rpath
In computing, rpath designates the run-time search path hard-coded in an executable file or library. Dynamic linking loaders use the rpath to find required libraries.
Specifically, it encodes a path to shared libraries into the header of an executable (or another shared library). This RPATH header value (so named in the Executable and Linkable Format header standards) may either override or supplement the system default dynamic linking search paths.
The rpath of an executable or shared library is an optional entry in the .dynamic
section of the ELF executable or shared libraries, with the type DT_RPATH
, called the DT_RPATH
attribute. It can be stored there at link time by the linker. Tools such as chrpath
and patchelf
can create or modify the entry later.
Use of the DT_RPATH entry by the dynamic linker
The different dynamic linkers for ELF implement the use of the DT_RPATH
attribute in different ways.
GNU ld.so
The dynamic linker of the GNU C Library searches for shared libraries in the following locations:[1]
- The (colon-separated) paths in the
DT_RPATH
dynamic section attribute of the binary if present and theDT_RUNPATH
attribute does not exist. - The (colon-separated) paths in the environment variable
LD_LIBRARY_PATH
, unless the executable is asetuid/setgid
binary, in which case it is ignored.LD_LIBRARY_PATH
can be overridden by calling the dynamic linker with the option--library-path
(e.g./lib/ld-linux.so.2 --library-path $HOME/mylibs myprogram
). - The (colon-separated) paths in the
DT_RUNPATH
dynamic section attribute of the binary if present. - Lookup based on the
ldconfig
cache file (often located at/etc/ld.so.cache
) which contains a compiled list of candidate libraries previously found in the augmented library path (set by/etc/ld.so.conf
). If, however, the binary was linked with the-z nodefaultlib
linker option, libraries in the default library paths are skipped. - In the trusted default path
/lib
, and then/usr/lib
. If the binary was linked with the-z nodefaultlib
linker option, this step is skipped.
Notes:
- The option
--inhibit-rpath LIST
of the dynamic linker instructs it to ignoreDT_RPATH
andDT_RUNPATH
attributes of the object names in LIST. To specify a main program in the LIST, give empty string. - Libraries specified by the environment variable
LD_PRELOAD
and then those listed in/etc/ld.so.preload
are loaded before the search begins. A preload can thus be used to replace some (or all) of the requested library's normal functionalities, or it can simply be used to supply a library that would otherwise not be found. - Static libraries are searched and linked into the ELF file at link time and are not searched at run time.
The role of GNU ld
The GNU Linker (GNU ld) implements a feature which it calls "new-dtags", which can be used to insert an rpath that has lower precedence than the LD_LIBRARY_PATH
environment variable.
[2]
If the new-dtags feature is enabled in the linker (--enable-new-dtags
), GNU ld
, besides setting the DT_RPATH
attribute, also sets the DT_RUNPATH
attribute to the same string. At run time, if the dynamic linker finds a DT_RUNPATH
attribute, it ignores the value of the DT_RPATH
attribute, with the effect that LD_LIBRARY_PATH
is checked first and the paths in the DT_RUNPATH
attribute are only searched afterwards.
The ld dynamic linker does not search DT_RUNPATH
locations for transitive dependencies, unlike DT_RPATH
. [3]
Instead of specifying the -rpath
to the linker, the environment variable LD_RUN_PATH
can be set to the same effect.
Solaris ld.so
The dynamic linker of Solaris, specifically /lib/ld.so
of SunOS 5.8 and similar systems looks for libraries in the directories specified in the LD_LIBRARY_PATH variable before looking at the DT_RPATH
attribute. Sun Microsystems was the first to introduce dynamic library loading. Sun later added the rpath option to ld and used it in essential libraries as an added security feature. GNU ld did the same to support Sun-style dynamic libraries.
Example
$ cc -shared -Wl,-soname,termcap.so.4,-rpath,/lib/termcap.so.4 -o termcap.so.4
$ objdump -a -x termcap.so.4
NEEDED libc.so.6
SONAME termcap.so.4
RPATH /lib/termcap.so.4
In this example, GNU or Sun ld (ld.so) will REFUSE to load termcap for a program needing it unless the file termcap.so
is in /lib/
and named termcap.so.4
. LD_LIBRARY_PATH is ignored. If /lib/termcap.so.4
is removed to remediate, the shell dies (one cannot load an alternate termcap.so
and a rescue disk is needed, but also if a new termcap.so.4
has RPATH /lib, ld.so will refuse to use to load it unless it clobbered /lib/termcap.so.4
). But there's another issue: it isn't safe to copy over some libs in /lib
as they are "in use," further restricting the would-be lib tester. Furthermore, SONAME termcap.so.4 vs. SONAME termcap.so means programs needing basic termcap.so
are denied because the library above deleted the ABI access to basic support.
$ cc -shared -Wl,-soname,libtermcap.so.2 -o libtermcap.so.2
$ objdump -a -x termcap.so.2
NEEDED libc.so.6
SONAME termcap.so.2
Old Linux/Sun used the above, which allows a user to direct any program to use any termcap.so
they specify in LD_LIBRARY_PATH, or what is found in /usr/local/lib(n) using the search rules such as ld.so.conf
. However, GNU ld always uses /lib
or /usr/lib
regardless before LD_LIBRARY_PATH, so first /lib/termcap.so
is moved to /usr/local/lib
and that mentioned in ld.so.conf
, which enables use of moving libs
and ld.so.conf
or use of LD_LIBRARY_PATH to use. A preferred practice is to use "SONAME termcap.so" and have programs check version (all libs do support that) to use features available, but that was often skipped in old releases due to slow computing speed and lack of time to code correctly.
That being said, test this kind of thing thoroughly on a given platform before deciding to rely on it. Release administrators today are not guaranteed to respect past guidelines or documentation. Some UNIX varieties link and load in a completely different way. rpath is specific to ld shipped with a particular distribution.
Lastly, as said, rpath is a security feature however "mandatory access control" (MAC) and other techniques can be as effective or more effective than rpath to control lib reading and writing.
Control over rpath using today's compilers is often nearly impossible given lengthy and convoluted make(1) scripting. Worse, some build scripts ignore --disable-rpath even though they present it as an option. It would be time-consuming and frustrating, and probably unfeasible, to fix build scripting in every odd program to compile.
A simple sh(1) "wrapper" can call the real ld, named ld.bin. The wrapper can filter in/out -rpath option before invoking ld.
#!/bin/sh
# - filter ld options here -
ld.bin $opts
However, note that some builds incorrectly use rpath instead of rpath-link or LD_LIBRARY_PATH or $(TOP)/dir/foo.so to locate intermediate products that stay in the build directory - thus backwardly demand rpath in the final product, which is a new issue concerning "what is rpath".
References
- "Linux / Unix Command: ld.so". man7.org. Retrieved 19 February 2018.
- "Shared Libraries: distribution and build-system issues". Official website of the Haskell Compiler. Retrieved 4 April 2019.
- https://bugs.launchpad.net/ubuntu/+source/eglibc/+bug/1253638
External links
- chrpath - a tool to change the
DT_RPATH
attribute of an executable and convert it to anDT_RUNPATH
attribute - FreeBSD devel/chrpath Port - Tool to modify DT_RPATH in existing ELF binaries
- patchELF - a small utility to modify the dynamic linker and
DT_RUNPATH
attribute of ELF executables.