Linking and Loading
Fred Prussack
CS 518
L&L: Overview
Wake-up Questions
Terms and Definitions / General
Information
Loading
Linking
– Static vs. Dynamic
– ELF
– Other Linking Information/Processing
References
L&L: Topics Not Covered
Windows
Alternate binary file formats
Different versions of glibc
Different versions of the kernel (from 2.4)
L&L: Questions for the Sleepy
What is the name of the compile time
linker in Linux?
– ld
What is the name of the run-time linker in
Linux?
– ld.so
Where is the loader located in Linux?
– Part kernel / part ld.so
L&L: Terms
Linking
– Taking object files and creating loadable
modules with correct references to required
libraries, data, and procedures
Loading
– Placing a program image into main memory
for execution
L&L: General Information
Static libraries generally named xxx.a (archives)
Dynamic libraries generally named xxx.so
(shared objects)
Object files generally named xxx.o
ELF: Executable and Linking Format. Currently
the most common object file format on Linux
systems. Other formats: a.out, COFF, etc…
L&L: Static vs. Dynamic
Fully statically compiled executables
– Provide for faster load->execution time due to
no run-time linking requirement
– Generate larger executables requiring more
disk space
Executables with dynamic dependencies
– Require run-time linking and thus potential
time implications
– Allows for easier and better code re-use
L&L: Loading
do_execve()
Searches all the registered
binary handlers
search_binary_handler
load_elf_binary
start_thread
Loads current binary
and elf interpreter
sets up correct
registers
Question: What does the instruction pointer have in it now?
Answer: Entry point of the ELF interpreter
L&L: Loading/Linking
At this point ld.so now has control
Determine what libraries need to be
loaded for this binary
Determine dependencies for these
libraries
In what order are these loaded and what
type of list is produced from this
dependency list?
L&L: Linking
Basic job to clean up unresolved symbols
At compile time this can be accomplished
by executing ld with object files to produce
an executable
At run-time this is accomplished by loading
all required shared libraries (.so’s) and
fixing unresolved symbols found in the
libraries
L&L: Dynamic Linking
Load Time Dynamic Linking Done By ld.so
– Most likely on your system it is ld-linux.so which links
to ld-2.3.2.so
– All possibly resolved symbols are resolved during
compilation/first link (run of ld). Remaining unresolved
symbols are done at time of load
Lazy Binding (LD_BIND_NOW)
Run Time Dynamic (inline) Linking
– Allows applications to, during run time, open shared
object files and execute their functions
<dlfcn.h>
L&L: [more] Questions for the Sleepy
In what package is ld.so distributed and
built from?
– Glibc
Can gcc be made to not link files
automatically?
– Yes, of course! Use the –c option.
L&L: ELF File Format
Currently the standard binary format for Linux
since the late 90’s. Created in late 80’s.
Three types of object files
– Shared Object Files (.so; shared object file)
– Relocatable Object Files (.o; object file)
– Executable Object Files (executable binary file)
First 4 characters of this type of file is
[backspace (ascii 127)]ELF
L&L: ELF File Format
First the ELF Header – 52 bytes in length on a
32 bit system
Sections and Segments for libraries and binaries
Various ELF segments
–
–
–
–
text: program instructions
data: initialized data
plt: procedure linkage table
got: global offset table
Checking for NEEDED entries in the dynamic
segment will let ld know what it needs to load
L&L: ld.so & Library Location
ld.so must be able to correctly locate the
identified libraries in the executable. It does this
by looking for them in the following order:
– DT_RPATH (-rpath-link option)
Section in ELF file
– LD_LIBRARY_PATH
Environment Variable
– /etc/ld.so.cache
Compiled list of files to load
– /lib; /usr/lib
– /etc/ld.so.conf
L&L: ld.so processing
Loop all the program headers to find
necessary info
– PHDR (program header): where the program
headers start; This must be found first.
– DYNAMIC: indicates where to find the
dynamic segment (what must be loaded)
NEEDED: Name of file needed
– INTERP: used to find the interpreter – which
generally turns out to be ld.so
L&L: ld.so processing
Load all required libraries found in
NEEDED portions of the DYNAMIC
segment
Get all necessary information from
library
– Dynamic header; phdr; load header
L&L: ld.so info
Read-Only
Read-Write
L&L: ld.so processing
What about when we actually call a
function that hasn’t been loaded?
– First need to resolve addressing issues
– Probably best to permanently fix them
– Then we need to call the actual procedure
L&L: ld.so processing
Next Procedure Run
PLT0: pushl GOT + 4
jmp *GOT + 8
PLTN: jmp *GOT+n
push #reloc_offset
jmp PLT0
Question: What is the name of the fix routine?
Answer: fixup
library
reloc_offset
Stack
Procedure Start Loc
Routine to fix
GOT then
jump to
procedure
after locating
correct
symbol
L&L: ld.so misc. info
You can run ld.so from the command
line with an executable
– This provides a great ability to test out new
ld.so’s if necessary
– /lib/ld-linux.so [executable [args…]]
L&L: linking helper tools
ldd – list the dynamic dependancies
readelf – displays information from ELF files
objdump – show information from object files
nm – show symbol information from object files
strip – removes symbols from object files
LD_DEBUG/LD_DEBUG_OUTPUT – shows
debug output from ld.so
L&L: References
Stallings, William. Operating Systems Internals and Design Principles, 4th Edition. Upper Saddle
River, NJ: Prentice-Hall, 2001
http://efrw01.frascati.enea.it/Software/Unix/IstrFTU/cern-cnl-2001003-25-link.html
http://www.iecc.com/linker/linker10.html
http://www.ibiblio.org/oswg/oswg-nightly/oswg/en_GB.ISO_88591/books/linux-c-programming/GCC-HOWTO/x796.html
http://linux.about.com/library/cmd/blcmdl2_execve.htm
http://www.iecc.com/linker/
http://www.suse.de/~bastian/Export/linking.txt
http://linux.about.com/library/cmd/blcmdl8_ld.so.htm
http://www.linuxjournal.com/node/6463
http://www.ibiblio.org/oswg/oswg-nightly/oswg/en_GB.ISO_88591/books/linux-c-programming/GCC-HOWTO/x575.html
L&L: References (cont.)
http://www.moses.uklinux.net/patches/lki-single.html
http://whatis.techtarget.com/definition/0,,sid9_gci212493,00.html
http://encyclopedia.thefreedictionary.com/position%20independent%
20code
http://www.faqs.org/docs/Linux-HOWTO/Program-LibraryHOWTO.html
http://sources.redhat.com/autobook/autobook/autobook_71.html
http://www.educ.umu.se/~bjorn/linux/howto/ELF-HOWTO-1.html
http://www.tcfs.it/docs/manpages/BSD/gcc-howto-6.html
http://www.cs.ucdavis.edu/~haungs/paper/
http://www-106.ibm.com/developerworks/linux/library/ldll.html?dwzone=linux