cmd/sharefs: implement shared filesystem

This is for passing files between applications, similar to android /sdcard.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/sharefs/fuse.go

@@ -0,0 +1,265 @@
+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)
+	}
+}