[PATCH v3 2/4] fuzz: add an instrumentation filter

[Alexander Bulekov <alxndr@bu.edu>]

By default, -fsanitize=fuzzer instruments all code with coverage
information. However, this means that libfuzzer will track coverage over
hundreds of source files that are unrelated to virtual-devices. This
means that libfuzzer will optimize inputs for coverage observed in timer
code, memory APIs etc. This slows down the fuzzer and stores many inputs
that are not relevant to the actual virtual-devices.

With this change, clang versions that support the
"-fsanitize-coverage-allowlist" will only instrument a subset of the
compiled code, that is directly related to virtual-devices.

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
---
 configure                               | 11 +++++++++++
 scripts/oss-fuzz/instrumentation-filter | 14 ++++++++++++++
 2 files changed, 25 insertions(+)
 create mode 100644 scripts/oss-fuzz/instrumentation-filter

diff --git a/configure b/configure
index debd50c085..db8bee939a 100755
--- a/configure
+++ b/configure
@@ -5176,6 +5176,11 @@ if test "$fuzzing" = "yes" && test -z "${LIB_FUZZING_ENGINE+xxx}"; then
     error_exit "Your compiler doesn't support -fsanitize=fuzzer"
     exit 1
   fi
+  have_clang_coverage_filter=no
+  echo > $TMPTXT
+  if compile_prog "$CPU_CFLAGS -Werror -fsanitize=fuzzer -fsanitize-coverage-allowlist=$TMPTXT" ""; then
+      have_clang_coverage_filter=yes
+  fi
 fi
 
 # Thread sanitizer is, for now, much noisier than the other sanitizers;
@@ -6101,6 +6106,12 @@ if test "$fuzzing" = "yes" ; then
     # rule for the fuzzer adds these to the link_args. They need to be
     # configurable, to support OSS-Fuzz
     FUZZ_EXE_LDFLAGS="-fsanitize=fuzzer"
+
+    # Specify a filter to only instrument code that is directly related to
+    # virtual-devices.
+    if test "$have_clang_coverage_filter" = "yes" ; then
+        QEMU_CFLAGS="$QEMU_CFLAGS -fsanitize-coverage-allowlist=$source_path/scripts/oss-fuzz/instrumentation-filter"
+    fi
   else
     FUZZ_EXE_LDFLAGS="$LIB_FUZZING_ENGINE"
   fi
diff --git a/scripts/oss-fuzz/instrumentation-filter b/scripts/oss-fuzz/instrumentation-filter
new file mode 100644
index 0000000000..44e853159c
--- /dev/null
+++ b/scripts/oss-fuzz/instrumentation-filter
@@ -0,0 +1,14 @@
+# Code that we actually want the fuzzer to target
+# See: https://clang.llvm.org/docs/SanitizerCoverage.html#disabling-instrumentation-without-source-modification
+#
+src:*/hw/*
+src:*/include/hw/*
+src:*/slirp/*
+
+# We don't care about coverage over fuzzer-specific code, however we should
+# instrument the fuzzer entry-point so libFuzzer always sees at least some
+# coverage - otherwise it will exit after the first input
+src:*/tests/qtest/fuzz/fuzz.c
+
+# Enable instrumentation for all functions in those files
+fun:*
-- 
2.28.0