How to create static linked shared libraries

Question

For my master's thesis i'm trying to adapt a shared library approach for an ARM Cortex-M3 embedded system. As our targeted board has no MMU I think that it would make no sense to use "normal" dynamic shared libraries. Because .text is executed directly from flash and .data is copied to RAM at boot time I can't address .data relative to the code thus GOT too. GOT would have to be accessed through an absolute address which has to be defined at link time. So why not assigning fixed absolute addresses to all symbols at link time...?

From the book "Linkers and Loaders" I got aware of "static linked shared libraries, that is, libraries where program and data addresses in libraries are bound to executables at link time". The linked chapter describes how such libraries could be created in general and gives references to Unix System V, BSD/OS; but also mentions Linux and it's uselib() system call. Unfortunately the book gives no information how to actually create such libraries such as tools and/or compiler/linker switches. Apart from that book I hardly found any other information about such libraries "in the wild". The only thing I found in this regard was prelink for Linux. But as this operates on "normal" dynamic libraries thats not really what I'm searching for.

I fear that the use of these kind of libaries is very specific, so that no common tools exists to create them. Although the mentioned uselib() syscall in this context makes me wondering. But I wanted to make sure that I haven't overlooked anything before starting to hack my own linker... ;) So could anyone give me more information about such libraries?

Furthermore I'm wondering if there is any gcc/ld switch which links and relocates a file but keeps the relocation entries in the file - so that it could be re-relocated? I found the "-r" option, but that completely skips the relocation process. Does anyone have an idea?

edit:

Yes, I'm also aware of linker scripts. With gcc libfoo.c -o libfoo -nostdlib -e initLib -Ttext 0xdeadc0de I managed to get some sort of linked & relocated object file. But so far I haven't found any possibility to link a main program against this and use it as shared library. (The "normal way" of linking a dynamic shared library will be refused by the linker.)

Answer 1

Concepts

Minimum concept of what such a shared library maybe about.

• same code
• different data

There are variations on this. Do you support linking between libraries. Are the references a DAG structure or fully cyclic? Do you want to put the code in ROM, or support code updates? Do you wish to load libraries after a process is initially run? The last one is generally the difference between static shared libraries and dynamic shared libraries. Although many people will forbid references between libraries as well.

Facilities

Eventually, everything will come down to the addressing modes of the processor. In this case, the ARM thumb. The loader is generally coupled to the OS and the binary format in use. Your tool chain (compiler and linker) must also support the binary format and can generate the needed code.

Support for accessing data via a register is intrinsic in the APCS (the ARM Procedure calling standard). In this case, the data is accessed via the sb (for static base) which is register R9. The static base and stack checking are optional features. I believe you may need to configure/compile GCC to enable or disable these options.

The options -msingle-pic-base and -mpic-register are in the GCC manual. The idea is that an OS will initially allocate separate data for each library user and then load/reload the sb on a context switch. When code runs to a library, the data is accessed via the sb for that instances data.

Gcc's arm.c code has the require_pic_register() which does code generation for data references in a shared library. It may correspond to the ARM ATPCS shared library mechanics.^{See Sec 5.5}

You may circumvent the tool chain by using macros and inline assembler and possibly function annotations, like naked and section. However, the library and possibly the process need code modification in this case; Ie, non-standard macros like EXPORT(myFunction), etc.

One possibility

If the system is fully specified (a ROM image), you can make the offsets you can pre-generate data offsets that are unique for each library in the system. This is done fairly easily with a linker script. Use the NOLOAD and put the library data in some phony section. It is even possible to make the main program a static shared library. For instance, you are making a network device with four Ethernet ports. The main application handles traffic on one port. You can spawn four instances of the application with different data to indicate which port is being handled.

If you have a large mix/match of library types, the foot print for the library data may become large. In this case you need to re-adjust the sb when calls are made through a wrapper function on the external API to the library.

  void *__wrap_malloc(size_t size)  /* Wrapped version. */
  {
       /* Locals on stack */
       unsigned int new_sb = glob_libc; /* accessed via current sb. */
       void * rval;
       unsigned int old_sb;

       volatile asm(" mov %0, sb\n" : "=r" (old_sb);
       volatile asm(" mov sb, %0\n" :: "r" (new_sb);
       rval = __real_malloc(size);
       volatile asm(" mov sb, %0\n" :: "r" (old_sb);
       return rval;
  }

See the GNU ld --wrap option. This complexity is needed if you have a larger homogenous set of libraries. If your libraries consists of only 'libc/libsupc++', then you may not need to wrap anything.

The ARM ATPCS has veneers inserted by the compiler that do the equivalent,

LDR a4, [PC, #4] ; data address
MOV SB, a4
LDR a4, [PC, #4] ; function-entry
BX a4
DCD data-address
DCD function-entry

The size of the library data using this technique is 4k (possibly 8k, but that might need compiler modification). The limit is via ldr rN, [sb, #offset], were ARM limits offset to 12bits. Using the wrapping, each library has a 4k limit.

If you have multiple libraries that are not known when the original application builds, then you need to wrap each one and place a GOT type table via the OS loader at a fixed location in the main applications static base. Each application will require space for a pointer for each library. If the library is not used by the application, then the OS does not need to allocate the space and that pointer can be NULL.

The library table can be accessed via known locations in .text, via the original processes sb or via a mask of the stack. For instance, if all processes get a 2K stack, you can reserve the lower 16 words for a library table. sp & ~0x7ff will give an implicit anchor for all tasks. The OS will need to allocate task stacks as well.

Note, this mechanism is different than the ATPCS, which uses sb as a table to get offsets to the actual library data. As the memory is rather limited for the Cortex-M3 described it is unlikely that each individual library will need to use more than 4k of data. If the system supports an allocator this is a work around to this limitation.

References

• Xflat technical overview - Technical discussion from the Xflat authors; Xflat is a uCLinux binary format that supports shared libraries. A very good read.
• Linkage table and GOT - SO on PLT and GOT.
• ARM EABI - The normal ARM binary format.
• Assemblers and Loader, by David Solomon. Especially, pg262 A.3 Base Registers
• ARM ATPCS, especially Section 5.5, Shared Libraries, pg18.
• bFLT is another uCLinux binary format that supports shared libraries.

Answer 2

How much RAM do you have attached? Cortex-M systems have only a few dozen kiB on-chip and for the rest they require external SRAM.

I can't address .data relative to the code

You don't have to. You can place the library symbol jump table in the .data segment (or a segment that behaves similarly) at a fixed position.

thus GOT too. GOT would have to be accessed through an absolute address which has to be defined at link time. So why not assigning fixed absolute addresses to all symbols at link time...?

Nothing prevents you from having a second GOT placed at a fixed location, that's writable. You have to instruct your linker where and how to create it. For this you give the linker a so called "linker script", which is kind of a template-blueprint for the memory layout of the final program.

Answer 3

I'll try to answer your question before commenting about your intentions.

To compile a file in linux/solaris/any platform that uses ELF binaries:

gcc -o libFoo.so.1.0.0 -shared -fPIC foo1.c foo2.c foo3.c ... -Wl,-soname=libFoo.so.1

I'll explain all the options next:

-o libFoo.so.1.0.0

is the name we are going to give to the shared library file, once linked.

-shared

means that you have a shared object file at end, so there can be unsolved references after compilation and linked, that would be solved in late binding.

-fPIC

instructs the compiler to generate position independent code, so the library can be linked in a relocatable fashion.

-Wl,-soname=libFoo.so.1

has two parts: first, -Wl instructs the compiler to pass the next option (separated by comma) to the linker. The option is -soname=libFoo.so.1. This option, tells the linker the soname used for this library. The exact value of the soname is free style string, but there's a convenience custom to use the name of the library and the major version number. This is important, as when you do static linking of a shared library, the soname of the library gets stuck to the executable, so only a library with that soname can be loaded to assist this executable. Traditionally, when only the implementation of a library changes, we change only the name of the library, without changing the soname part, as the interface of the library doesn't change. But when you change the interface, you are building a new, incompatible one, so you must change the soname part, as it doesn't get in conflict with other 'versions' of it.

To link to a shared library is the same than to link to a static one (one that has .a as extension) Just put it on the command file, as in:

gcc -o bar bar.c libFoo.so.1.0.0

Normally, when you get some library in the system, you get one file and one or two symbolic links to it in /usr/lib directory:

/usr/lib/libFoo.so.1.0.0
/usr/lib/libFoo.so.1 --> /usr/lib/libFoo.so.1.0.0
/usr/lib/libFoo.so --> /usr/lib/libFoo.so.1

The first is the actual library called on executing your program. The second is a link with the soname as the name of the file, just to be able to do the late binding. The third is the one you must have to make

gcc -o bar bar.c -lFoo

work. (gcc and other ELF compilers search for libFoo.so, then for libFoo.a, in /usr/lib directory)

After all, there's an explanation of the concept of shared libraries, that perhaps will make you to change your image about statically linked shared code.

Dynamic libraries are a way for several programs to share the functionalities of them (that means the code, perhaps the data also). I think you are a little disoriented, as I feel you have someway misinterpreted what a statically linked shared library means.

static linking refers to the association of a program to the shared libraries it's going to use before even launching it, so there's a hardwired link between the program and all the symbols the library has. Once you launch the program, the linking process begins and you get a program running with all of its statically linked shared libraries. The references to the shared library are resolved, as the shared library is given a fixed place in the virtual memory map of the process. That's the reason the library has to be compiled with the -fPIC option (relocatable code) as it can be placed differently in the virtual space of each program.

On the opposite, dynamic linking of shared libraries refers to the use of a library (libdl.so) that allows you to load (once the program is executing) a shared library (even one that has not been known about before), search for its public symbols, solve references, load more libraries related to this one (and solve recursively as the linker could have done) and allow the program to make calls to symbols on it. The program doesn't even need to know the library was there on compiling or linking time.

Shared libraries is a concept related to the sharing of code. A long time ago, there was UNIX, and it made a great advance to share the text segment (whit the penalty of not being able for a program to modify its own code) of a program by all instances of it, so you have to wait for it to load just the first time. Nowadays, the concept of code sharing has extended to the library concept, and you can have several programs making use of the same library (perhaps libc, libdl or libm) The kernel makes a count reference of all the programs that are using it, and it just gets unloaded when no other program is using it.

using shared libraries has only one drawback: the compiler must create relocatable code to generate a shared library as the space used by one program for it can be used for another library when we try to link it to another program. This imposes normally a restriction in the set of op codes to be generated or imposes the use of one/several registers to cope with the mobility of code (there's no mobility but several linkings can make it to be situated at different places)

Believe me, using static code just derives you to making bigger executables, as you cannot share effectively the code, but with a shared library.