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Linking and Loading

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

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Linking and Loading

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  1. Linking and Loading Fred Prussack CS 518

  2. L&L: Overview • Wake-up Questions • Terms and Definitions / General Information • Loading • Linking • Static vs. Dynamic • ELF • Other Linking Information/Processing • References

  3. L&L: Topics Not Covered • Windows • Alternate binary file formats • Different versions of glibc • Different versions of the kernel (from 2.4)

  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

  5. 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

  6. 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…

  7. 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

  8. L&L: Loading do_execve() Searches all the registered binary handlers search_binary_handler Loads current binary and elf interpreter load_elf_binary sets up correct registers start_thread Question: What does the instruction pointer have in it now? Answer: Entry point of the ELF interpreter

  9. 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?

  10. 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

  11. 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>

  12. 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.

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. L&L: ld.so info Read-Only Read-Write

  19. 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

  20. L&L: ld.so processing Next Procedure Run PLT0: pushl GOT + 4 jmp *GOT + 8 PLTN: jmp *GOT+n push #reloc_offset jmp PLT0 library Routine to fix GOT then jump to procedure after locating correct symbol reloc_offset Stack Procedure Start Loc Question: What is the name of the fix routine? Answer: fixup

  21. 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…]]

  22. 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

  23. 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-2001-003-25-link.html • http://www.iecc.com/linker/linker10.html • http://www.ibiblio.org/oswg/oswg-nightly/oswg/en_GB.ISO_8859-1/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_8859-1/books/linux-c-programming/GCC-HOWTO/x575.html

  24. 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-Library-HOWTO.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/l-dll.html?dwzone=linux

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