理解LD_ASSUME_KERNEL的用法

Question

我试图理解我的系统上的env变量LD_ASSUME_KERNEL (Debian/bullseye+bpo)。

手风琴，手风琴：

$ man pthreads

我应该能够运行这样的东西，但是在我的系统中，我得到的是：

% LD_ASSUME_KERNEL=2.2.5 ldd /bin/ls
/bin/bash: error while loading shared libraries: libdl.so.2: cannot open shared object file: No such file or directory

这有点太低了，我无法理解到底是怎么回事。我不明白LD_ASSUME_KERNEL实现在我的系统上是否被破坏了，或者我没有正确地阅读文档。

其他一些尝试失败了：

% LD_TRACE_LOADED_OBJECTS=1 LD_ASSUME_KERNEL=2.2.5 ldd
        linux-vdso.so.1 (0x00007ffe3f7e0000)
        libtinfo.so.6 => /lib/x86_64-linux-gnu/libtinfo.so.6 (0x00007f5399001000)
        libdl.so.2 => not found
        libc.so.6 => not found
        libc.so.6 => not found

和

% LD_TRACE_LOADED_OBJECTS=1 LD_ASSUME_KERNEL=2.4.19 ldd
        linux-vdso.so.1 (0x00007ffeaacb9000)
        libtinfo.so.6 => /lib/x86_64-linux-gnu/libtinfo.so.6 (0x00007f861cb18000)
        libdl.so.2 => not found
        libc.so.6 => not found
        libc.so.6 => not found

同时：

% LD_TRACE_LOADED_OBJECTS=1 ldd
        linux-vdso.so.1 (0x00007ffc929a9000)
        libtinfo.so.6 => /lib/x86_64-linux-gnu/libtinfo.so.6 (0x00007fa319a29000)
        libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fa319a23000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fa31985e000)
        /lib64/ld-linux-x86-64.so.2 (0x00007fa319aaa000)

参考文献：

$ man pthreads
[...]
   Selecting the threading implementation: LD_ASSUME_KERNEL
       On systems with a glibc that supports both LinuxThreads and NPTL (i.e., glibc 2.3.x), the LD_ASSUME_KERNEL environment variable can be used to override  the  dynamic  linker's  default  choice  of threading implementation.  This variable tells the dynamic linker to assume that it is running on top of a particular kernel version.  By specifying a kernel version that does not provide the support required by NPTL, we can force the use of LinuxThreads.  (The most likely reason for doing this is to run a (broken) application that depends on some nonconformant behavior in  LinuxThreads.)
       For example:

           bash$ $( LD_ASSUME_KERNEL=2.2.5 ldd /bin/ls | grep libc.so | \
                           awk '{print $3}' ) | egrep -i 'threads|nptl'
                   linuxthreads-0.10 by Xavier Leroy

同样的情况也适用于：

$ man ld.so
[...]
       LD_ASSUME_KERNEL (since glibc 2.2.3)
              Each  shared object can inform the dynamic linker of the minimum kernel ABI version that it requires.  (This requirement is encoded in an ELF note section that is viewable via readelf -n as
              a section labeled NT_GNU_ABI_TAG.)  At run time, the dynamic linker determines the ABI version of the running kernel and will reject loading shared objects that specify minimum ABI versions
              that exceed that ABI version.

              LD_ASSUME_KERNEL  can  be  used to cause the dynamic linker to assume that it is running on a system with a different kernel ABI version.  For example, the following command line causes the
              dynamic linker to assume it is running on Linux 2.2.5 when loading the shared objects required by myprog:

                  $ LD_ASSUME_KERNEL=2.2.5 ./myprog

              On systems that provide multiple versions of a shared object (in different directories in the search path) that have different minimum kernel ABI version requirements, LD_ASSUME_KERNEL  can
              be used to select the version of the object that is used (dependent on the directory search order).

              Historically,  the  most common use of the LD_ASSUME_KERNEL feature was to manually select the older LinuxThreads POSIX threads implementation on systems that provided both LinuxThreads and
              NPTL (which latter was typically the default on such systems); see pthreads(7).

% apt-cache policy manpages
manpages:
  Installed: 5.10-1
  Candidate: 5.10-1
  Version table:
 *** 5.10-1 500
        500 http://deb.debian.org/debian bullseye/main amd64 Packages
        500 http://deb.debian.org/debian bullseye/main i386 Packages
        100 /var/lib/dpkg/status

作为附带说明，输出始终是相同的：

/lib/x86_64-linux-gnu/libc.so.6
LD_ASSUME_KERNEL=2.2.5 /lib/x86_64-linux-gnu/libc.so.6
LD_ASSUME_KERNEL=2.4.19 /lib/x86_64-linux-gnu/libc.so.6

我得到：

GNU C Library (Debian GLIBC 2.31-13+deb11u3) stable release version 2.31.
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Compiled by GNU CC version 10.2.1 20210110.
libc ABIs: UNIQUE IFUNC ABSOLUTE
For bug reporting instructions, please see:
.

Answer 1

这里的重要部分是引用了手册中的这段话：

在运行时，动态链接器确定正在运行的内核的ABI版本，并拒绝加载指定超过该ABI版本的最小ABI版本的共享对象。

file将告诉您给定库的最小ABI版本是什么；因为您正在运行x86_64，所以在许多情况下，您应该会发现最小ABI版本是3.2.0：

$ file -L /lib/x86_64-linux-gnu/libc.so.6
/lib/x86_64-linux-gnu/libc-2.31.so: ELF 64-bit LSB shared object, x86-64, version 1 (GNU/Linux), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=2e5abcee94f3bcbed7bba094f341070a2585a2ba, for GNU/Linux 3.2.0, stripped

在您的系统中，例外是libtinfo.so.6，它不声明最小的ABI：

$ file -L /lib/x86_64-linux-gnu/libtinfo.so.6
/lib/x86_64-linux-gnu/libtinfo.so.6: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=d6920dbdd057f44edaf4c1fbce191b5854dfd9e6, stripped

将LD_ASSUME_KERNEL设置为小于3.2.0的内容会指示动态链接器拒绝任何需要3.2.0或更高版本的库；因为您的系统没有提供其他版本的受影响库，因此ld.so失败。