Direct Syscall Implementation

Extract syscall numbers (SSNs) from ntdll to implement direct syscalls and bypass usermode hooks. Modern EDRs hook ntdll functions; direct syscalls bypass these by extracting the SSN from the stub and calling the syscall instruction directly.

Syscall Stub Pattern (x64)

4C 8B D1        mov r10, rcx
B8 XX 00 00 00  mov eax, SSN  ; XX is the syscall number
0F 05           syscall
C3              ret

Input

Hex bytes of a syscall stub

Output

The syscall number (decimal)