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/*
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Copyright 2015 Google LLC All rights reserved.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at:
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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*/
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/*
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american fuzzy lop - high-performance binary-only instrumentation
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-----------------------------------------------------------------
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Written by Andrew Griffiths <agriffiths@google.com> and
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Michal Zalewski <lcamtuf@google.com>
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Idea & design very much by Andrew Griffiths.
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This code is a shim patched into the separately-distributed source
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code of QEMU 2.10.0. It leverages the built-in QEMU tracing functionality
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to implement AFL-style instrumentation and to take care of the remaining
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parts of the AFL fork server logic.
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The resulting QEMU binary is essentially a standalone instrumentation
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tool; for an example of how to leverage it for other purposes, you can
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have a look at afl-showmap.c.
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*/
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#include <sys/shm.h>
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#include "../../config.h"
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/***************************
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* VARIOUS AUXILIARY STUFF *
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***************************/
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/* A snippet patched into tb_find_slow to inform the parent process that
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we have hit a new block that hasn't been translated yet, and to tell
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it to translate within its own context, too (this avoids translation
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overhead in the next forked-off copy). */
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#define AFL_QEMU_CPU_SNIPPET1 do { \
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afl_request_tsl(pc, cs_base, flags); \
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} while (0)
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/* This snippet kicks in when the instruction pointer is positioned at
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_start and does the usual forkserver stuff, not very different from
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regular instrumentation injected via afl-as.h. */
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#define AFL_QEMU_CPU_SNIPPET2 do { \
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if(itb->pc == afl_entry_point) { \
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afl_setup(); \
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afl_forkserver(cpu); \
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} \
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afl_maybe_log(itb->pc); \
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} while (0)
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/* We use one additional file descriptor to relay "needs translation"
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messages between the child and the fork server. */
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#define TSL_FD (FORKSRV_FD - 1)
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/* This is equivalent to afl-as.h: */
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static unsigned char *afl_area_ptr;
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/* Exported variables populated by the code patched into elfload.c: */
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abi_ulong afl_entry_point, /* ELF entry point (_start) */
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afl_start_code, /* .text start pointer */
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afl_end_code; /* .text end pointer */
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/* Set in the child process in forkserver mode: */
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static unsigned char afl_fork_child;
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unsigned int afl_forksrv_pid;
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/* Instrumentation ratio: */
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static unsigned int afl_inst_rms = MAP_SIZE;
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/* Function declarations. */
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static void afl_setup(void);
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static void afl_forkserver(CPUState*);
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static inline void afl_maybe_log(abi_ulong);
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static void afl_wait_tsl(CPUState*, int);
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static void afl_request_tsl(target_ulong, target_ulong, uint64_t);
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/* Data structure passed around by the translate handlers: */
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struct afl_tsl {
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target_ulong pc;
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target_ulong cs_base;
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uint64_t flags;
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};
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/* Some forward decls: */
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TranslationBlock *tb_htable_lookup(CPUState*, target_ulong, target_ulong, uint32_t);
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static inline TranslationBlock *tb_find(CPUState*, TranslationBlock*, int);
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/*************************
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* ACTUAL IMPLEMENTATION *
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*************************/
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/* Set up SHM region and initialize other stuff. */
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static void afl_setup(void) {
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char *id_str = getenv(SHM_ENV_VAR),
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*inst_r = getenv("AFL_INST_RATIO");
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int shm_id;
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if (inst_r) {
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unsigned int r;
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r = atoi(inst_r);
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if (r > 100) r = 100;
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if (!r) r = 1;
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afl_inst_rms = MAP_SIZE * r / 100;
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}
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if (id_str) {
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shm_id = atoi(id_str);
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afl_area_ptr = shmat(shm_id, NULL, 0);
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if (afl_area_ptr == (void*)-1) exit(1);
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/* With AFL_INST_RATIO set to a low value, we want to touch the bitmap
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so that the parent doesn't give up on us. */
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if (inst_r) afl_area_ptr[0] = 1;
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}
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if (getenv("AFL_INST_LIBS")) {
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afl_start_code = 0;
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afl_end_code = (abi_ulong)-1;
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}
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/* pthread_atfork() seems somewhat broken in util/rcu.c, and I'm
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not entirely sure what is the cause. This disables that
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behaviour, and seems to work alright? */
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rcu_disable_atfork();
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}
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/* Fork server logic, invoked once we hit _start. */
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static void afl_forkserver(CPUState *cpu) {
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static unsigned char tmp[4];
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if (!afl_area_ptr) return;
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/* Tell the parent that we're alive. If the parent doesn't want
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to talk, assume that we're not running in forkserver mode. */
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if (write(FORKSRV_FD + 1, tmp, 4) != 4) return;
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afl_forksrv_pid = getpid();
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/* All right, let's await orders... */
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while (1) {
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pid_t child_pid;
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int status, t_fd[2];
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/* Whoops, parent dead? */
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if (read(FORKSRV_FD, tmp, 4) != 4) exit(2);
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/* Establish a channel with child to grab translation commands. We'll
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read from t_fd[0], child will write to TSL_FD. */
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if (pipe(t_fd) || dup2(t_fd[1], TSL_FD) < 0) exit(3);
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close(t_fd[1]);
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child_pid = fork();
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if (child_pid < 0) exit(4);
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if (!child_pid) {
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/* Child process. Close descriptors and run free. */
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afl_fork_child = 1;
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close(FORKSRV_FD);
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close(FORKSRV_FD + 1);
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close(t_fd[0]);
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return;
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}
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/* Parent. */
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close(TSL_FD);
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if (write(FORKSRV_FD + 1, &child_pid, 4) != 4) exit(5);
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/* Collect translation requests until child dies and closes the pipe. */
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afl_wait_tsl(cpu, t_fd[0]);
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/* Get and relay exit status to parent. */
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if (waitpid(child_pid, &status, 0) < 0) exit(6);
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if (write(FORKSRV_FD + 1, &status, 4) != 4) exit(7);
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}
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}
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/* The equivalent of the tuple logging routine from afl-as.h. */
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static inline void afl_maybe_log(abi_ulong cur_loc) {
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static __thread abi_ulong prev_loc;
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/* Optimize for cur_loc > afl_end_code, which is the most likely case on
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Linux systems. */
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if (cur_loc > afl_end_code || cur_loc < afl_start_code || !afl_area_ptr)
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return;
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/* Looks like QEMU always maps to fixed locations, so ASAN is not a
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concern. Phew. But instruction addresses may be aligned. Let's mangle
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the value to get something quasi-uniform. */
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cur_loc = (cur_loc >> 4) ^ (cur_loc << 8);
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cur_loc &= MAP_SIZE - 1;
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/* Implement probabilistic instrumentation by looking at scrambled block
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address. This keeps the instrumented locations stable across runs. */
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if (cur_loc >= afl_inst_rms) return;
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afl_area_ptr[cur_loc ^ prev_loc]++;
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prev_loc = cur_loc >> 1;
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}
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/* This code is invoked whenever QEMU decides that it doesn't have a
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translation of a particular block and needs to compute it. When this happens,
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we tell the parent to mirror the operation, so that the next fork() has a
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cached copy. */
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static void afl_request_tsl(target_ulong pc, target_ulong cb, uint64_t flags) {
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struct afl_tsl t;
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if (!afl_fork_child) return;
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t.pc = pc;
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t.cs_base = cb;
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t.flags = flags;
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if (write(TSL_FD, &t, sizeof(struct afl_tsl)) != sizeof(struct afl_tsl))
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return;
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}
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/* This is the other side of the same channel. Since timeouts are handled by
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afl-fuzz simply killing the child, we can just wait until the pipe breaks. */
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static void afl_wait_tsl(CPUState *cpu, int fd) {
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struct afl_tsl t;
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TranslationBlock *tb;
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while (1) {
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/* Broken pipe means it's time to return to the fork server routine. */
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if (read(fd, &t, sizeof(struct afl_tsl)) != sizeof(struct afl_tsl))
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break;
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tb = tb_htable_lookup(cpu, t.pc, t.cs_base, t.flags);
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if(!tb) {
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mmap_lock();
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tb_lock();
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tb_gen_code(cpu, t.pc, t.cs_base, t.flags, 0);
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mmap_unlock();
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tb_unlock();
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}
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}
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close(fd);
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}
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Reference in new issue