ldd命令浅析

妖狐艹你老母 2022-06-16 02:49 1153阅读 0赞

# 简介 #

ldd是linux下常用的查看程序链接了那些动态库的命令，这个命令其实是一个脚步

# 内容 #

下面是ubuntu14.04中ldd脚本的内容，位于/usr/bin中

#! /bin/bash
    # Copyright (C) 1996-2014 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
    # <http://www.gnu.org/licenses/>.
    
    
    # This is the `ldd' command, which lists what shared libraries are
    # used by given dynamically-linked executables. It works by invoking the
    # run-time dynamic linker as a command and setting the environment
    # variable LD_TRACE_LOADED_OBJECTS to a non-empty value.
    
    # We should be able to find the translation right at the beginning.
    TEXTDOMAIN=libc
    TEXTDOMAINDIR=/usr/share/locale
    
    RTLDLIST="/lib/ld-linux.so.2 /lib64/ld-linux-x86-64.so.2 /libx32/ld-linux-x32.so.2"
    warn=
    bind_now=
    verbose=
    
    while test $# -gt 0; do
      case "$1" in
      --vers | --versi | --versio | --version)
        echo 'ldd (Ubuntu EGLIBC 2.19-0ubuntu6.11) 2.19'
        printf $"Copyright (C) %s 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. " "2014"
        printf $"Written by %s and %s. " "Roland McGrath" "Ulrich Drepper"
        exit 0
        ;;
      --h | --he | --hel | --help)
        echo $"Usage: ldd [OPTION]... FILE... --help print this help and exit --version print version information and exit -d, --data-relocs process data relocations -r, --function-relocs process data and function relocations -u, --unused print unused direct dependencies -v, --verbose print all information "
        printf $"For bug reporting instructions, please see:\\n%s.\\n" \
          "<https://bugs.launchpad.net/ubuntu/+source/eglibc/+bugs>"
        exit 0
        ;;
      -d | --d | --da | --dat | --data | --data- | --data-r | --data-re | \
      --data-rel | --data-relo | --data-reloc | --data-relocs)
        warn=yes
        shift
        ;;
      -r | --f | --fu | --fun | --func | --funct | --functi | --functio | \
      --function | --function- | --function-r | --function-re | --function-rel | \
      --function-relo | --function-reloc | --function-relocs)
        warn=yes
        bind_now=yes
        shift
        ;;
      -v | --verb | --verbo | --verbos | --verbose)
        verbose=yes
        shift
        ;;
      -u | --u | --un | --unu | --unus | --unuse | --unused)
        unused=yes
        shift
        ;;
      --v | --ve | --ver)
        echo >&2 $"ldd: option \`$1' is ambiguous"
        exit 1
        ;;
      --)       # Stop option processing.
        shift; break
        ;;
      -*)
        echo >&2 'ldd:' $"unrecognized option" "\`$1'"
        echo >&2 $"Try \`ldd --help' for more information."
        exit 1
        ;;
      *)
        break
        ;;
      esac
    done
    
    nonelf ()
    {
      # Maybe extra code for non-ELF binaries.
      return 1;
    }
    
    add_env="LD_TRACE_LOADED_OBJECTS=1 LD_WARN=$warn LD_BIND_NOW=$bind_now"
    add_env="$add_env LD_LIBRARY_VERSION=\$verify_out"
    add_env="$add_env LD_VERBOSE=$verbose"
    if test "$unused" = yes; then
      add_env="$add_env LD_DEBUG=\"$LD_DEBUG${LD_DEBUG:+,}unused\""
    fi
    
    # The following command substitution is needed to make ldd work in SELinux
    # environments where the RTLD might not have permission to write to the
    # terminal. The extra "x" character prevents the shell from trimming trailing
    # newlines from command substitution results. This function is defined as a
    # subshell compound list (using "(...)") to prevent parameter assignments from
    # affecting the calling shell execution environment.
    try_trace() (
      output=$(eval $add_env '"$@"' 2>&1; rc=$?; printf 'x'; exit $rc)
      rc=$?
      printf '%s' "${output%x}"
      return $rc
    )
    
    case $# in
    0)
      echo >&2 'ldd:' $"missing file arguments"
      echo >&2 $"Try \`ldd --help' for more information."
      exit 1
      ;;
    1)
      single_file=t
      ;;
    *)
      single_file=f
      ;;
    esac
    
    result=0
    for file do
      # We don't list the file name when there is only one.
      test $single_file = t || echo "${file}:"
      case $file in
      */*) :
           ;;
      *) file=./$file
         ;;
      esac
      if test ! -e "$file"; then
        echo "ldd: ${file}:" $"No such file or directory" >&2
        result=1
      elif test ! -f "$file"; then
        echo "ldd: ${file}:" $"not regular file" >&2
        result=1
      elif test -r "$file"; then
        RTLD=
        ret=1
        for rtld in ${RTLDLIST}; do
          if test -x $rtld; then
        dummy=`$rtld 2>&1` 
        if test $? = 127; then
          verify_out=`${rtld} --verify "$file"`
          ret=$?
          case $ret in
          [02]) RTLD=${rtld}; break;;
          esac
        fi
          fi
        done
        case $ret in
        0|2)
          try_trace "$RTLD" "$file" || result=1
          ;;
        1)
          # This can be a non-ELF binary or no binary at all.
          nonelf "$file" || {
        echo $" not a dynamic executable"
        result=1
          }
          ;;
        *)
          echo 'ldd:' ${RTLD} $"exited with unknown exit code" "($ret)" >&2
          exit 1
          ;;
        esac
      else
        echo 'ldd:' $"error: you do not have read permission for" "\`$file'" >&2
        result=1
      fi
    done
    
    exit $result
    # Local Variables:
    # mode:ksh
    # End:

# 解析 #

ldd命令其实是依靠设置环境变量实现的，  
如:LD\_TRACE\_LOADED\_OBJECTS 只要设置其值非空即可。  
撤销该环境变量，ls即又可以恢复正常使用：

![这里写图片描述][SouthEast]

***LD\_TRACE\_LOADED\_OBJECTS环境变量不为空时，任何可执行程序在运行时，它都会只显示模块的dependency，而程序并不真正执行***

ldd默认开启的环境变量是：LD\_TRACE\_LOADED\_OBJECTS=1  
其他的变量(和值)分别对应一些选项：  
\-d, –data-relocs -> LD\_WARN=yes  
\-r, –function-relocs ->LD\_WARN和LD\_BIND\_NOW=yes  
\-u, –unused -> LD\_DEBUG=”unused”  
\-v, –verbose -> LD\_VERBOSE=yes  
LD\_TRACE\_LOADED\_OBJECTS为必要环境变量，其他视具体情况。

[SouthEast]: /images/20220616/f2ef90854b9c437bb3df75bb0e16707b.png