日々の破片

_ soのトリビア

土曜のRHG読書会ふつぱいら編で、mputさんからlibcは実行できるという話が出た。

手元のLinuxで実行可能ビットが立っているsoを起動すると以下となった。

$ ls -l /lib/*.so|grep rwx
-rwxr-xr-x 1 root root   72800 2008-08-04 16:14 /lib/klibc-oHYcLOsqK031xhhY6ecG96fp_1A.so
-rwxr-xr-x 1 root root  131584 2009-01-30 02:56 /lib/ld-2.8.90.so
-rwxr-xr-x 1 root root 1502520 2009-01-30 02:56 /lib/libc-2.8.90.so
-rwxr-xr-x 1 root root  130214 2009-01-30 02:56 /lib/libpthread-2.8.90.so
$ /lib/klibc-oHYcLOsqK031xhhY6ecG96fp_1A.so 
Segmentation fault
$ /lib/ld-2.8.90.so 
Usage: ld.so [OPTION]... EXECUTABLE-FILE [ARGS-FOR-PROGRAM...]
You have invoked `ld.so', the helper program for shared library executables.
This program usually lives in the file `/lib/ld.so', and special directives
in executable files using ELF shared libraries tell the system's program
loader to load the helper program from this file.  This helper program loads
the shared libraries needed by the program executable, prepares the program
to run, and runs it.  You may invoke this helper program directly from the
command line to load and run an ELF executable file; this is like executing
that file itself, but always uses this helper program from the file you
specified, instead of the helper program file specified in the executable
file you run.  This is mostly of use for maintainers to test new versions
of this helper program; chances are you did not intend to run this program.
 
  --list                list all dependencies and how they are resolved
  --verify              verify that given object really is a dynamically linked
                        object we can handle
  --library-path PATH   use given PATH instead of content of the environment
                        variable LD_LIBRARY_PATH
  --inhibit-rpath LIST  ignore RUNPATH and RPATH information in object names
                        in LIST
$ /lib/libc-2.8.90.so 
GNU C Library development release version 2.8.90, by Roland McGrath et al.
Copyright (C) 2008 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 4.3.2.
Compiled on a Linux >>2.6.24-16-server<< system on 2009-01-29.
Available extensions:
	crypt add-on version 2.1 by Michael Glad and others
	GNU Libidn by Simon Josefsson
	Native POSIX Threads Library by Ulrich Drepper et al
	BIND-8.2.3-T5B
For bug reporting instructions, please see:
.
$ /lib/libpthread-2.8.90.so 
Native POSIX Threads Library by Ulrich Drepper et al
Copyright (C) 2006 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.
Forced unwind support included.

いきなりSEGVするklibcはともかく、他のsoは少なくとも著作権表示はする。

ldはusage表示までするので実際に引数を与えてみる。

$ /lib/ld-2.8.90.so --list /usr/local/bin/ruby
	linux-vdso.so.1 =>  (0x00007fff237ff000)
	librt.so.1 => /lib/librt.so.1 (0x00007fe91b3ff000)
	libdl.so.2 => /lib/libdl.so.2 (0x00007fe91b1fb000)
	libcrypt.so.1 => /lib/libcrypt.so.1 (0x00007fe91afc2000)
	libm.so.6 => /lib/libm.so.6 (0x00007fe91ad3d000)
	libc.so.6 => /lib/libc.so.6 (0x00007fe91a9cb000)
	libpthread.so.0 => /lib/libpthread.so.0 (0x00007fe91a7ae000)
	/lib64/ld-linux-x86-64.so.2 => /lib/ld-2.8.90.so (0x00007fe91b620000)

lddの出力に似ている。

$ ldd /usr/local/bin/ruby
	linux-vdso.so.1 =>  (0x00007fff0c9fe000)
	librt.so.1 => /lib/librt.so.1 (0x00007fc40442e000)
	libdl.so.2 => /lib/libdl.so.2 (0x00007fc40422a000)
	libcrypt.so.1 => /lib/libcrypt.so.1 (0x00007fc403ff2000)
	libm.so.6 => /lib/libm.so.6 (0x00007fc403d6d000)
	libc.so.6 => /lib/libc.so.6 (0x00007fc4039fb000)
	libpthread.so.0 => /lib/libpthread.so.0 (0x00007fc4037df000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fc404637000)

というよりも全く同じだ。

$ file /usr/bin/ldd
/usr/bin/ldd: Bourne-Again shell script text executable

lddは中でld.soを呼び出しているのだった。