如何强制链接到旧的 libc `fcntl` 而不是 `fcntl64`？

Question

好像GLIBC 2.28 (released August 2018) made a fairly aggressive change to fcntl. The definition was changed in <fcntl.h> to no longer be an external function, but a #define to fcntl64.

结果是，如果您在使用此 glibc 的系统上编译代码——如果它完全使用 fcntl() ——生成的二进制文件将不会在2018 年 8 月之前的系统。这会影响各种各样的应用程序...fcntl() 的手册页显示它是一小部分子函数的入口点：

https://linux.die.net/man/2/fcntl

如果您能告诉链接器您需要哪个特定版本的 GLIBC 函数，那就太好了。但我发现的最接近的是这个技巧在另一个 post:

的回答中描述

Answer to "Linking against older symbol version in a .so file"

这有点复杂。 fcntl 是可变的，没有 vffcntl 而需要 va_list。在这种情况下 you cannot forward an invocation of a variadic function。 :-(

当一个人拥有故意降低依赖性的稳定代码时，在当前 Ubuntu 上构建它是一种失望......然后让可执行文件拒绝运行在另一个 Ubuntu 仅在一年前（接近当天）发布。对此有什么办法？

Answer 1

What recourse does one have for this?

GLIBC 无法 #define USE_FCNTL_NOT_FCNTL64 这一事实说明了很多。无论是对是错，大多数 OS+ 工具链制造商似乎已经决定，将新系统的旧版本系统的二进制文件作为目标不是高优先级。

阻力最小的方法是让虚拟机围绕最旧的 OS+ 构建项目的工具链。每当您认为二进制文件在旧系统上运行时，就可以使用它来制作二进制文件。

但是...

如果您认为您的用法是在不受偏移大小更改影响的 fcntl() 调用的子集中（也就是说您不使用字节范围锁）
或者愿意审查您的代码以使用向后兼容的结构定义
并且不害怕巫术

...然后继续阅读。

The name is different, and fcntl is variadic without a vffcntl that takes a va_list. In such situations you cannot forward an invocation of a variadic function.

...然后要应用 the wrapping trick mentioned，您必须逐行查看 fcntl() 的接口文档，按原样解压可变参数，然后使用新的可变参数调用。

幸运的是，情况并没有那么困难（fcntl 采用 0 或 1 个具有记录类型的参数）。为了尝试为其他人省去一些麻烦，这里是代码。务必将 --wrap=fcntl64 传递给链接器（-Wl,--wrap=fcntl64 如果不直接调用 ld）：

asm (".symver fcntl64, fcntl@GLIBC_2.2.5");

extern "C" int __wrap_fcntl64(int fd, int cmd, ...)
{
    int result;
    va_list va;
    va_start(va, cmd);

    switch (cmd) {
      //
      // File descriptor flags
      //
      case F_GETFD: goto takes_void;
      case F_SETFD: goto takes_int;

      // File status flags
      //
      case F_GETFL: goto takes_void;
      case F_SETFL: goto takes_int;

      // File byte range locking, not held across fork() or clone()
      //
      case F_SETLK: goto takes_flock_ptr_INCOMPATIBLE;
      case F_SETLKW: goto takes_flock_ptr_INCOMPATIBLE;
      case F_GETLK: goto takes_flock_ptr_INCOMPATIBLE;

      // File byte range locking, held across fork()/clone() -- Not POSIX
      //
      case F_OFD_SETLK: goto takes_flock_ptr_INCOMPATIBLE;
      case F_OFD_SETLKW: goto takes_flock_ptr_INCOMPATIBLE;
      case F_OFD_GETLK: goto takes_flock_ptr_INCOMPATIBLE;

      // Managing I/O availability signals
      //
      case F_GETOWN: goto takes_void;
      case F_SETOWN: goto takes_int;
      case F_GETOWN_EX: goto takes_f_owner_ex_ptr;
      case F_SETOWN_EX: goto takes_f_owner_ex_ptr;
      case F_GETSIG: goto takes_void;
      case F_SETSIG: goto takes_int;

      // Notified when process tries to open or truncate file (Linux 2.4+)
      //
      case F_SETLEASE: goto takes_int;
      case F_GETLEASE: goto takes_void;

      // File and directory change notification
      //
      case F_NOTIFY: goto takes_int;

      // Changing pipe capacity (Linux 2.6.35+)
      //
      case F_SETPIPE_SZ: goto takes_int;
      case F_GETPIPE_SZ: goto takes_void;

      // File sealing (Linux 3.17+)
      //
      case F_ADD_SEALS: goto takes_int;
      case F_GET_SEALS: goto takes_void;

      // File read/write hints (Linux 4.13+)
      //
      case F_GET_RW_HINT: goto takes_uint64_t_ptr;
      case F_SET_RW_HINT: goto takes_uint64_t_ptr;
      case F_GET_FILE_RW_HINT: goto takes_uint64_t_ptr;
      case F_SET_FILE_RW_HINT: goto takes_uint64_t_ptr;

      default:
        fprintf(stderr, "fcntl64 workaround got unknown F_XXX constant")
    }

  takes_void:
    va_end(va);
    return fcntl64(fd, cmd);

  takes_int:
    result = fcntl64(fd, cmd, va_arg(va, int));
    va_end(va);
    return result;

  takes_flock_ptr_INCOMPATIBLE:
    //
    // !!! This is the breaking case: the size of the flock
    // structure changed to accommodate larger files.  If you
    // need this, you'll have to define a compatibility struct
    // with the older glibc and make your own entry point using it,
    // then call fcntl64() with it directly (bear in mind that has
    // been remapped to the old fcntl())
    // 
    fprintf(stderr, "fcntl64 hack can't use glibc flock directly");
    exit(1);

  takes_f_owner_ex_ptr:
    result = fcntl64(fd, cmd, va_arg(va, struct f_owner_ex*));
    va_end(va);
    return result;

  takes_uint64_t_ptr:
    result = fcntl64(fd, cmd, va_arg(va, uint64_t*));
    va_end(va);
    return result;
}

请注意，根据您实际构建的版本，如果这些部分不可用，您可能必须 #ifdef 将其中一些标记部分删除。

This affects quite a variety of applications...the manual page for fcntl() shows that it's the entry point for a small universe of sub-functions

...这可能应该给人们上一课：避免通过可变参数滥用创建这样的 "kitchen sink" 函数。

Answer 2

How to force linkage to older libc fcntl instead of fcntl64?

针对旧版本 libc 进行编译。期间.

因为glibc不是forward compatible, it is only backwards-compatible:

The GNU C Library is designed to be a backwards compatible, portable, and high performance ISO C library. It aims to follow all relevant standards including ISO C11, POSIX.1-2008, and IEEE 754-2008.

没有任何向前兼容性的保证，你不知道还有什么不能正常工作。

如何强制链接到旧的 libc `fcntl` 而不是 `fcntl64`？

How to force linkage to older libc `fcntl` instead of `fcntl64`?

c

glibc

variadic-functions

ld

fcntl