[PATCH v2 2/4] fuzz: add an instrumentation filter

[Alexander Bulekov]

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 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                               |  4 ++++
 scripts/oss-fuzz/instrumentation-filter | 14 ++++++++++++++
 2 files changed, 18 insertions(+)
 create mode 100644 scripts/oss-fuzz/instrumentation-filter

diff --git a/configure b/configure
index debd50c085..40412bcfcf 100755
--- a/configure
+++ b/configure
@@ -6089,6 +6089,10 @@ if test "$fuzzing" = "yes" ; then
   # If LIB_FUZZING_ENGINE is set, assume we are running on OSS-Fuzz, and the
   # needed CFLAGS have already been provided
   if test -z "${LIB_FUZZING_ENGINE+xxx}" ; then
+    # Specify a filter to only instrument code that is directly related to
+    # virtual-devices.
+       QEMU_CFLAGS="$QEMU_CFLAGS 
-fsanitize-coverage-allowlist=$source_path/scripts/oss-fuzz/instrumentation-filter"
+
     # Add CFLAGS to tell clang to add fuzzer-related instrumentation to all the
     # compiled code.
     QEMU_CFLAGS="$QEMU_CFLAGS -fsanitize=fuzzer-no-link"
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