AFL 採用 forkserver 技術,只需進行一次 execve() 函數執行,之後的 fuzz 進程通過寫時拷⻉技術從已經停止的 fuzz 進程鏡像直接拷⻉。AFL主要是在對代碼進行插樁時，將forkserver的彙編代碼插入目標程序中執行，彙編代碼與流程圖如下：
"__afl_forkserver:\n"
"\n"
" /* Enter the fork server mode to avoid the overhead of execve() calls. We\n"
" push rdx (area ptr) twice to keep stack alignment neat. */\n"
"\n"
" pushq %rdx\n"
" pushq %rdx\n"
"\n"
" /* Phone home and tell the parent that we're OK. (Note that signals with\n"
" no SA_RESTART will mess it up). If this fails, assume that the fd is\n"
" closed because we were execve()d from an instrumented binary, or because\n"
" the parent doesn't want to use the fork server. */\n"
"\n"
" movq $4, %rdx /* length */\n"
" leaq __afl_temp(%rip), %rsi /* data */\n"
" movq $" STRINGIFY((FORKSRV_FD + 1)) ", %rdi /* file desc */\n"
CALL_L64("write")
"\n"
" cmpq $4, %rax\n"
" jne __afl_fork_resume\n"
"\n"
"__afl_fork_wait_loop:\n"
"\n"
" /* Wait for parent by reading from the pipe. Abort if read fails. */\n"
"\n"
" movq $4, %rdx /* length */\n"
" leaq __afl_temp(%rip), %rsi /* data */\n"
" movq $" STRINGIFY(FORKSRV_FD) ", %rdi /* file desc */\n"
CALL_L64("read")
" cmpq $4, %rax\n"
" jne __afl_die\n"
"\n"
" /* Once woken up, create a clone of our process. This is an excellent use\n"
" case for syscall(__NR_clone, 0, CLONE_PARENT), but glibc boneheadedly\n"
" caches getpid() results and offers no way to update the value, breaking\n"
" abort(), raise(), and a bunch of other things :-( */\n"
"\n"
CALL_L64("fork")
" cmpq $0, %rax\n"
" jl __afl_die\n"
" je __afl_fork_resume\n"
"\n"
" /* In parent process: write PID to pipe, then wait for child. */\n"
"\n"
" movl %eax, __afl_fork_pid(%rip)\n"
"\n"
" movq $4, %rdx /* length */\n"
" leaq __afl_fork_pid(%rip), %rsi /* data */\n"
" movq $" STRINGIFY((FORKSRV_FD + 1)) ", %rdi /* file desc */\n"
CALL_L64("write")
"\n"
" movq $0, %rdx /* no flags */\n"
" leaq __afl_temp(%rip), %rsi /* status */\n"
" movq __afl_fork_pid(%rip), %rdi /* PID */\n"
CALL_L64("waitpid")
" cmpq $0, %rax\n"
" jle __afl_die\n"
"\n"
" /* Relay wait status to pipe, then loop back. */\n"
"\n"
" movq $4, %rdx /* length */\n"
" leaq __afl_temp(%rip), %rsi /* data */\n"
" movq $" STRINGIFY((FORKSRV_FD + 1)) ", %rdi /* file desc */\n"
CALL_L64("write")
"\n"
" jmp __afl_fork_wait_loop\n"
"\n"
"__afl_fork_resume:\n"
"\n"
" /* In child process: close fds, resume execution. */\n"
"\n"
" movq $" STRINGIFY(FORKSRV_FD) ", %rdi\n"
CALL_L64("close")
``` 