pwnlib.elf.elf — ELF Files

Exposes functionality for manipulating ELF files

Stop hard-coding things! Look them up at runtime with pwnlib.elf.

Example Usage

>>> e = ELF('/bin/cat')
>>> print hex(e.address) 
0x400000
>>> print hex(e.symbols['write']) 
0x401680
>>> print hex(e.got['write']) 
0x60b070
>>> print hex(e.plt['write']) 
0x401680

You can even patch and save the files.

>>> e = ELF('/bin/cat')
>>> e.read(e.address+1, 3)
'ELF'
>>> e.asm(e.address, 'ret')
>>> e.save('/tmp/quiet-cat')
>>> disasm(file('/tmp/quiet-cat','rb').read(1))
'   0:   c3                      ret'

Module Members

class pwnlib.elf.elf.ELF(path, checksec=True)

Bases: elftools.elf.elffile.ELFFile

Encapsulates information about an ELF file.

Example

>>> bash = ELF(which('bash'))
>>> hex(bash.symbols['read'])
0x41dac0
>>> hex(bash.plt['read'])
0x41dac0
>>> u32(bash.read(bash.got['read'], 4))
0x41dac6
>>> print bash.disasm(bash.plt.read, 16)
0:   ff 25 1a 18 2d 00       jmp    QWORD PTR [rip+0x2d181a]        # 0x2d1820
6:   68 59 00 00 00          push   0x59
b:   e9 50 fa ff ff          jmp    0xfffffffffffffa60

asm(address, assembly)

Assembles the specified instructions and inserts them into the ELF at the specified address.

This modifies the ELF in-pace. The resulting binary can be saved with ELF.save()

bss(offset=0) → int

Returns:Address of the .bss section, plus the specified offset.

checksec(banner=True)

Prints out information in the binary, similar to checksec.sh.

Parameters:banner (bool) – Whether to print the path to the ELF binary.

debug(argv=[], *a, **kw) → tube

Debug the ELF with gdb.debug().

Parameters:

• argv (list) – List of arguments to the binary
• *args – Extra arguments to gdb.debug()
• **kwargs – Extra arguments to gdb.debug()

Returns:

tube – See gdb.debug()

disable_nx()

Disables NX for the ELF.

Zeroes out the PT_GNU_STACK program header p_type field.

disasm(address, n_bytes) → str

Returns a string of disassembled instructions at the specified virtual memory address

dynamic_by_tag(tag) → tag

Parameters:tag (str) – Named DT_XXX tag (e.g. 'DT_STRTAB'). Returns:elftools.elf.dynamic.DynamicTag

dynamic_string(offset) → bytes

Fetches an enumerated string from the DT_STRTAB table.

Parameters:offset (int) – String index Returns:str – String from the table as raw bytes.

dynamic_value_by_tag(tag) → int

Retrieve the value from a dynamic tag a la DT_XXX.

If the tag is missing, returns None.

fit(address, *a, **kw)

Writes fitted data into the specified address.

See: packing.fit()

flat(address, *a, **kw)

Writes a full array of values to the specified address.

See: packing.flat()

static from_assembly(assembly) → ELF

Given an assembly listing, return a fully loaded ELF object which contains that assembly at its entry point.

Parameters:

• assembly (str) – Assembly language listing
• vma (int) – Address of the entry point and the module’s base address.

Example

>>> e = ELF.from_assembly('nop; foo: int 0x80', vma = 0x400000)
>>> e.symbols['foo'] = 0x400001
>>> e.disasm(e.entry, 1)
'  400000:       90                      nop'
>>> e.disasm(e.symbols['foo'], 2)
'  400001:       cd 80                   int    0x80'

static from_bytes(bytes) → ELF

Given a sequence of bytes, return a fully loaded ELF object which contains those bytes at its entry point.

Parameters:

• bytes (str) – Shellcode byte string
• vma (int) – Desired base address for the ELF.

Example

>>> e = ELF.from_bytes('\x90\xcd\x80', vma=0xc000)
>>> print(e.disasm(e.entry, 3))
    c000:       90                      nop
    c001:       cd 80                   int    0x80

get_machine_arch()

Return the machine architecture, as detected from the ELF header. Not all architectures are supported at the moment.

get_section_by_name(name)

Get a section from the file, by name. Return None if no such section exists.

iter_segments_by_type(t)

Yields:Segments matching the specified type.

num_sections()

Number of sections in the file

num_segments()

Number of segments in the file

offset_to_vaddr(offset) → int

Translates the specified offset to a virtual address.

Parameters:offset (int) – Offset to translate Returns:int – Virtual address which corresponds to the file offset, or None.

Examples

This example shows that regardless of changes to the virtual address layout by modifying ELF.address, the offset for any given address doesn’t change.

>>> bash = ELF('/bin/bash')
>>> bash.address == bash.offset_to_vaddr(0)
True
>>> bash.address += 0x123456
>>> bash.address == bash.offset_to_vaddr(0)
True

p16(address, data, *a, **kw)

Writes a 16-bit integer data to the specified address

p32(address, data, *a, **kw)

Writes a 32-bit integer data to the specified address

p64(address, data, *a, **kw)

Writes a 64-bit integer data to the specified address

p8(address, data, *a, **kw)

Writes a 8-bit integer data to the specified address

pack(address, data, *a, **kw)

Writes a packed integer data to the specified address

process(argv=[], *a, **kw) → process

Execute the binary with process. Note that argv is a list of arguments, and should not include argv[0].

Parameters:

• argv (list) – List of arguments to the binary
• *args – Extra arguments to process
• **kwargs – Extra arguments to process

Returns:

process

read(address, count) → bytes

Read data from the specified virtual address

Parameters:

• address (int) – Virtual address to read
• count (int) – Number of bytes to read

Returns:

A str object, or None.

Examples

The simplest example is just to read the ELF header.

>>> bash = ELF(which('bash'))
>>> bash.read(bash.address, 4)
'\x7fELF'

ELF segments do not have to contain all of the data on-disk that gets loaded into memory.

First, let’s create an ELF file has some code in two sections.

>>> assembly = '''
... .section .A,"awx"
... .global A
... A: nop
... .section .B,"awx"
... .global B
... B: int3
... '''
>>> e = ELF.from_assembly(assembly, vma=False)

By default, these come right after eachother in memory.

>>> e.read(e.symbols.A, 2)
'\x90\xcc'
>>> e.symbols.B - e.symbols.A
1

Let’s move the sections so that B is a little bit further away.

>>> objcopy = pwnlib.asm._objcopy()
>>> objcopy += [
...     '--change-section-vma', '.B+5',
...     '--change-section-lma', '.B+5',
...     e.path
... ]
>>> subprocess.check_call(objcopy)
0

Now let’s re-load the ELF, and check again

>>> e = ELF(e.path)
>>> e.symbols.B - e.symbols.A
6
>>> e.read(e.symbols.A, 2)
'\x90\x00'
>>> e.read(e.symbols.A, 7)
'\x90\x00\x00\x00\x00\x00\xcc'
>>> e.read(e.symbols.A, 10)
'\x90\x00\x00\x00\x00\x00\xcc\x00\x00\x00'

Everything is relative to the user-selected base address, so moving things around keeps everything working.

>>> e.address += 0x1000
>>> e.read(e.symbols.A, 10)
'\x90\x00\x00\x00\x00\x00\xcc\x00\x00\x00'

save(path=None)

Save the ELF to a file

>>> bash = ELF(which('bash'))
>>> bash.save('/tmp/bash_copy')
>>> copy = file('/tmp/bash_copy')
>>> bash = file(which('bash'))
>>> bash.read() == copy.read()
True

search(needle, writable = False) → generator

Search the ELF’s virtual address space for the specified string.

Notes

Does not search empty space between segments, or uninitialized data. This will only return data that actually exists in the ELF file. Searching for a long string of NULL bytes probably won’t work.

Parameters:

• needle (str) – String to search for.
• writable (bool) – Search only writable sections.

Yields:

An iterator for each virtual address that matches.

Examples

An ELF header starts with the bytes \x7fELF, so we sould be able to find it easily.

>>> bash = ELF('/bin/bash')
>>> bash.address + 1 == next(bash.search('ELF'))
True

We can also search for string the binary.

>>> len(list(bash.search('GNU bash'))) > 0
True

section(name) → bytes

Gets data for the named section

Parameters:name (str) – Name of the section Returns:str – String containing the bytes for that section

string(address) → str

Reads a null-terminated string from the specified address

Returns:A str with the string contents (NUL terminator is omitted), or an empty string if no NUL terminator could be found.

u16(address, *a, **kw)

Unpacks an integer from the specified address.

u32(address, *a, **kw)

Unpacks an integer from the specified address.

u64(address, *a, **kw)

Unpacks an integer from the specified address.

u8(address, *a, **kw)

Unpacks an integer from the specified address.

unpack(address, *a, **kw)

Unpacks an integer from the specified address.

vaddr_to_offset(address) → int

Translates the specified virtual address to a file offset

Parameters:address (int) – Virtual address to translate Returns:int – Offset within the ELF file which corresponds to the address, or None.

Examples

>>> bash = ELF(which('bash'))
>>> bash.vaddr_to_offset(bash.address)
0
>>> bash.address += 0x123456
>>> bash.vaddr_to_offset(bash.address)
0
>>> bash.vaddr_to_offset(0) is None
True

write(address, data)

Writes data to the specified virtual address

Parameters:

• address (int) – Virtual address to write
• data (str) – Bytes to write

Note

This routine does not check the bounds on the write to ensure that it stays in the same segment.

Examples

>>> bash = ELF(which('bash'))
>>> bash.read(bash.address+1, 3)
'ELF'
>>> bash.write(bash.address, "HELO")
>>> bash.read(bash.address, 4)
'HELO'

address

Address of the lowest segment loaded in the ELF.

When updated, the addresses of the following fields are also updated:

• symbols
• got
• plt
• functions

However, the following fields are NOT updated:

• segments
• sections

Example

>>> bash = ELF('/bin/bash')
>>> read = bash.symbols['read']
>>> text = bash.get_section_by_name('.text').header.sh_addr
>>> bash.address += 0x1000
>>> read + 0x1000 == bash.symbols['read']
True
>>> text == bash.get_section_by_name('.text').header.sh_addr
True

Type:int

arch = None

Architecture of the file (e.g. 'i386', 'arm').

See: ContextType.arch

Type:str

asan

Whether the current binary was built with Address Sanitizer (ASAN).

Type:bool

aslr

Whether the current binary is position-independent.

Type:bool

bits = 32

Bit-ness of the file

Type:int

build = None

Linux kernel build commit, if this is a Linux kernel image

Type:str

buildid

GNU Build ID embedded into the binary

Type:str

bytes = 4

Pointer width, in bytes

Type:int

canary

Whether the current binary uses stack canaries.

Type:bool

config = None

Linux kernel configuration, if this is a Linux kernel image

Type:dict

data

Raw data of the ELF file.

See:

get_data()

Type:str

dwarf

DWARF info for the elf

elftype

ELF type (EXEC, DYN, etc)

Type:str

endian = 'little'

Endianness of the file (e.g. 'big', 'little')

Type:str

entry

Address of the entry point for the ELF

Type:int

entrypoint

Address of the entry point for the ELF

Type:int

execstack

Whether the current binary uses an executable stack.

This is based on the presence of a program header PT_GNU_STACK being present, and its setting.

PT_GNU_STACK

The p_flags member specifies the permissions on the segment containing the stack and is used to indicate wether the stack should be executable. The absense of this header indicates that the stack will be executable.

In particular, if the header is missing the stack is executable. If the header is present, it may explicitly mark that the stack is executable.

This is only somewhat accurate. When using the GNU Linker, it usees DEFAULT_STACK_PERMS to decide whether a lack of PT_GNU_STACK should mark the stack as executable:

/* On most platforms presume that PT_GNU_STACK is absent and the stack is
 * executable.  Other platforms default to a nonexecutable stack and don't
 * need PT_GNU_STACK to do so.  */
uint_fast16_t stack_flags = DEFAULT_STACK_PERMS;

By searching the source for DEFAULT_STACK_PERMS, we can see which architectures have which settings.

$ git grep '#define DEFAULT_STACK_PERMS' | grep -v PF_X
sysdeps/aarch64/stackinfo.h:31:#define DEFAULT_STACK_PERMS (PF_R|PF_W)
sysdeps/nios2/stackinfo.h:31:#define DEFAULT_STACK_PERMS (PF_R|PF_W)
sysdeps/tile/stackinfo.h:31:#define DEFAULT_STACK_PERMS (PF_R|PF_W)

Type:bool

executable = None

True if the ELF is an executable

executable_segments

List of all segments which are executable.

See:

ELF.segments

Type:list

file = None

Open handle to the ELF file on disk

Type:file

fortify

Whether the current binary was built with Fortify Source (-DFORTIFY).

Type:bool

functions = {}

dotdict of name to Function for each function in the ELF

got = {}

dotdict of name to address for all Global Offset Table (GOT) entries

libc

If this ELF imports any libraries which contain 'libc[.-], and we can determine the appropriate path to it on the local system, returns a new ELF object pertaining to that library.

If not found, the value will be None.

Type:ELF

library = None

True if the ELF is a shared library

linker = None

Path to the linker for the ELF

memory = None

IntervalTree which maps all of the loaded memory segments

mmap = None

Memory-mapped copy of the ELF file on disk

Type:mmap.mmap

msan

Whether the current binary was built with Memory Sanitizer (MSAN).

Type:bool

native = None

Whether this ELF should be able to run natively

non_writable_segments

List of all segments which are NOT writeable.

See:

ELF.segments

Type:list

nx

Whether the current binary uses NX protections.

Specifically, we are checking for READ_IMPLIES_EXEC being set by the kernel, as a result of honoring PT_GNU_STACK in the kernel.

The Linux kernel directly honors PT_GNU_STACK to mark the stack as executable.

case PT_GNU_STACK:
    if (elf_ppnt->p_flags & PF_X)
        executable_stack = EXSTACK_ENABLE_X;
    else
        executable_stack = EXSTACK_DISABLE_X;
    break;

Additionally, it then sets read_implies_exec, so that all readable pages are executable.

if (elf_read_implies_exec(loc->elf_ex, executable_stack))
    current->personality |= READ_IMPLIES_EXEC;

Type:bool

os = None

Operating system of the ELF

packed

Whether the current binary is packed with UPX.

Type:bool

path = '/path/to/the/file'

Path to the file

Type:str

pie

Whether the current binary is position-independent.

Type:bool

plt = {}

dotdict of name to address for all Procedure Linkate Table (PLT) entries

relro

Whether the current binary uses RELRO protections.

This requires both presence of the dynamic tag DT_BIND_NOW, and a GNU_RELRO program header.

The ELF Specification describes how the linker should resolve symbols immediately, as soon as a binary is loaded. This can be emulated with the LD_BIND_NOW=1 environment variable.

DT_BIND_NOW

If present in a shared object or executable, this entry instructs the dynamic linker to process all relocations for the object containing this entry before transferring control to the program. The presence of this entry takes precedence over a directive to use lazy binding for this object when specified through the environment or via dlopen(BA_LIB).

(page 81)

Separately, an extension to the GNU linker allows a binary to specify a PT_GNU_RELRO program header, which describes the region of memory which is to be made read-only after relocations are complete.

Finally, a new-ish extension which doesn’t seem to have a canonical source of documentation is DF_BIND_NOW, which has supposedly superceded DT_BIND_NOW.

DF_BIND_NOW

If set in a shared object or executable, this flag instructs the dynamic linker to process all relocations for the object containing this entry before transferring control to the program. The presence of this entry takes precedence over a directive to use lazy binding for this object when specified through the environment or via dlopen(BA_LIB).

>>> path = pwnlib.data.elf.relro.path
>>> for test in glob(os.path.join(path, 'test-*')):
...     e = ELF(test)
...     expected = os.path.basename(test).split('-')[2]
...     actual = str(e.relro).lower()
...     assert actual == expected

Type:bool

rpath

Whether the current binary has an RPATH.

Type:bool

runpath

Whether the current binary has a RUNPATH.

Type:bool

rwx_segments

List of all segments which are writeable and executable.

See:

ELF.segments

Type:list

sections

A list of elftools.elf.sections.Section objects for the segments in the ELF.

Type:list

segments

A list of elftools.elf.segments.Segment objects for the segments in the ELF.

Type:list

start

Address of the entry point for the ELF

Type:int

statically_linked = None

True if the ELF is statically linked

sym

Alias for ELF.symbols

Type:dotdict

symbols = {}

dotdict of name to address for all symbols in the ELF

ubsan

Whether the current binary was built with Undefined Behavior Sanitizer (UBSAN).

Type:bool

version = None

Linux kernel version, if this is a Linux kernel image

Type:tuple

writable_segments

List of all segments which are writeable.

See:

ELF.segments

Type:list

class pwnlib.elf.elf.Function(name, address, size, elf=None)

Encapsulates information about a function in an ELF binary.

Parameters:

• name (str) – Name of the function
• address (int) – Address of the function
• size (int) – Size of the function, in bytes
• elf (ELF) – Encapsulating ELF object

address = None

Address of the function in the encapsulating ELF

elf = None

Encapsulating ELF object

name = None

Name of the function

size = None

Size of the function, in bytes

class pwnlib.elf.elf.dotdict

Wrapper to allow dotted access to dictionary elements.

Is a real dict object, but also serves up keys as attributes when reading attributes.

Supports recursive instantiation for keys which contain dots.

Example

>>> x = pwnlib.elf.elf.dotdict()
>>> isinstance(x, dict)
True
>>> x['foo'] = 3
>>> x.foo
3
>>> x['bar.baz'] = 4
>>> x.bar.baz
4