Binary Loader
What is done by binary loader?
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Read executable from the filesystem
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Parse the binary header
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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.
Executable
●
●
●
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.
What’s in the executable file
●
Headers
–
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Object Code
–
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Position Independent code(PIC)
Symbols
–
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Data or instructures
Relocation
–
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Architecture, version, entry point, index table
Index to the data inside object code.
Debug information
Microsoft .COM format
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0-0xff: PSP
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0x100-xxxx
–
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The whole .COM executable will be loaded here.
No headers, symbol table and debug
information.
A.out
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Contains
–
–
–
–
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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
Relocation
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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
●
●
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Extern: 1 if it is a external symbols
Pcrel: It is relative to the PC.
Others.
Symbol and string table
●
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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.
ELF(Executable and Linkable Format)
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A ELF header
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zero or more program tables
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zero or more section tables
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support dlopen,dlsym
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Support real dynamic libraries
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References
–
http://www.linuxjournal.com/article.php?sid=1059
ELF:header
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ELF magic
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Type, machine,version
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entry:start point of program
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ehsize: the size of header(sizeof(struct elfhdr))
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shnum: The number of sectionss.
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shoff: The starting point of the section table
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shentsize: The size of each section
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phoff,shoff,flags
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phentsize,phnum
largo% readelf -S hello.o
There are 11 section headers, starting at offset
name
type
VM addr
flag
[0]
NULL
00000000 00000 00000
[1] .text
PROGBITS
00000000 00040 00014
[2] .rel.text
REL
00000000 00370 00010
[3] .data
PROGBITS
00000000 00054 00000
[4] .bss
NOBITS
00000000 00054 00000
[5] .note
NOTE
00000000 00054 00014
[6] .rodata PROGBITS
00000000 00068 0000d
[7] .comment
PROGBITS 00000000 00075 00012
[8] .shstrtab
STRTAB
00000000 00087 0004d
[9] .symtab SYMTAB
00000000 000d4 000c0
[a] .strtab STRTAB
00000000 00194 00024
1b8:
off
00
00
08
00
00
00
00
00
00
10
00
/
/
/
/
/
/
/
/
/
/
/
size
0
6
0
3
3
0
2
0
0
0
0
0
0
9
0
0
0
0
0
0
a
0
0
0
1
0
0
0
0
0
0
a
0
0
10
4
4
4
1
1
1
1
4
1
Type of sections
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PROGBITS: Program contents.
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NOBITS: BSS
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SYMTAB and DYNSYM: Symbol tables
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STRTAB: A string table
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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.
Typical sections
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.interp: The dynamic linker
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.hash,.dynsym,.dynstr: tables used by DLL
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.plt:jump tables to functions in libraries(RO)
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–
items are point to the DLL
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lazy binding(LD_BIND_NOW)
.got: The global offset table(RW)
–
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the DLL will change the value of this section
.text,.data,.bss
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
PLT and GOT
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Procedure Linkage Table(PLT)
–
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Function jump table
Global Offset Table(GOT)
–
Data jump table
XIP(eXecute In Place)
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Save memory(Especially for NOMMU system)
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Fast startup time(less memory copy)
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Requirements
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no writable data in text segment
XIP Example
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Eamples- uCLinux fs/binfmt_flat.c
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Allocate the memory for
●
●
●
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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);
Relocation Information
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Global Offset Table(GOT)
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Contains pointers to all global data and codes
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We need to recalculate all addresses
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;
}
}
}
Header of FLAT binary
Text
DATA
Relocs
BSS
Stack
MAGIC
version
entry
data start
data end
bss end
stack size
reloc start
reloc count
flags
reserved
FLAT_FLAG_RAM
FLAT_FLAG_GOTPIC
FLAT_FLAG_GZIP
Relocation Information(Cont)
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relocation table
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This is created by elf2flt
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The gcc will assume the following binary
striucture
–
text segment
–
data segment
–
bss segment
The link script must implement this order
XIP relocation
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Two memory segment
–
text segment: point to filesystem directly.
–
data,bss segment
The filesystem must put the entire binary in
contiguous blocks.
–
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Otherwise, do_mmap will copy all blocks into
contiguous in the RAM.
The mmnommu/filemap.c: generic_file_mmap