path: root/cmd/sharefs/fuse.go

package main

/*
#cgo pkg-config: --static fuse3

#include "fuse-helper.h"
#include <stdlib.h>
#include <string.h>

extern void *sharefs_init(struct fuse_conn_info *conn, struct fuse_config *cfg);
void sharefs_destroy(void *private_data);

typedef void (*closure)();
static inline struct fuse_opt _FUSE_OPT_END() { return (struct fuse_opt)FUSE_OPT_END; };
static inline int _fuse_main(int argc, char *argv[], const struct fuse_operations *op, void *user_data) { return fuse_main(argc, argv, op, user_data); }
*/
import "C"
import (
	"fmt"
	"log"
	"os"
	"path/filepath"
	"strconv"
	"syscall"
	"unsafe"
)

type (
	// closure represents a C function pointer.
	closure = C.closure

	// fuseArgs represents the fuse_args structure.
	fuseArgs = C.struct_fuse_args
)

var (
	// initFailed is set to true by sharefs_init if initialisation was unsuccessful.
	initFailed bool

	// initSource is pathname to the writable source directory used by sharefs_init to populate sharefs_private.
	initSource string
	// initSetUidGid is the uid and gid to set by sharefs_init when running as root.
	initSetUidGid [2]int
)

//export sharefs_init
func sharefs_init(_ *C.struct_fuse_conn_info, cfg *C.struct_fuse_config) unsafe.Pointer {
	private_data := C.malloc(C.size_t(unsafe.Sizeof(C.struct_sharefs_private{})))
	priv := (*C.struct_sharefs_private)(private_data)

	if os.Geteuid() == 0 {
		if initSetUidGid[0] <= 0 || initSetUidGid[1] <= 0 {
			log.Println("setuid and setgid must not be 0")
			goto fail
		}
		if err := syscall.Setresgid(initSetUidGid[1], initSetUidGid[1], initSetUidGid[1]); err != nil {
			log.Printf("cannot set gid: %v", err)
			goto fail
		}
		if err := syscall.Setgroups(nil); err != nil {
			log.Printf("cannot set supplementary groups: %v", err)
			goto fail
		}
		if err := syscall.Setresuid(initSetUidGid[0], initSetUidGid[0], initSetUidGid[0]); err != nil {
			log.Printf("cannot set uid: %v", err)
			goto fail
		}
	}

	cfg.use_ino = C.true
	cfg.direct_io = C.false
	// getattr is context-dependent
	cfg.attr_timeout = 0
	cfg.entry_timeout = 0
	cfg.negative_timeout = 0

	// all future filesystem operations happen through this dirfd
	if fd, err := syscall.Open(initSource, syscall.O_DIRECTORY|syscall.O_RDONLY, 0); err != nil {
		log.Printf("cannot open %q: %v", initSource, err)
		goto fail
	} else if err = syscall.Fchdir(fd); err != nil {
		log.Printf("cannot enter %q: %s", initSource, err)
		goto fail
	} else {
		priv.dirfd = C.int(fd)
	}

	return private_data

fail:
	C.free(private_data)
	C.fuse_exit(C.fuse_get_context().fuse)
	initFailed = true
	return nil
}

//export sharefs_destroy
func sharefs_destroy(private_data unsafe.Pointer) {
	if private_data != nil {
		defer C.free(private_data)
		priv := (*C.struct_sharefs_private)(private_data)

		if err := syscall.Close(int(priv.dirfd)); err != nil {
			log.Printf("cannot close source directory: %v", err)
		}
	}
}

// showHelp prints the help message.
func showHelp() {
	fmt.Printf("usage: %s [options] <mountpoint>\n\n", executableName)
	C.fuse_cmdline_help()
	C.fuse_lowlevel_help()
	fmt.Println("    -o source=/data/media  source directory to be mounted")
	fmt.Println("    -o setuid=1023         uid to run as when starting as root")
	fmt.Println("    -o setgid=1023         gid to run as when starting as root")
}

// parseOpts parses fuse options via fuse_opt_parse.
func parseOpts(args *C.struct_fuse_args) (
	source string,
	setuid, setgid int,
	ret int,
) {
	var unsafeOpts struct {
		// Pathname to writable source directory.
		source *C.char

		// Decimal string representation of uid to set when running as root.
		setuid *C.char
		// Decimal string representation of gid to set when running as root.
		setgid *C.char
	}

	if C.fuse_opt_parse(args, unsafe.Pointer(&unsafeOpts), &[]C.struct_fuse_opt{
		{templ: C.CString("source=%s"), offset: C.ulong(unsafe.Offsetof(unsafeOpts.source)), value: 0},
		{templ: C.CString("setuid=%s"), offset: C.ulong(unsafe.Offsetof(unsafeOpts.setuid)), value: 0},
		{templ: C.CString("setgid=%s"), offset: C.ulong(unsafe.Offsetof(unsafeOpts.setgid)), value: 0},

		C._FUSE_OPT_END(),
	}[0], nil) == -1 {
		ret = 1
		return
	}

	if unsafeOpts.source != nil {
		defer C.free(unsafe.Pointer(unsafeOpts.source))
	}
	if unsafeOpts.setuid != nil {
		defer C.free(unsafe.Pointer(unsafeOpts.setuid))
	}
	if unsafeOpts.setgid != nil {
		defer C.free(unsafe.Pointer(unsafeOpts.setgid))
	}

	if unsafeOpts.source == nil {
		showHelp()
		ret = 1
		return
	} else {
		source = C.GoString(unsafeOpts.source)
	}

	if unsafeOpts.setuid == nil {
		setuid = -1
	} else if v, err := strconv.Atoi(C.GoString(unsafeOpts.setuid)); err != nil || v <= 0 {
		log.Println("invalid value for option setuid")
		ret = 1
		return
	} else {
		setuid = v
	}
	if unsafeOpts.setgid == nil {
		setgid = -1
	} else if v, err := strconv.Atoi(C.GoString(unsafeOpts.setgid)); err != nil || v <= 0 {
		log.Println("invalid value for option setgid")
		ret = 1
		return
	} else {
		setgid = v
	}

	return
}

// copyStrings returns a copy of s with null-termination.
func copyStrings(s ...string) **C.char {
	if len(s) == 0 {
		return nil
	}
	args := unsafe.Slice((**C.char)(C.malloc(C.size_t(uintptr(len(s))*unsafe.Sizeof(s[0])))), len(s))
	for i, arg := range s {
		args[i] = C.CString(arg)
	}
	return &args[0]
}

// unsafeAddArgument adds an argument to fuseArgs via fuse_opt_add_arg.
// The last byte of arg must be 0.
func unsafeAddArgument(args *fuseArgs, arg string) {
	C.fuse_opt_add_arg(args, (*C.char)(unsafe.Pointer(unsafe.StringData(arg))))
}

func _main(argc int, argv **C.char) int {
	args := C.struct_fuse_args{argc: C.int(argc), argv: argv, allocated: 1}

	// this causes the kernel to enforce access control based on
	// struct stat populated by sharefs_getattr
	unsafeAddArgument(&args, "-odefault_permissions\x00")

	{
		source, setuid, setgid, ret := parseOpts(&args)
		if ret != 0 {
			return ret
		}

		if a, err := filepath.Abs(source); err != nil {
			log.Println(err)
			return 1
		} else {
			initSource = a
		}

		if os.Geteuid() == 0 {
			if setuid <= 0 || setgid <= 0 {
				log.Println("setuid and setgid must not be 0")
				return 1
			}
		} else if setuid > 0 || setgid > 0 {
			log.Println("setuid and setgid has no effect when not starting as root")
			return 1
		}
		initSetUidGid[0], initSetUidGid[1] = setuid, setgid
	}

	// TODO(ophestra): spawn container here, set PR_SET_NO_NEW_PRIVS and enforce landlock
	fuse_main_return := C._fuse_main(args.argc, args.argv, &C.struct_fuse_operations{
		init:    closure(C.sharefs_init),
		destroy: closure(C.sharefs_destroy),

		// implemented in fuse-helper.c
		getattr:  closure(C.sharefs_getattr),
		readlink: closure(C.sharefs_readlink),
		readdir:  closure(C.sharefs_readdir),
		mkdir:    closure(C.sharefs_mkdir),
		unlink:   closure(C.sharefs_unlink),
		rmdir:    closure(C.sharefs_rmdir),
		rename:   closure(C.sharefs_rename),
		truncate: closure(C.sharefs_truncate),
		utimens:  closure(C.sharefs_utimens),
		create:   closure(C.sharefs_create),
		open:     closure(C.sharefs_open),
		read:     closure(C.sharefs_read),
		write:    closure(C.sharefs_write),
		statfs:   closure(C.sharefs_statfs),
		release:  closure(C.sharefs_release),
		fsync:    closure(C.sharefs_fsync),
	}, nil)

	if initFailed {
		return 1
	} else {
		return int(fuse_main_return)
	}
}