V8 Internals 101

To follow along, it is recommended to build d8 with debug mode. Steps on how to build d8 can be found here

Pointer Compression

In V8, pointer to an object is tagged with 1 on the least significant bit (LSB). This is done to distinguish between immediate values and pointer. Small immediate integer (SMI) are stored in 32-bit memory space with the LSB always set to 0.

The pointer itself is 32-bit wide which serves as an offset from the isolate_root. This is how sandboxing works in V8 heap. V8 would sum up isolate_root and the 32-bit offset value to get the actual memory address in the process.

In the example below, we could see that the elements pointer is 0x2932000006cd. The isolate_root is 0x293200000000, while the lower 32-bit, 0x000006cd is the offset and is the only value stored inside the heap.


Remember to subtract the pointer value by 1 when inspecting inside debugger

$ ./d8 --allow-natives-syntax
d8> let arr = [];
d8> %DebugPrint(arr);
DebugPrint: 0x2932001c9411: [JSArray]
 - map: 0x2932000ce6b1 <Map[16](PACKED_SMI_ELEMENTS)> [FastProperties]
 - prototype: 0x2932000ce925 <JSArray[0]>
 - elements: 0x2932000006cd <FixedArray[0]> [PACKED_SMI_ELEMENTS]
 - length: 0
 - properties: 0x2932000006cd <FixedArray[0]>
 - All own properties (excluding elements): {
    0x293200000d41: [String] in ReadOnlySpace: #length: 0x29320030f6f9 <AccessorInfo name= 0x293200000d41 <String[6]: #length>, data= 0x293200000061 <undefined>> (const accessor descriptor), location: descriptor

Further details on pointer compression can be found on this V8 blog.


First, let's take a look at how arrays are structured inside memory using GDB for this snippet of code.

let arr = [1.1, 2.2, 3.3, 4.4]
gdb -ex 'run' --args './d8 --allow-natives-syntax --shell ./script.js'
  • --allow-natives-syntax: allow us to invoke built-in function, e.g., %DebugPrint
  • --shell: drop into interactive mode after executing script.js
d8> %DebugPrint(arr)
DebugPrint: 0x3349001c94ad: [JSArray]
 - map: 0x3349000cefb1 <Map[16](PACKED_DOUBLE_ELEMENTS)> [FastProperties]
 - prototype: 0x3349000ce925 <JSArray[0]>
 - elements: 0x3349001c9485 <FixedDoubleArray[4]> [PACKED_DOUBLE_ELEMENTS]
 - length: 4
 - properties: 0x3349000006cd <FixedArray[0]>
 - All own properties (excluding elements): {
    0x334900000d41: [String] in ReadOnlySpace: #length: 0x33490030f6f9 <AccessorInfo name= 0x334900000d41 <String[6]: #length>, data= 0x334900000061 <undefined>> (const accessor descriptor), location: descriptor
 - elements: 0x3349001c9485 <FixedDoubleArray[4]> {
           0: 1.1
           1: 2.2
           2: 3.3
           3: 4.4

Inspecting the object directly in debugger, we could see that there are some familiar values 0xcefb1, 0x6cd, 0x1c9485

gef> tele 0x4f6001c94ad-0x1
0x3349001c94ac|+0x0000|+000: 0x000006cd000cefb1
0x3349001c94b4|+0x0008|+001: 0x00000008001c9485
0x3349001c94bc|+0x0010|+002: 0x0000030600000b51
0x3349001c94c4|+0x0018|+003: 0x0000000000000000
0x3349001c94cc|+0x0020|+004: 0x0000006100000100
0x3349001c94d4|+0x0028|+005: 0x0000006100000061
0x3349001c94dc|+0x0030|+006: 0x0000006100000061
0x3349001c94e4|+0x0038|+007: 0x0000006100000061
0x3349001c94ec|+0x0040|+008: 0x0000006100000061
0x3349001c94f4|+0x0048|+009: 0x0000006100000061

Recall that the upper 32-bit, 0x3349, is not present as it is the isolate_root value and is stored somewhere else in the memory. Next, remember that SMI is stored shifted to the left by 1. The length of our array is 4 and so the value in memory would be 4 << 1 = 0x8 which could be found at 0x3349001c94b8.

0x000x000cefb1map (pointer)
0x040x000006cdproperties (pointer)
0x080x001c9485elements (pointer)
0x0c0x00000008length (SMI)

Inspecting the array elements via d8 and debugger:

d8> %DebugPrintPtr(0x3349001c9485)
DebugPrint: 0x3349001c9485: [FixedDoubleArray]
 - map: 0x334900000851 <Map(FIXED_DOUBLE_ARRAY_TYPE)>
 - length: 4
           0: 1.1
           1: 2.2
           2: 3.3
           3: 4.4
gef> tele 0x3349001c9485-0x1
0x3349001c9484|+0x0000|+000: 0x0000000800000851
0x3349001c948c|+0x0008|+001: 0x3ff199999999999a
0x3349001c9494|+0x0010|+002: 0x400199999999999a
0x3349001c949c|+0x0018|+003: 0x400a666666666666
0x3349001c94a4|+0x0020|+004: 0x401199999999999a
0x3349001c94ac|+0x0028|+005: 0x000006cd000cefb1
0x3349001c94b4|+0x0030|+006: 0x00000008001c9485
0x3349001c94bc|+0x0038|+007: 0x0000030600000b51
0x3349001c94c4|+0x0040|+008: 0x0000000000000000
0x3349001c94cc|+0x0048|+009: 0x0000006100000100

Again we could see familiar values like 0x851 for map and 0x8 for length. The 4 64-bit values at offset 0x8 is actually 1.1, 2.2, 3.3, and 4.4 floating numbers in hexadecimal format.

gef> p/x 1.1
$1 = 0x3ff199999999999a
gef> p/x 2.2
$2 = 0x400199999999999a
gef> p/x 3.3
$3 = 0x400a666666666666
gef> p/x 4.4
$4 = 0x401199999999999a

At offset 0x28, we could see that it is actually the JSArray object that we inspected earlier.


Now, let's see when some of the elements changes type to Object and Integer. The elements are reallocated and the floating numbers are converted into objects.

d8> arr[0] = {}
d8> arr[1] = 1
d8> %DebugPrint(arr)
DebugPrint: 0x3349001c94ad: [JSArray]
 - map: 0x3349000cf031 <Map[16](PACKED_ELEMENTS)> [FastProperties]
 - prototype: 0x3349000ce925 <JSArray[0]>
 - elements: 0x3349001ca1b9 <FixedArray[4]> [PACKED_ELEMENTS]
 - length: 4
 - properties: 0x3349000006cd <FixedArray[0]>
 - All own properties (excluding elements): {
    0x334900000d41: [String] in ReadOnlySpace: #length: 0x33490030f6f9 <AccessorInfo name= 0x334900000d41 <String[6]: #length>, data= 0x334900000061 <undefined>> (const accessor descriptor), location: descriptor
 - elements: 0x3349001ca1b9 <FixedArray[4]> {
           0: 0x3349001ca19d <Object map = 0x3349000c4945>
           1: 1
           2: 0x3349001ca1dd <HeapNumber 3.3>
           3: 0x3349001ca1d1 <HeapNumber 4.4>
d8> %DebugPrintPtr(0x3349001ca1dd)
DebugPrint: 0x3349001ca1dd: [HeapNumber]
 - map: 0x3349000007b1 <Map[12](HEAP_NUMBER_TYPE)>
 - value: 3.3

Looking via debugger, this is how the new elements and HeapNumber look like inside memory.

gef> tele 0x3349001ca1b9-0x1
0x3349001ca1b8|+0x0000|+000: 0x0000000800000565
0x3349001ca1c0|+0x0008|+001: 0x00000002001ca19d
0x3349001ca1c8|+0x0010|+002: 0x001ca1d1001ca1dd

gef> tele 0x3349001ca1dd-0x1
0x3349001ca1dc|+0x0000|+000: 0x66666666000007b1
0x3349001ca1e4|+0x0008|+001: 0x000007b1400a6666
0x3349001ca1ec|+0x0010|+002: 0x400199999999999a
0x3349001ca1f4|+0x0018|+003: 0x9999999a000007b1
0x000x000cefb1map (pointer)
0x040x400a666666666666value (3.3)


Read this: https://jhalon.github.io/chrome-browser-exploitation-1/#object-representation

Elements Kinds

For arrays of kind PACKED_SMI_ELEMENTS, PACKED_DOUBLE_ELEMENTS, and PACKED_ELEMENTS, the elements is always allocated first which means that it can be found on lower memory address than the JSArray object itself. object.

let packed_smi_arr_1 = [1]
packed_smi_arr_1.push(2.2) // now this array becomes PACKED_DOUBLE_ELEMENTS kind
packed_smi_arr_1.push({}) // now this array becomes PACKED_ELEMENTS kind

let packed_smi_arr_2 = [1]
packed_smi_arr_2.push({}) // now this array becomes PACKED_ELEMENTS kind
packed_smi_arr_2.push(2.2) // stays on PACKED_ELEMENTS kind

let packed_double_arr = [1, 2.2]
let packed_arr_1 = [{}]
let packed_arr_2 = [0, {}]

There are also other kind of arrays, i.e., HOLEY_SMI_ELEMENTS, HOLEY_DOUBLE_ELEMENTS, and HOLEY_ELEMENTS. This are arrays that has the_hole_value as the element. The JSArray object is located at lower memory address than the elements.

let arr = Array(4)  // arr is HOLEY_SMI_ELEMENTS kind
arr[0] = 1.1        // arr transitions to HOLEY_DOUBLE_ELEMENTS kind
arr[1] = {}         // arr transitions to HOLEY_ELEMENTS kind

let foo = [1, 2.2]  // foo is PACKED_DOUBLE_ELEMENTS kind
delete foo[1]       // foo transitions to HOLEY_DOUBLE_ELEMENTS

Element kinds transition can be read on this blog

Garbage Collection

./d8 --trace-gc --expose-gc
d8> gc();  // major GC (mark and sweep)
d8> gc({type:'minor'});  // minor GC (scavenge)

Major GC

  • Covers the whole heap (ReadOnlySpace, OldSpace, NewSpace, etc.(?))
  • Marking is done from roots (typically variable on the most outer scope)
var b = [{foo:'bar'}, 1.1, {leet:'1337'}]
// b, {foo:'bar'}, and {leet:'1337'} lives in NewSpace
// HeapNumber(1.1) lives in OldSpace
// b, {foo:'bar'}, and {leet:'1337'} moves to OldSpace
// HeapNumber(1.1) lives in OldSpace
var b = [{foo:'bar'}, 1.1, {leet:'1337'}]
delete b[0];
// b and {leet:'1337'} lives in NewSpace
// {foo:'bar'} still lives in NewSpace
// HeapNumber(1.1) lives in OldSpace
// b and {leet:'1337'} moves to OldSpace
// {foo:'bar'} is garbage collected
// HeapNumber(1.1) still lives in OldSpace
delete b[2];
delete b[1];
// {leet:'1337'} still lives in OldSpace
// HeapNumber(1.1) still lives in OldSpace
// {leet:'1337'} is garbage collected
// HeapNumber(1.1) still lives in OldSpace

Minor GC

  • Only covers the NewSpace, specifically From-Space / Nursery, which is where objects are allocated to
  • First minor GC relocate reachable object from From-Space/Nursery to To-Space/Intermediate, then swap the labels between From-Space/Nursery and To-Space/Intermediate
  • Objects allocated after the first minor GC (let these objects be y) lives in the same space together with those that survives the first minor GC (let these objects be x)
  • The next minor GC, x objects that are still reachable are reallocated to OldSpace, and reachable y objects move to To-Space/Intermediate, then swap labels again
  • For subsequent minor GC, the same pattern follows
var b = [{foo:'bar'}, 1.1, {leet:'1337'}]
delete b[0];
// b and {leet:'1337'} lives in NewSpace
// {foo:'bar'} still lives in NewSpace
// HeapNumber(1.1) lives in OldSpace
var c = {idk:'new', kdi:'wen'}
// b and {leet:'1337'} moves to To-Space (NewSpace)
// {foo:'bar'} is garbage collected
// HeapNumber(1.1) lives in OldSpace
// c and its properties lives together with b and {leet:'1337'}
delete b[2];
// {leet:'1337'} is still around
// {leet:'1337'} is garbage collected
// b moves to OldSpace
// c and its properties still lives in NewSpace
// c and its properties moves to OldSpace



Building d8 for Debugging

git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git
export PATH="$(pwd)/depot_tools:$PATH"
fetch v8
cd v8
gclient sync
./tools/dev/v8gen.py x64.debug
ninja -C ./out.gn/x64.debug
cd ./out.gn/x64.debug