elfloader

ARMv7M ELF loader

The goal of this project is provide a loader for ELF file format for ARMv7-M (thumb-2) architecture (Aka Cortex-M, Cortex-R in Thumb2 mode) over bare-metal or RTOS enviroment.

This loader not required MMU or special OS support (only aligned memory alloc) and run with minimun memory overhead (only required parts of files is loaded in memory).

This is developed using gcc arm embedded compiler from GCC arm embedded (arm-none-eabi) but is successful tested with linaro arm-linux-gnueabihf in freestangin mode.

ELF creation

For correct handling, The code must be compiled with certain characteristics:

• No common section is allowed. All non-init data is in bss (CC -fno-common)
• Only word relocation is allowed. (CC -mword-relocation) This is not realy true, some compilers produce R_ARM_THB_CALL/JMP relocation and ignore word-relocation flags, therefore, the loader handling only two relocation:
  • R_ARM_ABS32 Emmited on every data access and some jmp (weak/extern)
  • R_ARM_THB_JMP/CALL Emmited on some short jumps (CC -mlong-call flag not fix it)
• Relocatable ELF is required (LD -r option)
• No start library (LD -nostartfiles)

An example of application is found in the app folder

Usage

The API is simple, call to #exec_elf function and enjoying.

    extern int exec_elf(const char *path, const ELFEnv_t *env);

This function take a path to a file, and ELFEnv_t is a struct containing:

	typedef struct {
	  const ELFSymbol_t *exported;
	  size_t exported_size;
	} ELFEnv_t;

• exported: Array of symbols to resolve in executable. ELFSymbol_t is a struct contains const char *name for C-String symbol name and void *ptr pointer to memory related to symbol (entry point of function, variable address, etc)
• size: Size of exported symbol array in elements number

Loader config

File handling macros

• LOADER_FD_T File descriptor type
• LOADER_OPEN_FOR_RD(path) Function to open file for read
• LOADER_FD_VALID(fd) Validity evaluation of file descriptor
• LOADER_READ(fd, buffer, size) Function to read buffer from fd
• LOADER_CLOSE(fd) Function to close file descriptor
• LOADER_SEEK_FROM_START(fd, off) Seek function over fd
• LOADER_TELL(fd) Tell position of fd cursor

Memory manager/access

• LOADER_ALIGN_ALLOC(size, align, perm) Aligned malloc function macro
• LOADER_FREE(ptr) Free memory function
• LOADER_CLEAR(ptr, size) Memory clearance (to 0) function
• LOADER_STREQ(s1, s2) String compare function (return !=0 if s1==s2)

Code execution

• LOADER_JUMP_TO(entry) Macro for jump to "entry" pointer (entry_t)

Debug/message

• DBG(...) printf style macro for debug
• ERR(msg) puts style macro used on severe error
• MSG(msg) puts style macro used on info/warning