[libFuzzer] Add -detect_exits flag.

[morehouse]

Summary

Allows users to disable exit reports. Motivated by
https://github.com/google/oss-fuzz/pull/1314.

[morehouse]

• Add documentation line.

[kcc]

why is this needed?

[morehouse]

In D45730#1070575, @kcc wrote:

why is this needed?

This was motivated by conversation regarding wget on OSS-Fuzz: https://github.com/google/oss-fuzz/pull/1314.

There, Tim Ruhsen mentioned he's getting exit reports and doesn't want them since wget calls exit frequently.

[kcc]

libFuzzer is in-process fuzzer, exit is not a libFuzzer-friendly thing.
I am reluctant to allow exits even under a flag -- it will discourage people from making their APIs more fuzzable.
For legacy code that nobody is willing to change we have the out-of-process AFL

[morehouse]

All right. If that's the route we want to go, I'll abandon this patch.

compiler-rt/lib/fuzzer/FuzzerDriver.cpp

int FuzzerDriver(int *argc, char ***argv, UserCallback Callback) {
   Options.HandleInt = Flags.handle_int;
   Options.HandleSegv = Flags.handle_segv;
   Options.HandleTerm = Flags.handle_term;
   Options.HandleXfsz = Flags.handle_xfsz;
   Options.HandleUsr1 = Flags.handle_usr1;
   Options.HandleUsr2 = Flags.handle_usr2;
   SetSignalHandler(Options);
 
+  if (Flags.detect_exits)
     std::atexit(Fuzzer::StaticExitCallback);
 
   if (Flags.minimize_crash)
     return MinimizeCrashInput(Args, Options);
 
   if (Flags.minimize_crash_internal_step)
     return MinimizeCrashInputInternalStep(F, Corpus);
 
   if (Flags.cleanse_crash)

compiler-rt/lib/fuzzer/FuzzerFlags.def

 FUZZER_FLAG_INT(handle_int, 1, "If 1, try to intercept SIGINT.")
 FUZZER_FLAG_INT(handle_term, 1, "If 1, try to intercept SIGTERM.")
 FUZZER_FLAG_INT(handle_xfsz, 1, "If 1, try to intercept SIGXFSZ.")
 FUZZER_FLAG_INT(handle_usr1, 1, "If 1, try to intercept SIGUSR1.")
 FUZZER_FLAG_INT(handle_usr2, 1, "If 1, try to intercept SIGUSR2.")
 FUZZER_FLAG_INT(close_fd_mask, 0, "If 1, close stdout at startup; "
     "if 2, close stderr; if 3, close both. "
     "Be careful, this will also close e.g. stderr of asan.")
+FUZZER_FLAG_INT(detect_exits, 1, "If 1, report when fuzz targets exit.")
 FUZZER_FLAG_INT(detect_leaks, 1, "If 1, and if LeakSanitizer is enabled "
     "try to detect memory leaks during fuzzing (i.e. not only at shut down).")
 FUZZER_FLAG_INT(purge_allocator_interval, 1, "Purge allocator caches and "
     "quarantines every <N> seconds. When rss_limit_mb is specified (>0), "
     "purging starts when RSS exceeds 50% of rss_limit_mb. Pass "
     "purge_allocator_interval=-1 to disable this functionality.")
 FUZZER_FLAG_INT(trace_malloc, 0, "If >= 1 will print all mallocs/frees. "
     "If >= 2 will also print stack traces.")

compiler-rt/test/fuzzer/exit-report.test

 RUN: %cpp_compiler %S/SimpleTest.cpp -o %t-SimpleTest
-RUN: not %t-SimpleTest 2>&1 | FileCheck %s
+RUN: not %t-SimpleTest -detect_exits=1 2>&1 | FileCheck %s --check-prefix=REPORT
+RUN: %t-SimpleTest -detect_exits=0 2>&1 | FileCheck %s --check-prefix=NO_REPORT
 
-CHECK: ERROR: libFuzzer: fuzz target exited
-CHECK: SUMMARY: libFuzzer: fuzz target exited
-CHECK: Test unit written to
+REPORT: ERROR: libFuzzer: fuzz target exited
+REPORT: SUMMARY: libFuzzer: fuzz target exited
+REPORT: Test unit written to
+
+NO_REPORT-NOT: ERROR: libFuzzer: fuzz target exited
+NO_REPORT-NOT: SUMMARY: libFuzzer: fuzz target exited
+NO_REPORT-NOT: Test unit written to

llvm/docs/LibFuzzer.rst

``-exact_artifact_path``
   Ignored if empty (the default).  If non-empty, write the single artifact on
   failure (crash, timeout) as ``$(exact_artifact_path)``. This overrides
   ``-artifact_prefix`` and will not use checksum in the file name. Do not use
   the same path for several parallel processes.
 ``-print_pcs``
   If 1, print out newly covered PCs. Defaults to 0.
 ``-print_final_stats``
   If 1, print statistics at exit.  Defaults to 0.
+``-detect_exits``
+  If 1, report when fuzz targets exit.  Defaults to 1.
 ``-detect_leaks``
   If 1 (default) and if LeakSanitizer is enabled
   try to detect memory leaks during fuzzing (i.e. not only at shut down).
 ``-close_fd_mask``
   Indicate output streams to close at startup. Be careful, this will
   remove diagnostic output from target code (e.g. messages on assert failure).
 
    - 0 (default): close neither ``stdout`` nor ``stderr``