lock(&single_file.lock);
> + if (to_set & FUSE_SET_ATTR_MODE)
> + single_file.mode = (single_file.mode & S_IFMT) |
> + (attr->st_mode & ~S_IFMT);
> + if (to_set & FUSE_SET_ATTR_ATIME) {
> + if (to_set & FUSE_SET_ATTR_ATIME_NOW)
> + single_file.atime = now;
> + else
> + single_file.atime = attr->st_atim;
> + }
> + if (to_set & FUSE_SET_ATTR_MTIME) {
> + if (to_set & FUSE_SET_ATTR_MTIME_NOW)
> + single_file.mtime = now;
> + else
> + single_file.mtime = attr->st_mtim;
> + }
> + pthread_mutex_unlock(&single_file.lock);
> +
> + single_file_ll_getattr(req, ino, fi);
> + return;
> +deny:
> + fuse_reply_err(req, EPERM);
> +}
> +
> +void single_file_ll_lookup(fuse_req_t req, fuse_ino_t parent, const char *name)
> +{
> + struct single_file_stat llstat;
> + bool filled;
> +
> + if (!is_single_file_child(parent, name)) {
> + fuse_reply_err(req, ENOENT);
> + return;
> + }
> +
> + memset(&llstat, 0, sizeof(llstat));
> + pthread_mutex_lock(&single_file.lock);
> + filled = sf_stat(SINGLE_FILE_INO, &llstat);
> + pthread_mutex_unlock(&single_file.lock);
> + if (!filled)
> + fuse_reply_err(req, ENOENT);
> + else
> + fuse_reply_entry(req, &llstat.entry);
> +}
> +
> +void single_file_ll_open(fuse_req_t req, fuse_ino_t ino,
> + struct fuse_file_info *fi)
> +{
> + if (ino != SINGLE_FILE_INO)
> + fuse_reply_err(req, EISDIR);
> + else if (single_file.ro && (fi->flags & O_ACCMODE) != O_RDONLY)
> + fuse_reply_err(req, EACCES);
> + else
> + fuse_reply_open(req, fi);
> +}
> +
> +void single_file_ll_fsync(fuse_req_t req, fuse_ino_t ino, int datasync,
> + struct fuse_file_info *fi)
> +{
> + int ret = 0;
> +
> + (void)datasync;
> + (void)fi;
> +
> + if (ino == SINGLE_FILE_INO) {
> + ret = fsync(single_file.backing_fd);
> + if (ret)
> + ret = errno;
> + }
> +
> + fuse_reply_err(req, ret);
> +}
> +
> +unsigned long long parse_num_blocks(const char *arg, int log_block_size)
> +{
> + char *p;
> + unsigned long long num;
> +
> + num = strtoull(arg, &p, 0);
> +
> + if (p[0] && p[1])
> + return 0;
> +
> + switch (*p) {
> + case 'T': case 't':
> + num <<= 10;
> + fallthrough;
> + case 'G': case 'g':
> + num <<= 10;
> + fallthrough;
> + case 'M': case 'm':
> + num <<= 10;
> + fallthrough;
> + case 'K': case 'k':
> + if (log_block_size < 0)
> + num <<= 10;
> + else
> + num >>= log_block_size;
> + break;
> + case 's':
> + if (log_block_size < 0)
> + num <<= 9;
> + else
> + num >>= (1+log_block_size);
> + break;
> + case '\0':
> + break;
> + default:
> + return 0;
> + }
> + return num;
> +}
> +
> +static int single_file_set_blocksize(const char *arg)
> +{
> + unsigned long long l = parse_num_blocks(arg, -1);
> +
> + if (l < 512 || l > INT32_MAX || (l & (l - 1)) != 0) {
> + fprintf(stderr, "%s: block size must be power of two between 512 and 2G.\n",
> + arg);
> + return -1;
> + }
> +
> + /* do not pass through to libfuse */
> + single_file.blocksize = l;
> + return 0;
> +}
> +
> +static int single_file_set_size(const char *arg)
> +{
> + unsigned long long l = parse_num_blocks(arg, -1);
> +
> + if (l < 1 || (l & 511) != 0 || l > INT64_MAX) {
> + fprintf(stderr, "%s: size must be multiple of 512 and larger than zero.\n",
> + arg);
> + return -1;
> + }
> +
> + /* do not pass through to libfuse */
> + single_file.isize = l;
> + return 0;
> +}
> +
> +int single_file_opt_proc(void *data, const char *arg, int key,
> + struct fuse_args *outargs)
> +{
> + (void)data;
> + (void)outargs;
> +
> + switch (key) {
> + case SINGLE_FILE_RO:
> + /* pass through to libfuse */
> + single_file.ro = true;
> + return 1;
> + case SINGLE_FILE_RW:
> + /* pass through to libfuse */
> + single_file.ro = false;
> + return 1;
> + case SINGLE_FILE_REQUIRE_BDEV:
> + single_file.require_bdev = true;
> + return 0;
> + case SINGLE_FILE_DIO:
> + single_file.allow_dio = true;
> + return 0;
> + case SINGLE_FILE_NODIO:
> + single_file.allow_dio = false;
> + return 0;
> + case SINGLE_FILE_SYNC:
> + single_file.sync = true;
> + return 0;
> + case SINGLE_FILE_NOSYNC:
> + single_file.sync = false;
> + return 0;
> + case SINGLE_FILE_BLOCKSIZE:
> + return single_file_set_blocksize(arg + 10);
> + case SINGLE_FILE_SIZE:
> + return single_file_set_size(arg + 5);
> + }
> +
> + return 1;
> +}
> +
> +int single_file_service_open(struct fuse_service *sf, const char *path)
> +{
> + int open_flags = single_file.ro ? O_RDONLY : O_RDWR;
> + int fd;
> + int ret;
> +
> +again:
> + if (single_file.require_bdev)
> + ret = fuse_service_request_blockdev(sf, path,
> + open_flags | O_EXCL, 0, 0,
> + single_file.blocksize);
> + else
> + ret = fuse_service_request_file(sf, path, open_flags | O_EXCL,
> + 0, 0);
> + if (ret)
> + return ret;
> +
> + if (!single_file.ro && open_flags == O_RDONLY)
> + single_file.ro = true;
> +
> + ret = fuse_service_receive_file(sf, path, &fd);
> + if (ret)
> + return ret;
> +
> + /* downgrade from rw to ro if necessary */
> + if ((fd == -EPERM || fd == -EACCES) && open_flags == O_RDWR) {
> + open_flags = O_RDONLY;
> + goto again;
> + }
> +
> + if (fd < 0) {
> + fprintf(stderr, "%s: opening file: %s.\n",
> + path, strerror(-fd));
> + return -1;
> + }
> +
> + single_file.backing_fd = fd;
> + return 0;
> +}
> +
> +int single_file_check_write(off_t pos, size_t *count)
> +{
> + if (pos >= single_file.isize)
> + return -EFBIG;
> +
> + if (*count > single_file.isize)
> + *count = single_file.isize;
> + if (pos >= single_file.isize - *count)
> + *count = single_file.isize - pos;
> +
> + return 0;
> +}
> +
> +void single_file_check_read(off_t pos, size_t *count)
> +{
> + int ret = single_file_check_write(pos, count);
> +
> + if (ret)
> + *count = 0;
> +}
> +
> +int single_file_configure(const char *device, const char *filename)
> +{
> + struct stat statbuf;
> + unsigned long long backing_size;
> + unsigned int proposed_blocksize;
> + int lbasize;
> + int ret;
> +
> + ret = fstat(single_file.backing_fd, &statbuf);
> + if (ret) {
> + perror(device);
> + return -1;
> + }
> + lbasize = statbuf.st_blksize;
> + backing_size = statbuf.st_size;
> +
> + if (S_ISBLK(statbuf.st_mode)) {
> +#ifdef BLKSSZGET
> + ret = ioctl(single_file.backing_fd, BLKSSZGET, &lbasize);
> + if (ret) {
> + perror(device);
> + return -1;
> + }
> +#endif
> +
> +#ifdef BLKGETSIZE64
> + ret = ioctl(single_file.backing_fd, BLKGETSIZE64, &backing_size);
> + if (ret) {
> + perror(device);
> + return -1;
> + }
> +#endif
> + }
> +
> + if (backing_size == 0) {
> + fprintf(stderr, "%s: backing file size zero?\n", device);
> + return -1;
> + }
> +
> + if (lbasize == 0) {
> + fprintf(stderr, "%s: blocksize zero?\n", device);
> + return -1;
> + }
> +
> + proposed_blocksize = single_file.blocksize ? single_file.blocksize :
> + sysconf(_SC_PAGESIZE);
> + if (lbasize > proposed_blocksize) {
> + fprintf(stderr, "%s: lba size %u smaller than blocksize %u\n",
> + device, lbasize, proposed_blocksize);
> + return -1;
> + }
> +
> + if (single_file.isize % proposed_blocksize > 0) {
> + fprintf(stderr, "%s: size parameter %llu not congruent with blocksize %u\n",
> + device, (unsigned long long)single_file.isize,
> + proposed_blocksize);
> + return -1;
> + }
> +
> + if (single_file.isize > backing_size) {
> + fprintf(stderr, "%s: file size %llu smaller than size param %llu\n",
> + device, backing_size,
> + (unsigned long long)single_file.isize);
> + return -1;
> + }
> +
> + if (!single_file.blocksize)
> + single_file.blocksize = proposed_blocksize;
> + if (!single_file.isize)
> + single_file.isize = backing_size;
> +
> + single_file.isize = round_down(single_file.isize, single_file.blocksize);
> + single_file.blocks = single_file.isize / single_file.blocksize;
> +
> + return single_file_configure_simple(filename);
> +}
> +
> +int single_file_configure_simple(const char *filename)
> +{
> + if (!single_file.blocksize)
> + single_file.blocksize = sysconf(_SC_PAGESIZE);
> +
> + if (filename) {
> + char *n = strdup(filename);
> +
> + if (!n) {
> + perror(filename);
> + return -1;
> + }
> +
> + if (single_file_name_set)
> + free((void *)single_file_name);
> + single_file_name = n;
> + single_file_name_set = true;
> + }
> +
> + return 0;
> +}
> +
> +void single_file_close(void)
> +{
> + close(single_file.backing_fd);
> + single_file.backing_fd = -1;
> +
> + if (single_file_name_set)
> + free((void *)single_file_name);
> + single_file_name_set = false;
> +}
> diff --git a/meson.build b/meson.build
> index 827ec45ad3ad75..de038df8d92071 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -77,6 +77,7 @@ endif
> if service_socket_perms == ''
> service_socket_perms = '0220'
> endif
> +private_cfg.set('FUSE_SERVICE_SOCKET_DIR_RAW', service_socket_dir)
> private_cfg.set_quoted('FUSE_SERVICE_SOCKET_DIR', service_socket_dir)
> private_cfg.set('FUSE_SERVICE_SOCKET_PERMS', service_socket_perms)
>
>
>