一个直接在Linux内核上设置时间的函数？

Question

我们正在试图找到一个函数，它可以直接通过linux内核来设置时间，而不需要使用用户空间。我们发现在文件clock_settime.c上有一个仅限于超级用户的函数：

* Copyright (C) 1991-2022 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

#include <errno.h>
#include <time.h>
#include <hurd.h>
#include <hurd/port.h>
#include <shlib-compat.h>

/* Set the current time of day.
   This call is restricted to the super-user.  */
int
__clock_settime (clockid_t clock_id, const struct timespec *ts)
{
  error_t err;
  mach_port_t hostpriv;
  time_value_t tv;

  if (clock_id != CLOCK_REALTIME
      || ! valid_nanoseconds (ts->tv_nsec))
    return __hurd_fail (EINVAL);

  err = __get_privileged_ports (&hostpriv, NULL);
  if (err)
    return __hurd_fail (EPERM);

  TIMESPEC_TO_TIME_VALUE (&tv, ts);
  err = __host_set_time (hostpriv, tv);
  __mach_port_deallocate (__mach_task_self (), hostpriv);

  return __hurd_fail (err);
}
libc_hidden_def (__clock_settime)

versioned_symbol (libc, __clock_settime, clock_settime, GLIBC_2_17);
/* clock_settime moved to libc in version 2.17;
   old binaries may expect the symbol version it had in librt.  */
#if SHLIB_COMPAT (libc, GLIBC_2_2, GLIBC_2_17)
strong_alias (__clock_settime, __clock_settime_2);
compat_symbol (libc, __clock_settime_2, clock_settime, GLIBC_2_2);
#endif

我们希望遵循函数__host_set_time声明，但是在任何地方都没有库引用，我们认为它可能是帮助我们解决问题的函数。有人知道它是从哪里来的吗?它是干什么的？提前谢谢。

Answer 1

你看错源树了。您引用的代码是GNU C库的一部分，它是用户空间。此外，该代码仅在Hurd操作系统上使用，而不是在Linux上使用。这个C库在sysdeps/unix/sysv/linux/clock_settime.c中实现了Linux，您将看到这只是调用宏，这些宏扩展为系统调用陷阱的内联程序集。

您应该搜索Linux内核本身的源代码，而不是C库。我将"clock_settime“放入https://elixir.bootlin.com/上的搜索框中，经过几次跳转，找到了do_settimeofday64，我相信这就是您要寻找的API。您还可能希望查阅实际系统调用入口点的代码，这些代码位于kernel/time/time.c中；它们是执行安全内存读取和安全检查，然后调用do_settimeofday64的包装器。