A customer was having trouble calling the Read­File function with the interop feature of their language. It crashed with

(df4.1ef4): Access violation - code c0000005 (first chance)

rax=0000000001260000 rbx=00000093834eb020 rcx=0000000000000264
rdx=00000093834eb028 rsi=0000000000000264 rdi=0000000000000000
rip=00007ff9eaddaf5f rsp=00000093834eaf50 rbp=00000093834eb029
 r8=000000000000003c  r9=00000093834eb020 r10=00000093834eb028
r11=0000000000000246 r12=ffffffff89010f67 r13=0000000000000001
r14=000000d900000000 r15=0000000000000000
iopl=0         nv up ei pl nz na po nc
cs=0033  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010206
KERNELBASE!ReadFile+0xaf:
00007ff9`eaddaf5f mov     [r14],103h ds:000000d9`00000000=????????????????

Their interop declaration looked something like this.

function ReadFile(
    handle: integer,
    buffer: array of byte,
    length: integer,
    actual: ref integer,
    overlapped: integer): integer;

// called as follows

var handle: integer; /* assume we get a handle somehow */
var buffer: array(256) of byte;
var actual: integer;
var result: integer;

result = ReadFile(handle, buffer, 256, actual, 0);

From the crash, we see that we are moving the value 0x0103 into an invalid pointer.

Can you solve the mystery?

Hint: The disassembly shows that this code is running in 64-bit mode.

Another hint: The value 0x103 is STATUS_PENDING.

Yet another hint: The invalid address happens to be an exact multiple of 4GB.

Okay, let’s put the hints together.

The last hint steers you to the answer: In the Read­File function, the handle and overlapped parameters are pointer-sized, whereas the length and actual parameters are 32-bit integers, but the interop declaration calls them all the same thing: integer. I’m guessing that in this language, integer is a 32-bit integer.

Due to the size mismatch, the caller puts a 32-bit zero on the stack, but the function expects a pointer and reads a 64-bit value. As a result, only the lower-order 32 bits end up zero, and the upper 32 bits contain uninitialized stack data.

The other clue that the problem is with the overlapped parameter is that the first use of the invalid pointer is to write STATUS_PENDING to it, which makes sense because that’s what gets written to an OVERLAPPED structure to indicate that the I/O is in progress.

I mentioned that the first parameter handle is also pointer-sized, yet nothing appears to go wrong with the handle. Why do we get away with an incorrect declaration for handle, but not for overlapped?

The Windows x86-64 calling convention puts the first four parameters in registers, so the handle parameter goes into the rcx register. The x86-64 architecture has the policy that if you load a 32-bit value into a 64-bit register, the upper 32 bits are set to zero by default.¹ The value being loaded into the rcx register almost certainly came from memory or another register, and the load of a 32-bit value into that register will implicitly zero-extend the value to 64 bits.

¹ There is a separate instruction for loading with sign extension.

Related reading: Which processors prefer sign-extended loads, and which prefer zero-extended loads?