unix_user_cgo.go

// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE.BSD file.
// SPDX-License-Identifier: BSD-3-Clause

//go:build (aix || darwin || dragonfly || freebsd || (!android && linux) || netbsd || openbsd || solaris) && cgo && !osusergo
// +build aix darwin dragonfly freebsd !android,linux netbsd openbsd solaris
// +build cgo
// +build !osusergo

package main

import (
	"fmt"
	"os/user"
	"strings"
	"syscall"
	"unsafe"
)

/*
#cgo solaris CFLAGS: -D_POSIX_PTHREAD_SEMANTICS
#include <unistd.h>
#include <sys/types.h>
#include <pwd.h>
#include <stdlib.h>

static int mygetpwuid_r(int uid, struct passwd *pwd,
	char *buf, size_t buflen, struct passwd **result) {
	return getpwuid_r(uid, pwd, buf, buflen, result);
}
*/
import "C"

func lookupUnixUid(uid int) (*unixUser, error) {
	var pwd C.struct_passwd
	var result *C.struct_passwd

	buf := alloc(userBuffer)
	defer buf.free()

	err := retryWithBuffer(buf, func() syscall.Errno {
		// mygetpwuid_r is a wrapper around getpwuid_r to avoid using uid_t
		// because C.uid_t(uid) for unknown reasons doesn't work on linux.
		return syscall.Errno(C.mygetpwuid_r(C.int(uid),
			&pwd,
			(*C.char)(buf.ptr),
			C.size_t(buf.size),
			&result))
	})
	if err != nil {
		return nil, fmt.Errorf("user: lookup userid %d: %v", uid, err)
	}
	if result == nil {
		return nil, user.UnknownUserIdError(uid)
	}
	return buildUser(&pwd), nil
}

func buildUser(pwd *C.struct_passwd) *unixUser {
	u := &unixUser{
		Uid:      uint64(pwd.pw_uid),
		Gid:      uint64(pwd.pw_gid),
		Username: C.GoString(pwd.pw_name),
		Name:     C.GoString(pwd.pw_gecos),
		HomeDir:  C.GoString(pwd.pw_dir),
		Shell:    C.GoString(pwd.pw_shell),
	}
	// The pw_gecos field isn't quite standardized. Some docs
	// say: "It is expected to be a comma separated list of
	// personal data where the first item is the full name of the
	// user."
	if i := strings.Index(u.Name, ","); i >= 0 {
		u.Name = u.Name[:i]
	}
	return u
}

type bufferKind C.int

const (
	userBuffer = bufferKind(C._SC_GETPW_R_SIZE_MAX)
)

func (k bufferKind) initialSize() C.size_t {
	sz := C.sysconf(C.int(k))
	if sz == -1 {
		// DragonFly and FreeBSD do not have _SC_GETPW_R_SIZE_MAX.
		// Additionally, not all Linux systems have it, either. For
		// example, the musl libc returns -1.
		return 1024
	}
	if !isSizeReasonable(int64(sz)) {
		// Truncate.  If this truly isn't enough, retryWithBuffer will error on the first run.
		return maxBufferSize
	}
	return C.size_t(sz)
}

type memBuffer struct {
	ptr  unsafe.Pointer
	size C.size_t
}

func alloc(kind bufferKind) *memBuffer {
	sz := kind.initialSize()
	return &memBuffer{
		ptr:  C.malloc(sz),
		size: sz,
	}
}

func (mb *memBuffer) resize(newSize C.size_t) {
	mb.ptr = C.realloc(mb.ptr, newSize)
	mb.size = newSize
}

func (mb *memBuffer) free() {
	C.free(mb.ptr)
}

// retryWithBuffer repeatedly calls f(), increasing the size of the
// buffer each time, until f succeeds, fails with a non-ERANGE error,
// or the buffer exceeds a reasonable limit.
func retryWithBuffer(buf *memBuffer, f func() syscall.Errno) error {
	for {
		errno := f()
		if errno == 0 {
			return nil
		} else if errno != syscall.ERANGE {
			return errno
		}
		newSize := buf.size * 2
		if !isSizeReasonable(int64(newSize)) {
			return fmt.Errorf("internal buffer exceeds %d bytes", maxBufferSize)
		}
		buf.resize(newSize)
	}
}

const maxBufferSize = 1 << 20

func isSizeReasonable(sz int64) bool {
	return sz > 0 && sz <= maxBufferSize
}