Reversing Rust String And Str Datatypes

Monday, April 13, 2020

Reversing Rust String And Str Datatypes

Lets build an app that uses several data-types in order to see how is stored from a low level perspective.

Rust string data-types

The two first main objects are "str" and String, lets check also the constructors.

Imports and functions

Even such a basic program links several libraries and occupy 2,568Kb, it's really not using the imports and expots the runtime functions even the main.

Even a simple string operation needs 544 functions on rust:

Main function

If you expected see a clear main function I regret to say that rust doesn't seem a real low-level language In spite of having a full control of the memory.

Ghidra turns crazy when tries to do the recursive parsing of the rust code, and finally we have the libc _start function, the endless loop after main is the way Ghidra decompiles the HLT instruction.

If we jump to main, we see a function call, the first parameter is rust_main as I named it below:

If we search "hello world" on the Defined Strings sections, matches at the end of a large string

After doing "clear code bytes" we can see the string and the reference:

We can see that the literal is stored in an non null terminated string, or most likely an array of bytes. we have a bunch of byte arrays and pointed from the code to the beginning.
Let's follow the ref. [ctrl]+[shift]+[f] and we got the references that points to the rust main function.

After several naming thanks to the Ghidra comments that identify the rust runtime functions, the rust main looks more understandable.
See below the ref to "hello world" that is passed to the string allocated hard-coding the size, because is non-null terminated string and there is no way to size this, this also helps to the rust performance, and avoid the c/c++ problems when you forgot the write the null byte for example miscalculating the size on a memcpy.

Regarding the string object, the allocator internals will reveal the structure in static.
alloc_string function call a function that calls a function that calls a function and so on, so this is the stack (also on static using the Ghidra code comments)

1. _$LT$alloc..string..String$u20$as$u20$core..convert..From$LT$$RF$str$GT$$GT$::from::h752d6ce1f15e4125
2. alloc::str::_$LT$impl$u20$alloc..borrow..ToOwned$u20$for$u20$str$GT$::to_owned::h649c495e0f441934
3. alloc::slice::_$LT$impl$u20$alloc..borrow..ToOwned$u20$for$u20$$u5b$T$u5d$$GT$::to_owned::h1eac45d28
4. alloc::slice::_$LT$impl$u20$$u5b$T$u5d$$GT$::to_vec::h25257986b8057640
5. alloc::slice::hack::to_vec::h37a40daa915357ad
6. core::slice::_$LT$impl$u20$$u5b$T$u5d$$GT$::len::h2af5e6c76291f524
7. alloc::vec::Vec$LT$T$GT$::extend_from_slice::h190290413e8e57a2
8. _$LT$alloc..vec..Vec$LT$T$GT$$u20$as$u20$alloc..vec..SpecExtend$LT$$RF$T$C$core..slice..Iter$LT$T$GT$$GT$$GT$::spec_extend::h451c2f92a49f9caa
...

Well I'm not gonna talk about the performance impact on stack but really to program well reusing code grants the maintainability and its good, and I'm sure that the rust developed had measured that and don't compensate to hardcode directly every constructor.

At this point we have two options, check the rust source code, or try to figure out the string object in dynamic with gdb.

Source code

Let's explain this group of substructures having rust source code in the hand.
The string object is defined at string.rs and it's simply an u8 type vector.

And the definition of vector can be found at vec.rs and is composed by a raw vector an the len which is the usize datatype.

The RawVector is a struct that helds the pointer to the null terminated string stored on an Unique object, and also contains the allocation pointer, here raw_vec.rs definition.

The cap field is the capacity of the allocation and a is the allocator:

Finally the Unique object structure contains a pointer to the null terminated string, and also a one byte marker core::marker::PhantomData

Dynamic analysis

The first parameter of the constructor is the interesting one, and in x64 arch is on RDI register, the extrange sequence RDI,RSI,RDX,RCX it sounds like ACDC with a bit of imagination (di-si-d-c)

So the RDI parámeter is the pointer to the string object:

So RDI contains the stack address pointer that points the the heap address 0x5578f030.
Remember to disable ASLR to correlate the addresses with Ghidra, there is also a plugin to do the synchronization.

Having symbols we can do:
p mystring

and we get the following structure:

String::String {
vec: alloc::vec::Vec {
buf: alloc::raw_vec::RawVec {
ptr: core::ptr::unique::Unique {
pointer: 0x555555790130 "hello world\000",
_marker: core::marker::PhantomData
},
cap: 11,
a: alloc::alloc::Global
},
len: 11
}
}

If the binary was compiled with symbols we can walk the substructures in this way:

(gdb) p mystring.vec.buf.ptr
$6 = core::ptr::unique::Unique {pointer: 0x555555790130 "hello world\000", _marker: core::marker::PhantomData}

(gdb) p mystring.vec.len

$8 = 11

If we try to get the pointer of each substructure we would find out that the the pointer is the same:

If we look at this pointer, we have two dwords that are the pointer to the null terminated string, and also 0xb which is the size, this structure is a vector.

The pionter to the c string is 0x555555790130

This seems the c++ string but, let's look a bit deeper:

RawVector
Vector:
(gdb) x/wx 0x7fffffffdf50
0x7fffffffdf50: 0x55790130 -> low dword c string pointer
0x7fffffffdf54: 0x00005555 -> hight dword c string pointer
0x7fffffffdf58: 0x0000000b -> len

0x7fffffffdf5c: 0x00000000
0x7fffffffdf60: 0x0000000b -> low cap (capacity)
0x7fffffffdf64: 0x00000000 -> hight cap
0x7fffffffdf68: 0xf722fe27 -> low a (allocator)
0x7fffffffdf6c: 0x00007fff -> hight a
0x7fffffffdf70: 0x00000005

So in this case the whole object is in stack except the null-terminated string.

Monday, April 13, 2020