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Binary Loader

Binary Loader. Read executable from the filesystem Parse the binary header Copy all segments into addresses specified in the binary header text,data,bss Call binary “interpreter” to initialize the binary Jump to the entry point of dynamic linker, not executable.

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Binary Loader

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  1. Binary Loader

  2. Read executable from the filesystem Parse the binary header Copy all segments into addresses specified in the binary header text,data,bss Call binary “interpreter” to initialize the binary Jump to the entry point of dynamic linker, not executable. What is done by binary loader?

  3. a.out The old and classic unix object format. It contains text,data and bss sections plus one symbol table and one string table. COFF The SVR3 object format. The header now comprises a section table ELF The successor to COFF Make the support of shared library easier. Executable

  4. Headers Architecture, version, entry point, index table Object Code Data or instructures Relocation Position Independent code(PIC) Symbols Index to the data inside object code. Debug information What’s in the executable file

  5. 0-0xff: PSP 0x100-xxxx The whole .COM executable will be loaded here. No headers, symbol table and debug information. Microsoft .COM format

  6. Contains a.out header Text section Data section Other sections The instruction(text) and data(data) section are seperated. Multiple process can share the same text A.out

  7. Mainly used by MMU-less system and some DLLs. An relocation entry(fixups) contains An address relative to the beginning of the section Length of fixups Index with different meaning according to Extern: 1 if it is a external symbols Pcrel: It is relative to the PC. Others. Relocation

  8. Each entry in the symbol table represent either a function or variable in the program. Each symbol entry hold a index to the string table. Symbol and string table

  9. A ELF header zero or more program tables zero or more section tables support dlopen,dlsym Support real dynamic libraries References http://www.linuxjournal.com/article.php?sid=1059 ELF(Executable and Linkable Format)

  10. ELF magic Type, machine,version entry:start point of program ehsize: the size of header(sizeof(struct elfhdr)) shnum: The number of sectionss. shoff: The starting point of the section table shentsize: The size of each section phoff,shoff,flags phentsize,phnum ELF:header

  11. largo% readelf -S hello.o There are 11 section headers, starting at offset 1b8: name type VM addr off size flag [0] NULL 00000000 00000 00000 00 / 0 0 0 0 [1] .text PROGBITS 00000000 00040 00014 00 / 6 0 0 10 [2] .rel.text REL 00000000 00370 00010 08 / 0 9 1 4 [3] .data PROGBITS 00000000 00054 00000 00 / 3 0 0 4 [4] .bss NOBITS 00000000 00054 00000 00 / 3 0 0 4 [5] .note NOTE 00000000 00054 00014 00 / 0 0 0 1 [6] .rodata PROGBITS 00000000 00068 0000d 00 / 2 0 0 1 [7] .comment PROGBITS 00000000 00075 00012 00 / 0 0 0 1 [8] .shstrtab STRTAB 00000000 00087 0004d 00 / 0 0 0 1 [9] .symtab SYMTAB 00000000 000d4 000c0 10 / 0 a a 4 [a] .strtab STRTAB 00000000 00194 00024 00 / 0 0 0 1

  12. PROGBITS: Program contents. NOBITS: BSS SYMTAB and DYNSYM: Symbol tables STRTAB: A string table REL and RELA: Relocation information. REL entries add the relocation value to the base value stored in the code or data, while RELA entries include the base value for relocation in the relocation entries themselves. DYNAMIC and HASH: Dynamic linking information and the runtime symbol hash table. Type of sections

  13. .interp: The dynamic linker .hash,.dynsym,.dynstr: tables used by DLL .plt:jump tables to functions in libraries(RO) items are point to the DLL lazy binding(LD_BIND_NOW) .got: The global offset table(RW) the DLL will change the value of this section .text,.data,.bss Typical sections

  14. ELF:program headers largo% readelf -l hello Elf file is Executable Entry point 0x8000400 There are 5 program headers, starting at offset 34: PHDR 0x00034 0x08000034 0x000a0 0x000a0 R E Interp 0x000d4 0x080000d4 0x00017 0x00017 R Requesting program interpreter [/lib/elf/ld-linux.so.1] Load 0x00000 0x08000000 0x00515 0x00515 R E Load 0x00518 0x08001518 0x000cc 0x000d4 RW Dynamic 0x0054c 0x0800154c 0x00098 0x00098 RW Shared library: [libc.so.4] 1

  15. Procedure Linkage Table(PLT) Function jump table Global Offset Table(GOT) Data jump table PLT and GOT

  16. Save memory(Especially for NOMMU system) Fast startup time(less memory copy) Requirements no writable data in text segment XIP(eXecute In Place)

  17. Eamples- uCLinux fs/binfmt_flat.c Allocate the memory for data segmenet bss segment stack relocation entries Shared Library headers extra = MAX(bss_len + stack_len, relocs * sizeof(unsigned long)); down_write(¤t->mm->mmap_sem); realdatastart = do_mmap(0, 0, data_len + extra + MAX_SHARED_LIBS * sizeof(unsigned long), PROT_READ|PROT_WRITE|PROT_EXEC, 0, 0); up_write(¤t->mm->mmap_sem); XIP Example

  18. Global Offset Table(GOT) Contains pointers to all global data and codes We need to recalculate all addresses Relocation Information if (flags & FLAT_FLAG_GOTPIC) { for (rp = (unsigned long *)datapos; *rp != 0xffffffff;rp++) { unsigned long addr; if (*rp) { addr = calc_reloc(*rp, libinfo, id, 0); if (addr == RELOC_FAILED) return -ENOEXEC; *rp = addr; } } }

  19. Text MAGIC version entry data start data end DATA bss end stack size FLAT_FLAG_RAM FLAT_FLAG_GOTPIC FLAT_FLAG_GZIP reloc start Relocs reloc count flags BSS reserved Stack Header of FLAT binary FLAT_FLAG_RAM

  20. relocation table This is created by elf2flt The gcc will assume the following binary striucture text segment data segment bss segment The link script must implement this order Relocation Information(Cont)

  21. Two memory segment text segment: point to filesystem directly. data,bss segment The filesystem must put the entire binary in contiguous blocks. Otherwise, do_mmap will copy all blocks into contiguous in the RAM. The mmnommu/filemap.c: generic_file_mmap XIP relocation

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