/* Copyright 2013 Google LLC All rights reserved. Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at: http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. */ /* american fuzzy lop - wrapper for GNU as --------------------------------------- Written and maintained by Michal Zalewski The sole purpose of this wrapper is to preprocess assembly files generated by GCC / clang and inject the instrumentation bits included from afl-as.h. It is automatically invoked by the toolchain when compiling programs using afl-gcc / afl-clang. Note that it's an explicit non-goal to instrument hand-written assembly, be it in separate .s files or in __asm__ blocks. The only aspiration this utility has right now is to be able to skip them gracefully and allow the compilation process to continue. That said, see experimental/clang_asm_normalize/ for a solution that may allow clang users to make things work even with hand-crafted assembly. Just note that there is no equivalent for GCC. */ #define AFL_MAIN #include "config.h" #include "types.h" #include "debug.h" #include "alloc-inl.h" #include "afl-as.h" #include #include #include #include #include #include #include #include #include static u8** as_params; /* Parameters passed to the real 'as' */ static u8* input_file; /* Originally specified input file */ static u8* modified_file; /* Instrumented file for the real 'as' */ static u8 be_quiet, /* Quiet mode (no stderr output) */ clang_mode, /* Running in clang mode? */ pass_thru, /* Just pass data through? */ just_version, /* Just show version? */ sanitizer; /* Using ASAN / MSAN */ static u32 inst_ratio = 100, /* Instrumentation probability (%) */ as_par_cnt = 1; /* Number of params to 'as' */ /* If we don't find --32 or --64 in the command line, default to instrumentation for whichever mode we were compiled with. This is not perfect, but should do the trick for almost all use cases. */ #ifdef WORD_SIZE_64 static u8 use_64bit = 1; #else static u8 use_64bit = 0; #ifdef __APPLE__ # error "Sorry, 32-bit Apple platforms are not supported." #endif /* __APPLE__ */ #endif /* ^WORD_SIZE_64 */ /* Examine and modify parameters to pass to 'as'. Note that the file name is always the last parameter passed by GCC, so we exploit this property to keep the code simple. */ static void edit_params(int argc, char** argv) { u8 *tmp_dir = getenv("TMPDIR"), *afl_as = getenv("AFL_AS"); u32 i; #ifdef __APPLE__ u8 use_clang_as = 0; /* On MacOS X, the Xcode cctool 'as' driver is a bit stale and does not work with the code generated by newer versions of clang that are hand-built by the user. See the thread here: http://goo.gl/HBWDtn. To work around this, when using clang and running without AFL_AS specified, we will actually call 'clang -c' instead of 'as -q' to compile the assembly file. The tools aren't cmdline-compatible, but at least for now, we can seemingly get away with this by making only very minor tweaks. Thanks to Nico Weber for the idea. */ if (clang_mode && !afl_as) { use_clang_as = 1; afl_as = getenv("AFL_CC"); if (!afl_as) afl_as = getenv("AFL_CXX"); if (!afl_as) afl_as = "clang"; } #endif /* __APPLE__ */ /* Although this is not documented, GCC also uses TEMP and TMP when TMPDIR is not set. We need to check these non-standard variables to properly handle the pass_thru logic later on. */ if (!tmp_dir) tmp_dir = getenv("TEMP"); if (!tmp_dir) tmp_dir = getenv("TMP"); if (!tmp_dir) tmp_dir = "/tmp"; as_params = ck_alloc((argc + 32) * sizeof(u8*)); as_params[0] = afl_as ? afl_as : (u8*)"as"; as_params[argc] = 0; for (i = 1; i < argc - 1; i++) { if (!strcmp(argv[i], "--64")) use_64bit = 1; else if (!strcmp(argv[i], "--32")) use_64bit = 0; #ifdef __APPLE__ /* The Apple case is a bit different... */ if (!strcmp(argv[i], "-arch") && i + 1 < argc) { if (!strcmp(argv[i + 1], "x86_64")) use_64bit = 1; else if (!strcmp(argv[i + 1], "i386")) FATAL("Sorry, 32-bit Apple platforms are not supported."); } /* Strip options that set the preference for a particular upstream assembler in Xcode. */ if (clang_mode && (!strcmp(argv[i], "-q") || !strcmp(argv[i], "-Q"))) continue; #endif /* __APPLE__ */ as_params[as_par_cnt++] = argv[i]; } #ifdef __APPLE__ /* When calling clang as the upstream assembler, append -c -x assembler and hope for the best. */ if (use_clang_as) { as_params[as_par_cnt++] = "-c"; as_params[as_par_cnt++] = "-x"; as_params[as_par_cnt++] = "assembler"; } #endif /* __APPLE__ */ input_file = argv[argc - 1]; if (input_file[0] == '-') { if (!strcmp(input_file + 1, "-version")) { just_version = 1; modified_file = input_file; goto wrap_things_up; } if (input_file[1]) FATAL("Incorrect use (not called through afl-gcc?)"); else input_file = NULL; } else { /* Check if this looks like a standard invocation as a part of an attempt to compile a program, rather than using gcc on an ad-hoc .s file in a format we may not understand. This works around an issue compiling NSS. */ if (strncmp(input_file, tmp_dir, strlen(tmp_dir)) && strncmp(input_file, "/var/tmp/", 9) && strncmp(input_file, "/tmp/", 5)) pass_thru = 1; } modified_file = alloc_printf("%s/.afl-%u-%u.s", tmp_dir, getpid(), (u32)time(NULL)); wrap_things_up: as_params[as_par_cnt++] = modified_file; as_params[as_par_cnt] = NULL; } /* Process input file, generate modified_file. Insert instrumentation in all the appropriate places. */ static void add_instrumentation(void) { static u8 line[MAX_LINE]; FILE* inf; FILE* outf; s32 outfd; u32 ins_lines = 0; u8 instr_ok = 0, skip_csect = 0, skip_next_label = 0, skip_intel = 0, skip_app = 0, instrument_next = 0; #ifdef __APPLE__ u8* colon_pos; #endif /* __APPLE__ */ if (input_file) { inf = fopen(input_file, "r"); if (!inf) PFATAL("Unable to read '%s'", input_file); } else inf = stdin; outfd = open(modified_file, O_WRONLY | O_EXCL | O_CREAT, 0600); if (outfd < 0) PFATAL("Unable to write to '%s'", modified_file); outf = fdopen(outfd, "w"); if (!outf) PFATAL("fdopen() failed"); while (fgets(line, MAX_LINE, inf)) { /* In some cases, we want to defer writing the instrumentation trampoline until after all the labels, macros, comments, etc. If we're in this mode, and if the line starts with a tab followed by a character, dump the trampoline now. */ if (!pass_thru && !skip_intel && !skip_app && !skip_csect && instr_ok && instrument_next && line[0] == '\t' && isalpha(line[1])) { fprintf(outf, use_64bit ? trampoline_fmt_64 : trampoline_fmt_32, R(MAP_SIZE)); instrument_next = 0; ins_lines++; } /* Output the actual line, call it a day in pass-thru mode. */ fputs(line, outf); if (pass_thru) continue; /* All right, this is where the actual fun begins. For one, we only want to instrument the .text section. So, let's keep track of that in processed files - and let's set instr_ok accordingly. */ if (line[0] == '\t' && line[1] == '.') { /* OpenBSD puts jump tables directly inline with the code, which is a bit annoying. They use a specific format of p2align directives around them, so we use that as a signal. */ if (!clang_mode && instr_ok && !strncmp(line + 2, "p2align ", 8) && isdigit(line[10]) && line[11] == '\n') skip_next_label = 1; if (!strncmp(line + 2, "text\n", 5) || !strncmp(line + 2, "section\t.text", 13) || !strncmp(line + 2, "section\t__TEXT,__text", 21) || !strncmp(line + 2, "section __TEXT,__text", 21)) { instr_ok = 1; continue; } if (!strncmp(line + 2, "section\t", 8) || !strncmp(line + 2, "section ", 8) || !strncmp(line + 2, "bss\n", 4) || !strncmp(line + 2, "data\n", 5)) { instr_ok = 0; continue; } } /* Detect off-flavor assembly (rare, happens in gdb). When this is encountered, we set skip_csect until the opposite directive is seen, and we do not instrument. */ if (strstr(line, ".code")) { if (strstr(line, ".code32")) skip_csect = use_64bit; if (strstr(line, ".code64")) skip_csect = !use_64bit; } /* Detect syntax changes, as could happen with hand-written assembly. Skip Intel blocks, resume instrumentation when back to AT&T. */ if (strstr(line, ".intel_syntax")) skip_intel = 1; if (strstr(line, ".att_syntax")) skip_intel = 0; /* Detect and skip ad-hoc __asm__ blocks, likewise skipping them. */ if (line[0] == '#' || line[1] == '#') { if (strstr(line, "#APP")) skip_app = 1; if (strstr(line, "#NO_APP")) skip_app = 0; } /* If we're in the right mood for instrumenting, check for function names or conditional labels. This is a bit messy, but in essence, we want to catch: ^main: - function entry point (always instrumented) ^.L0: - GCC branch label ^.LBB0_0: - clang branch label (but only in clang mode) ^\tjnz foo - conditional branches ...but not: ^# BB#0: - clang comments ^ # BB#0: - ditto ^.Ltmp0: - clang non-branch labels ^.LC0 - GCC non-branch labels ^.LBB0_0: - ditto (when in GCC mode) ^\tjmp foo - non-conditional jumps Additionally, clang and GCC on MacOS X follow a different convention with no leading dots on labels, hence the weird maze of #ifdefs later on. */ if (skip_intel || skip_app || skip_csect || !instr_ok || line[0] == '#' || line[0] == ' ') continue; /* Conditional branch instruction (jnz, etc). We append the instrumentation right after the branch (to instrument the not-taken path) and at the branch destination label (handled later on). */ if (line[0] == '\t') { if (line[1] == 'j' && line[2] != 'm' && R(100) < inst_ratio) { fprintf(outf, use_64bit ? trampoline_fmt_64 : trampoline_fmt_32, R(MAP_SIZE)); ins_lines++; } continue; } /* Label of some sort. This may be a branch destination, but we need to tread carefully and account for several different formatting conventions. */ #ifdef __APPLE__ /* Apple: L: */ if ((colon_pos = strstr(line, ":"))) { if (line[0] == 'L' && isdigit(*(colon_pos - 1))) { #else /* Everybody else: .L: */ if (strstr(line, ":")) { if (line[0] == '.') { #endif /* __APPLE__ */ /* .L0: or LBB0_0: style jump destination */ #ifdef __APPLE__ /* Apple: L / LBB */ if ((isdigit(line[1]) || (clang_mode && !strncmp(line, "LBB", 3))) && R(100) < inst_ratio) { #else /* Apple: .L / .LBB */ if ((isdigit(line[2]) || (clang_mode && !strncmp(line + 1, "LBB", 3))) && R(100) < inst_ratio) { #endif /* __APPLE__ */ /* An optimization is possible here by adding the code only if the label is mentioned in the code in contexts other than call / jmp. That said, this complicates the code by requiring two-pass processing (messy with stdin), and results in a speed gain typically under 10%, because compilers are generally pretty good about not generating spurious intra-function jumps. We use deferred output chiefly to avoid disrupting .Lfunc_begin0-style exception handling calculations (a problem on MacOS X). */ if (!skip_next_label) instrument_next = 1; else skip_next_label = 0; } } else { /* Function label (always instrumented, deferred mode). */ instrument_next = 1; } } } if (ins_lines) fputs(use_64bit ? main_payload_64 : main_payload_32, outf); if (input_file) fclose(inf); fclose(outf); if (!be_quiet) { if (!ins_lines) WARNF("No instrumentation targets found%s.", pass_thru ? " (pass-thru mode)" : ""); else OKF("Instrumented %u locations (%s-bit, %s mode, ratio %u%%).", ins_lines, use_64bit ? "64" : "32", getenv("AFL_HARDEN") ? "hardened" : (sanitizer ? "ASAN/MSAN" : "non-hardened"), inst_ratio); } } /* Main entry point */ int main(int argc, char** argv) { s32 pid; u32 rand_seed; int status; u8* inst_ratio_str = getenv("AFL_INST_RATIO"); struct timeval tv; struct timezone tz; clang_mode = !!getenv(CLANG_ENV_VAR); if (isatty(2) && !getenv("AFL_QUIET")) { SAYF(cCYA "afl-as " cBRI VERSION cRST " by \n"); } else be_quiet = 1; if (argc < 2) { SAYF("\n" "This is a helper application for afl-fuzz. It is a wrapper around GNU 'as',\n" "executed by the toolchain whenever using afl-gcc or afl-clang. You probably\n" "don't want to run this program directly.\n\n" "Rarely, when dealing with extremely complex projects, it may be advisable to\n" "set AFL_INST_RATIO to a value less than 100 in order to reduce the odds of\n" "instrumenting every discovered branch.\n\n"); exit(1); } gettimeofday(&tv, &tz); rand_seed = tv.tv_sec ^ tv.tv_usec ^ getpid(); srandom(rand_seed); edit_params(argc, argv); if (inst_ratio_str) { if (sscanf(inst_ratio_str, "%u", &inst_ratio) != 1 || inst_ratio > 100) FATAL("Bad value of AFL_INST_RATIO (must be between 0 and 100)"); } if (getenv(AS_LOOP_ENV_VAR)) FATAL("Endless loop when calling 'as' (remove '.' from your PATH)"); setenv(AS_LOOP_ENV_VAR, "1", 1); /* When compiling with ASAN, we don't have a particularly elegant way to skip ASAN-specific branches. But we can probabilistically compensate for that... */ if (getenv("AFL_USE_ASAN") || getenv("AFL_USE_MSAN")) { sanitizer = 1; inst_ratio /= 3; } if (!just_version) add_instrumentation(); if (!(pid = fork())) { execvp(as_params[0], (char**)as_params); FATAL("Oops, failed to execute '%s' - check your PATH", as_params[0]); } if (pid < 0) PFATAL("fork() failed"); if (waitpid(pid, &status, 0) <= 0) PFATAL("waitpid() failed"); if (!getenv("AFL_KEEP_ASSEMBLY")) unlink(modified_file); exit(WEXITSTATUS(status)); }