[libFuzzer] Experimental data flow tracer for fuzz targets.

[kcc]

Summary

Experimental data flow tracer for fuzz targets.
Allows to tell which bytes of the input affect which functions of the fuzz target.

We previously attempted to use DFSan directly in the libFuzzer process,
and that didn't work nicely.
Now we will try to collect the data flow information for the seed corpus
in a separate process (using this tracer), and then use it in the regular libFuzzer runs.

[Dor1s]

LGTM! Left some questions though, mostly for my own education, I guess 😛

lib/fuzzer/dataflow/DataFlow.cpp

73

nit: in other places these use CamelCase: Data, Size

159

nit: do we need trailing \n as on line 158?

160

I don't understand *start condition here. As per lines 161-162, *start would be 0, i.e. false when already initialized? Do we need !*start here?

179

probably a stupid question, but are we sure than __sanitizer_cov_trace_pc_guard gets called before any other hook? Otherwise, how can we be sure that CurrentFunc has a correct value when e.g. __dfsw___sanitizer_cov_trace_switch or __dfsw___sanitizer_cov_trace_* is executed?

189

just to confirm: the hooks defined below will get called by the runtime, when a particular comparison type is executed?

test/fuzzer/dataflow.test

27

I guess order of functions can differ, this is why we use a regexp rather than a particular function number?

[kcc]

lib/fuzzer/dataflow/DataFlow.cpp

159

doesn't matter, but I unified these.

160

Good point. This is a copy-pasto from the docs where the indexes start from 1, not from 0.
And I think They actually should start from 1 to avoid double-initializtion.
Good catch! Fixed.

179

Yes. __sanitizer_cov_trace_pc_guard is inserted into the beginning of a function entry block.

189

The hooks are inserted by the sanitizer coverage (and then modified by dfsan) -- they will be called before every instrumented comparison insn.

test/fuzzer/dataflow.test

27

Yes, the order of functions is undefined.
Note that the output doesn't contain the function names (it would be too expensive) -- it's just 'F' followed by a number.
See the comment at the top of DataFlow.cpp

[kcc]

address comments

[morehouse]

lib/fuzzer/dataflow/DataFlow.cpp

59

Nit: Why mix // comment style with /* ... */?

162

I think a simple counter variable would be clearer here.

183

Does DFSan require a second label here since there's 2 parameters in the original signature?

test/fuzzer/dataflow.test

28

IN_ABC-NOT?

[kcc]

lib/fuzzer/dataflow/DataFlow.cpp

59

This is actually a /* comment, see the first line.
Agree, with // it looks nicer, fixed.

183

The second label will be inserted, but I don't need it, since Cases is a constant.
Added a parameter called UnusedL just for clarity.

[kcc]

address more comments.

lib/fuzzer/dataflow/DataFlow.cpp

+/*===- DataFlow.cpp - a standalone DataFlow tracer                  -------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// An experimental data-flow tracer for fuzz targets.
+// It is based on DFSan and SanitizerCoverage.
+// https://clang.llvm.org/docs/DataFlowSanitizer.html
+// https://clang.llvm.org/docs/SanitizerCoverage.html#tracing-data-flow
+//
+// It executes the fuzz target on the given input while monitoring the
+// data flow for every instrumented comparison instruction.
+//
+// The output shows which functions depend on which bytes of the input.
+//
+// Build:
+//   1. Compile this file with -fsanitize=dataflow
+//   2. Build the fuzz target with -g -fsanitize=dataflow
+//       -fsanitize-coverage=trace-pc-guard,pc-table,func,trace-cmp
+//   3. Link those together with -fsanitize=dataflow
+//
+//  -fsanitize-coverage=trace-cmp inserts callbacks around every comparison
+//  instruction, DFSan modifies the calls to pass the data flow labels.
+//  The callbacks update the data flow label for the current function.
+//  See e.g. __dfsw___sanitizer_cov_trace_cmp1 below.
+//
+//  -fsanitize-coverage=trace-pc-guard,pc-table,func instruments function
+//  entries so that the comparison callback knows that current function.
+//
+//
+// Run:
+//   # Collect data flow for INPUT_FILE, write to OUTPUT_FILE (default: stdout)
+//   ./a.out INPUT_FILE [OUTPUT_FILE]
+//
+//   # Print all instrumented functions. llvm-symbolizer must be present in PATH
+//   ./a.out
+//
+// Example output:
+// ===============
+LEN:    5
+LABELS: 10
+L7 1 6
+L8 2 7
+L9 3 8
+L10 4 9
+F1 10
+F2 5
+// ===============
+"LEN:" indicates the number of bytes in the input.
+"LABELS:" indicates the number of DFSan labels created while running the input.
+  * The labels [1,LEN] correspond to the bytes of the input
+    (label 1 corresponds to byte 0, and so on)
+  * The label LEN+1 corresponds to the input size.
+  * The labels [LEN+2,LABELS] correspond to DFSan's union labels.
+"Li j k": describes the label 'i' as a union of labels 'j' and 'k'.
+"Ff l": tells that the function 'f' depends on the label 'l'.
+//===----------------------------------------------------------------------===*/
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <string.h>
+
+#include <execinfo.h>  // backtrace_symbols_fd
+
+#include <sanitizer/dfsan_interface.h>
+
+extern "C" {
+extern int LLVMFuzzerTestOneInput(const unsigned char *data, size_t size);
+__attribute__((weak)) extern int LLVMFuzzerInitialize(int *argc, char ***argv);
+} // extern "C"
+
+static size_t InputLen;
+static size_t NumFuncs;
+static const uintptr_t *FuncsBeg;
+static __thread size_t CurrentFunc;
+static dfsan_label *FuncLabels;  // Array of NumFuncs elements.
+
+// Prints all instrumented functions.
+int PrintFunctions() {
+  // We don't have the symbolizer integrated with dfsan yet.
+  // So use backtrace_symbols_fd and pipe it through llvm-symbolizer.
+  // TODO(kcc): this is pretty ugly and may break in lots of ways.
+  //      We'll need to make a proper in-process symbolizer work with DFSan.
+  FILE *Pipe = popen("sed 's/(+/ /g; s/).*//g' "
+                     "| llvm-symbolizer "
+                     "| grep 'dfs\\$' "
+                     "| sed 's/dfs\\$//g'", "w");
+  for (size_t I = 0; I < NumFuncs; I++) {
+    uintptr_t PC = FuncsBeg[I * 2];
+    void *const Buf[1] = {(void*)PC};
+    backtrace_symbols_fd(Buf, 1, fileno(Pipe));
+  }
+  pclose(Pipe);
+  return 0;
+}
+
+void PrintDataFlow(FILE *Out) {
+  fprintf(Out, "LEN:    %zd\n", InputLen);
+  fprintf(Out, "LABELS: %zd\n", dfsan_get_label_count());
+  for (dfsan_label L = InputLen + 2; L <= dfsan_get_label_count(); L++) {
+    auto *DLI = dfsan_get_label_info(L);
+    fprintf(Out, "L%d %d %d\n", L, DLI->l1, DLI->l2);
+  }
+  for (size_t I = 0; I < NumFuncs; I++)
+    if (FuncLabels[I])
+      fprintf(Out, "F%zd %d\n", I, FuncLabels[I]);
+}
+
+int main(int argc, char **argv) {
+  if (LLVMFuzzerInitialize)
+    LLVMFuzzerInitialize(&argc, &argv);
+  if (argc == 1)
+    return PrintFunctions();
+  assert(argc == 2 || argc == 3);
+
+  const char *Input = argv[1];
+  fprintf(stderr, "INFO: reading '%s'\n", Input);
+  FILE *In = fopen(Input, "r");
+  assert(In);
+  fseek(In, 0, SEEK_END);
+  InputLen = ftell(In);
+  fseek(In, 0, SEEK_SET);
+  unsigned char *Buf = (unsigned char*)malloc(InputLen);
+  size_t NumBytesRead = fread(Buf, 1, InputLen, In);
+  assert(NumBytesRead == InputLen);
+  fclose(In);
+
+  fprintf(stderr, "INFO: running '%s'\n", Input);
+  for (size_t I = 1; I <= InputLen; I++) {
+    dfsan_label L = dfsan_create_label("", nullptr);
+    assert(L == I);
+    dfsan_set_label(L, Buf + I - 1, 1);
+  }
+  dfsan_label SizeL = dfsan_create_label("", nullptr);
+  assert(SizeL == InputLen + 1);
+  dfsan_set_label(SizeL, &InputLen, sizeof(InputLen));
+
+  LLVMFuzzerTestOneInput(Buf, InputLen);
+  free(Buf);
+
+  bool OutIsStdout = argc == 2;
+  fprintf(stderr, "INFO: writing dataflow to %s\n",
+          OutIsStdout ? "<stdout>" : argv[2]);
+  FILE *Out = OutIsStdout ? stdout : fopen(argv[2], "w");
+  PrintDataFlow(Out);
+  if (!OutIsStdout) fclose(Out);
+}
+
+extern "C" {
+
+void __sanitizer_cov_trace_pc_guard_init(uint32_t *start,
+                                         uint32_t *stop) {
+  assert(NumFuncs == 0 && "This tool does not support DSOs\n");
+  assert(start < stop && "The code is not instrumented for coverage");
+  if (start == stop || *start) return;  // Initialize only once.
+  for (uint32_t *x = start; x < stop; x++)
+    *x = x - start;
+  NumFuncs += stop - start;
+  FuncLabels = (dfsan_label*)calloc(NumFuncs, sizeof(dfsan_label));
+  fprintf(stderr, "INFO: %zd instrumented function(s) observed\n", NumFuncs);
+}
+
+void __sanitizer_cov_pcs_init(const uintptr_t *pcs_beg,
+                              const uintptr_t *pcs_end) {
+  assert(NumFuncs == (pcs_end - pcs_beg) / 2);
+  FuncsBeg = pcs_beg;
+}
+
+void __sanitizer_cov_trace_pc_indir(uint64_t x){}  // unused.
+
+void __sanitizer_cov_trace_pc_guard(uint32_t *guard){
+  uint32_t FuncNum = *guard;
+  assert(FuncNum < NumFuncs);
+  CurrentFunc = FuncNum;
+}
+
+void __dfsw___sanitizer_cov_trace_switch(uint64_t Val, uint64_t *Cases,
+                                         dfsan_label L1) {
+  assert(CurrentFunc < NumFuncs);
+  FuncLabels[CurrentFunc] = dfsan_union(FuncLabels[CurrentFunc], L1);
+}
+
+#define HOOK(Name, Type)                                                       \
+  void Name(Type Arg1, Type Arg2, dfsan_label L1, dfsan_label L2) {            \
+    assert(CurrentFunc < NumFuncs);                                            \
+    FuncLabels[CurrentFunc] =                                                  \
+        dfsan_union(FuncLabels[CurrentFunc], dfsan_union(L1, L2));             \
+  }
+
+HOOK(__dfsw___sanitizer_cov_trace_const_cmp1, uint8_t)
+HOOK(__dfsw___sanitizer_cov_trace_const_cmp2, uint16_t)
+HOOK(__dfsw___sanitizer_cov_trace_const_cmp4, uint32_t)
+HOOK(__dfsw___sanitizer_cov_trace_const_cmp8, uint64_t)
+HOOK(__dfsw___sanitizer_cov_trace_cmp1, uint8_t)
+HOOK(__dfsw___sanitizer_cov_trace_cmp2, uint16_t)
+HOOK(__dfsw___sanitizer_cov_trace_cmp4, uint32_t)
+HOOK(__dfsw___sanitizer_cov_trace_cmp8, uint64_t)
+
+} // extern "C"

test/fuzzer/ThreeFunctionsTest.cpp

+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+
+// Find "FUZZME", the target has 3 different functions.
+#include <assert.h>
+#include <cstddef>
+#include <cstdint>
+#include <cstdlib>
+#include <cstdio>
+
+__attribute__((noinline))
+static bool Func1(const uint8_t *Data, size_t Size) {
+  // assumes Size >= 5, doesn't check it.
+  return Data[4] == 'M';
+}
+
+__attribute__((noinline))
+bool Func2(const uint8_t *Data, size_t Size) {
+  return Size >= 6 && Data[5] == 'E';
+}
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
+  if (Size >= 5
+      && Data[0] == 'F'
+      && Data[1] == 'U'
+      && Data[2] == 'Z'
+      && Data[3] == 'Z'
+      && Func1(Data, Size)
+      && Func2(Data, Size)) {
+        fprintf(stderr, "BINGO\n");
+        abort();
+  }
+  return 0;
+}

test/fuzzer/dataflow.test

+# Tests the data flow tracer.
+REQUIRES: linux
+
+# Build the tracer and the test.
+RUN: %no_fuzzer_cpp_compiler -c -fno-sanitize=all -fsanitize=dataflow -fsanitize-coverage=trace-pc-guard,pc-table,func,trace-cmp   %S/ThreeFunctionsTest.cpp -o %t-ThreeFunctionsTest.o
+RUN: %no_fuzzer_cpp_compiler    -fno-sanitize=all -fsanitize=dataflow  %t-ThreeFunctionsTest.o %S/../../lib/fuzzer/dataflow/DataFlow.cpp -o  %t-ThreeFunctionsTestDF
+
+# Dump the function list.
+RUN:  %t-ThreeFunctionsTestDF 2>&1 | FileCheck %s --check-prefix=FUNC_LIST
+FUNC_LIST-DAG: LLVMFuzzerTestOneInput
+FUNC_LIST-DAG: Func1
+FUNC_LIST-DAG: Func2
+
+# Prepare the inputs.
+RUN: rm -rf %t/IN
+RUN: mkdir -p %t/IN
+RUN: echo -n ABC    > %t/IN/ABC
+RUN: echo -n FUABC  > %t/IN/FUABC
+RUN: echo -n FUZZR  > %t/IN/FUZZR
+RUN: echo -n FUZZM  > %t/IN/FUZZM
+RUN: echo -n FUZZMU > %t/IN/FUZZMU
+
+# ABC: No data is used, the only used label is 4 (corresponds to the size)
+RUN:%t-ThreeFunctionsTestDF %t/IN/ABC    | FileCheck %s --check-prefix=IN_ABC
+IN_ABC: LEN: 3
+IN_ABC: LABELS: 4
+IN_ABC: F{{[012]}} 4
+IN_ABC-NO: F
+
+# FUABC: First 3 bytes are checked, Func1/Func2 are not called.
+RUN:%t-ThreeFunctionsTestDF %t/IN/FUABC  | FileCheck %s --check-prefix=IN_FUABC
+IN_FUABC: LEN: 5
+IN_FUABC: LABELS:
+IN_FUABC: L{{.*}} 1
+IN_FUABC: L{{.*}} 2
+IN_FUABC: L{{.*}} 3
+IN_FUABC-NOT: L{{.*}} 4
+IN_FUABC: F{{[012]}}
+IN_FUABC-NOT: F
+
+# FUZZR: 5 bytes are used (4 in one function, 5-th in the other), Func2 is not called.
+RUN:%t-ThreeFunctionsTestDF %t/IN/FUZZR  | FileCheck %s --check-prefix=IN_FUZZR
+IN_FUZZR: LEN: 5
+IN_FUZZR: LABELS:
+IN_FUZZR: L{{.*}} 1
+IN_FUZZR: L{{.*}} 2
+IN_FUZZR: L{{.*}} 3
+IN_FUZZR: L[[L0:[0-9]*]] 4
+IN_FUZZR-DAG: F{{[012]}} 5
+IN_FUZZR-DAG: F{{[012]}} [[L0]]
+IN_FUZZR-NOT: F
+
+# FUZZM: 5 bytes are used, both Func1 and Func2 are called, Func2 depends only on size (label 6).
+RUN:%t-ThreeFunctionsTestDF %t/IN/FUZZM  | FileCheck %s --check-prefix=IN_FUZZM
+IN_FUZZM: LEN: 5
+IN_FUZZM: LABELS:
+IN_FUZZM: L{{.*}} 1
+IN_FUZZM: L{{.*}} 2
+IN_FUZZM: L{{.*}} 3
+IN_FUZZM: L{{.*}} 4
+IN_FUZZM-DAG: F{{[012]}} 6
+IN_FUZZM-DAG: F{{[012]}} 5
+IN_FUZZM-DAG: F
+
+# FUZZMU: 6 bytes are used, both Func1 and Func2 are called, Func2 depends on byte 6 and size (label 7)
+RUN:%t-ThreeFunctionsTestDF %t/IN/FUZZMU  | FileCheck %s --check-prefix=IN_FUZZMU
+IN_FUZZMU: LEN: 6
+IN_FUZZMU: LABELS:
+IN_FUZZMU: L{{.*}} 1
+IN_FUZZMU: L{{.*}} 2
+IN_FUZZMU: L{{.*}} 3
+IN_FUZZMU: L{{.*}} 4
+IN_FUZZMU: L[[L2:[0-9]*]] 6 7
+IN_FUZZMU-DAG: F{{[012]}} 5
+IN_FUZZMU-DAG: F{{[012]}} [[L2]]
+IN_FUZZMU-DAG: F