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Modified Aug 3/99Pat Beirne
All rights reserved
Overview
In the old days, applications were build by compiling many .c files into.o files. These files often had inter-related references that weren't resolvedat compile time. The information on these references are stored withinthe .o files in areloc (relocation) object.Later, at link time, the linker would merge all the .o files, buildinga table of where symbols are ultimately located. Then the linker wouldrun through the set of relocs, filling them in.
A reloc consists of three parts:
- where in memory the fix is to be made
- the symbol which is involved in the fix
- an algorithm that the linker should use to create the fixup
These relocs are scattered through the .o files, and are used at linktime create the correct binary executable file. Once all the relocs areresolved, the linker is pretty well done its job.
At least this is the way things used to work, in the days of staticlinking.
With the introduction of run-time linking, the designers of the ELFformat decided that relocs are a suitable entity to hold run-time resolutioninformation. So now we have executable files which still have relocs inthem, even after linking.
However, new algorithms are required to signal how these fixups areto be done. Hence the introduction of a new family of reloc numbers (i.e.algorithms)
Relocs and Memory Space
What is a reloc? Binary executables often need certain bits of informationfixed up before they execute. ELF binaries carry a list of relocs whichdescribe these fixups. Each reloc contains:the address in the binary that is to get the fixup(offset)
the algorithm to calculate the fixup (type)
a symbol (string and object len)
At fixup time, the algorithm uses the offset & symbol, along withthe value currently in the file, to calculate a new value to be depositedinto memory. See Appendix B
One of the targets of the ELF binary system is a separation of codeand data. The code of apps and libraries is marked read-only andexecutable.The data is markedread-write, and not-executable.
The code is read-only so that multiple processes can use thecode, having loaded the code into memory only once. Each process has itsown page tables, mapping the code into its own memory. The code is nevermodified, and appears identical in each process space. Naturally, the codemust be position independent; each process can load the app into a differentaddress.
The code segment is allowed to contain constant pointers and strings(.rodata).
The data segment is read-write and is mapped into each processspace differently. [In Linux, each data segment is loaded from the samebase mmap, but it is markedcopy-on-write; after the first write,each process has its own copy of the data.] Therefore, relocs can onlypoint to the data segment.
This half-and-half nature of ELF binaries leads us to an interestingdesign point. Some of the relocs that we wish to make are in the data segment.These are easy to do: we can add relative offsets, or write absolute addresseswith no problem. But the fixups in the code area are more difficult. TheELF reloc design forces us to make the code relocs 'bounce off' an entryin the data area, known as the GOT (global offset table).
In other words, if code needs to refer to a global object, it insteadrefers to an entry in the GOT[], and at run-time, the GOT entry is fixed-upto point to the intended data. In this manner, the code space need neverbe fixed-up at run time. If the code needs to refer to a local object,it refers to it 'relative to the &GOT[0]'; this too is position independent.NOTE 1
If the code needs to jump to a subroutine in a different module, thelinker creates an array of jump-stubs, called the PLT (procedure linkuptable). These jump-stubs jump indirect, using an entry in the GOT[] toimplement the far call.
Finally, ELF implements run time linking by deferring function resolutionuntil the function is called. This means that calls to library functionsgo through a fixup process the first time that they are called.
The rest of this paper explains the operation of these concepts.
NOTE 1: Relative (GOTOFF) code is made 'relativeto the start of the GOT table'. Instead, it could have been made 'relativeto the load address of the module', which would have been cleaner in myopinion. But there are reasons that some architectures chose the former,so we'll stick with it.
Reloc Design
Relocs are used in many places in the design cycle:- in .o files intended for executables
- in .o files intended for shared libraries
- in executables
- in shared libraries (.so files)
1-i) relative, from 'here' to a symbol (R_*_PC32), used forbranches
1-ii) abolute, to a symbol (R_*_32) NOTE 2
2) Object files which are going to be part of a library are a littledifferent. For one thing, they must be compiled as PIC code, using the-fpicflag. Next, there must be a distinction between local data/functions andglobal data/functions. Finally, relocs in the code/.rodata sections mustuse got-based relocs, because the code/.rodata area of the final libaryfile cannot be modified at run time. The relocs are:
in code:
2-i) reference to local symbol: use the relative distance from theGOT to the local symbol (R_*_GOTOFF); these relocs can exist inthe code area, because they will be fully resolved at link time
2-ii) reference to a global symbol: create an entry in the GOT andlet the run-time system deposit the symbol's address into the GOT for us(R_*_GOT32)
2-iii) In addition, relative calls to subroutiine (R_*_PC32)can be used.
in data:
2-iii) reference to symbol (R_*_32) [NOTE: symbols which areglobal have a reloc that references the symbol by name; symbols which arelocal can have a reloc that simply references the section number, and havea section-offset contained in the reloc. See NOTE 2]
3) Executables need to be able to refer to global data (such as errno)as if there is only one copy. ELF systems do this by copying global symbolsdown into the application .bss space. Then the executable and all the librariespoint to this single copy. To realize this, we need relocs:
3-i) reach into a library to a symbol and copy down the data into ourown .bss space (R_*_COPY)
3-ii) pointer to global data (R_*_GLOB_DAT)
3-iii) pointer to library function (R_*_JMP_SLOT)
Notice that all of these relocs must modifiy only the data sectionof the executable; the code section is read-only! All the relocs from the.o file have either been resolved, or mutated into one of the above 3.
4) Shared libraries are the most complex. By the time the library islinked, all the R_*_GOTOFF relocs (from the .o files) are resolved.
4-i) All the R_*_GOT32 relocs are resolved, pointing at GOTentries. At link time, these GOT entries get relocs of their own, pointingto the global data/function. (R_*_GLOB_DAT/R_*_JMP_SLOT respectively).
4-ii) There will be times when local data structures need to hold absolutepointers to local data. Put the module-relative address of the symbol inthe library; at run-time, add the module-load address to it (R_*_RELATIVE)
NOTE 3
Again, notice that all of these relocs must modifiy only the data sectionof the executable; the code section is read-only!
Jackpot Jump Slot Machine
When the linker creates 3) and 4) above, the linker actually createscode and data that was not explicit in the .o files. There is a .plt sectioncreated in the code segment, which is an array of function stubs used tohandle the run-time resolution of library calls. There is a .got sectioncreated in the data segment, which holds pointers to global symbols. Bothof these synthetic sections are 'helpers' to the code segment, since thecode segment cannot be modified at run-time.
To make all this happen, the object files must contain information aboutwhether a symbol is global or local, function or data, and the object size.(The old a.out scheme did not require all this extra info)
NOTE 2 At this point, I'll mention that globalrelocs must neccessarily involve the three aspects of a reloc:
- where in memory the reloc is to be made
- the symbol involved in the reloc
- the algorithm used to make the fixup.
For instance, in this ARM code
The code on the 3rd line (the call) needs to be fixed up, but that'seasy, since it's a PC relative fixup.
If the .o file has no idea where .Lextern is, it must neccessarilycreate a reloc which refers to
symbol Lextern.
The word at .L3 needs a fixup as well. If the .o file can determinethe location of a local symbol, such as L2, then it is allowed to replacethe symbol with a section-plus-offset. The offset is stored in thereloc target address, and the section is an entry in the reloc symbol tableThis reduces the number of symbols in the symbol table, making run-timelinking easier.
NOTE 3 Notice that the R_*_GOTOFF and R_*_GOT32relocs include an offset from &GOT[0}, which is usually about halfwaythrough the module. The R_ARM_RELATIVE relocs, on the other hand, containsan offset from the beginning of the module. Why? Tradition.
Jump Tables
As much as possible, ELF dynamic linking defers the resolution of jump/calladdresses until the last minute. The technique is based on the followingconstraints.- The calling technique should not force a change in the assembly code producedfor apps; it MAY cause changes in the way assembly code is produced forpic-code (i.e. libraries)
- The technique must be such that all executable areas must not be modifiedat run time; and any modified data areas must not be executed.
- start in the code
- go through the PLT
- using a pointer from the GOT
1) In the code:This is typical code using the relative jump or call. The target is anentry in the PLT. Note that this call is identical to a normal call.
2) In the PLT: The PLT is a synthetic area, created by the linker. Itexists in both executables and libraries. It is an array of stubs, oneper imported function call.
On i386 architecture, this code looks like:A subroutine call to PLT[n+1] will result jumping indirect through GOT[n+3].When first invoked, GOT[n+3] points back to PLT[n+1]+6, which is the push/jmpsequence. Going through the PLT[0], the resolver uses the argument on thestack to determine 'n' and resolves the symbol 'n'. The resolver code thenrepairs GOT[n+3] to point directly at the target subroutine.
The first PLT entry is slightly different, and is used to form a trampolineto the fixup code.Flow is directed to the resolver routine. 'n' is already on the stack,and &GOT[1] gets added on the stack. This way the resolver (locatedin ld-linux.so.2) can determine which library is asking for its service.
3) In the GOT: The GOT (global offset table) contains helper pointersfor both PLT fixups and GOT fixup. The first 3 entries are special/reserved.The next M entries belong to the PLT fixups. The next D entries belongto various data fixups.
The GOT is a synthetic area, created by the linker. It exists in bothexecutables and libraries.
When the GOT is first set up, all the GOT entries relating to PLT fixupsare pointing to code back at PLT[0].
The special entries in the GOT are
GOT[0] = linked list pointer usedby the dyn-loader
GOT[1] = pointer to the reloctable for this module
GOT[2] = pointer to the fixup/resolvercode, located in the ld-linux.so.2 library
followed by
GOT[3] .... GOT[3+M] = indirectfunction call helpers, one per imported function
GOT[3+M+1] ...... GOT[end]= indirect pointers for global data references, one per imported global
Remember that each library and executable gets its own PLT and GOT array.
Memory & Load Addresses
In a typical Linux system, the addresses 0-3fff.ffff (3 gigs)are available for the user program space.Exectuable binary files include header information that indicates aload address. Libraries, because they are position-independent, don't needa load address, but contain a 0 in this field.
i386In this case, the ld-linux.so.2 is loaded as an app. Sinceit was built as a library, it tries to load at 0. [In ArmLinux, this isforbidden, so the kernel pushes it up to 0x1000.] Once ld-linux.so.2loads, it reads it argv[1] and loads the foo_app at itspreferred location (0x0800.0000). Other libraries are loaded up a the mmaparea. So, in this case, the user memory map appears as
Notice that the small malloc space is much smaller in this case, butthis is supposed to be for load-testing and diagnostics, so it's not toobad.
Please, if you need more text, let me know: patb@corel.ca
Appendix A: Relocs in i386
Here is some analysis of the i386 design:
Intel
in .o files; these are the old relocs......| R_386_PC32 | determine the destinance from this memory location to the 'symbol',then add it to the value currently at this dword; deposit the result backinto the dword |
| R_386_PLT32 | create a new entry in the PLT[] and GOT[] determine the distance from here to the PLT[] entry and store thatdistance as a dword at this location at final link, rename the reloc to a R_386_JMP_SLOT, keeping the same'symbol' and point it at the GOT[] entry |
Executable files that are built 'static' have no relocs in them. Theyrun standalone.
In executable files which are intended to run with shared libraries......| R_386_COPY | read a string of bytes from the 'symbol' address and deposit a copyinto this location; the 'symbol' object has an intrinsic length i.e. move initialized data from a library down into the app data space |
| this reloc is, in a sense, the complement of the R_386_COPY above | |
| R_386_RELATIVE | at dynamic link time, read the dword at this location, add it to therun-time start address of this module; deposit the result back into thisdword |
Note that R_386_32 relocs can appear in libraries as well. These mustbe executed carefully!
R_386_COPY and R_386_GLOB_DATA can be considered complements ofeach other. Suppose you have a global data object defined in a dynamiclibrary. The library will have the binary version of the object in its.data space. When the application is built, the linker puts a R_386_COPYreloc in there to copy the data down to the application's .bss space. Inturn, the library never references the original global object; it referencesthe copy that is in the application data space, through a correspondingR_386_GLOB_DATA. Wierd, huh? After loading and copying, the original data(from the library) is never used; only the copy (in the app data space).
To make the whole dynamic linking operation happen, the linker introducesseveral 'synthetic' constructs into the target when you build an app ora library:| R_386_COPY=5, |