[PATCH-for-6.1 v6] fuzz: add an instrumentation filter

* [PATCH-for-6.1 v6] fuzz: add an instrumentation filter
@ 2021-07-09 20:08 Alexander Bulekov
  2021-07-13 10:37 ` Darren Kenny
  0 siblings, 1 reply; 2+ messages in thread
From: Alexander Bulekov @ 2021-07-09 20:08 UTC (permalink / raw)
  To: qemu-devel
  Cc: Thomas Huth, Alexander Bulekov, Philippe Mathieu-Daudé,
	darren.kenny, Bandan Das, Stefan Hajnoczi, Paolo Bonzini

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>
---

Hi,
Resending this one, as this did not work with the OSS-Fuzz containers.
(The instrumentation filter support was only checked/applied when
LIB_FUZZING_ENGINE was not set). I had to shift a couple things around
in the configure script, since this was last reviewed.
Thank you

 configure                                     | 28 +++++++++++++++----
 .../oss-fuzz/instrumentation-filter-template  | 15 ++++++++++
 2 files changed, 37 insertions(+), 6 deletions(-)
 create mode 100644 scripts/oss-fuzz/instrumentation-filter-template

diff --git a/configure b/configure
index e799d908a3..99d6182af9 100755
--- a/configure
+++ b/configure
@@ -4943,13 +4943,21 @@ fi
 
 ##########################################
 # checks for fuzzer
-if test "$fuzzing" = "yes" && test -z "${LIB_FUZZING_ENGINE+xxx}"; then
+if test "$fuzzing" = "yes" ; then
   write_c_fuzzer_skeleton
-  if compile_prog "$CPU_CFLAGS -Werror -fsanitize=fuzzer" ""; then
-    have_fuzzer=yes
-  else
-    error_exit "Your compiler doesn't support -fsanitize=fuzzer"
-    exit 1
+  if test -z "${LIB_FUZZING_ENGINE+xxx}"; then
+    if compile_prog "$CPU_CFLAGS -Werror -fsanitize=fuzzer" ""; then
+      have_fuzzer=yes
+    else
+      error_exit "Your compiler doesn't support -fsanitize=fuzzer"
+      exit 1
+    fi
+  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
 
@@ -5843,6 +5851,14 @@ if test "$fuzzing" = "yes" ; then
   else
     FUZZ_EXE_LDFLAGS="$LIB_FUZZING_ENGINE"
   fi
+
+  # Specify a filter to only instrument code that is directly related to
+  # virtual-devices.
+  if test "$have_clang_coverage_filter" = "yes" ; then
+    cp "$source_path/scripts/oss-fuzz/instrumentation-filter-template" \
+       instrumentation-filter
+    QEMU_CFLAGS="$QEMU_CFLAGS -fsanitize-coverage-allowlist=instrumentation-filter"
+  fi
 fi
 
 if test "$plugins" = "yes" ; then
diff --git a/scripts/oss-fuzz/instrumentation-filter-template b/scripts/oss-fuzz/instrumentation-filter-template
new file mode 100644
index 0000000000..76d2b6139a
--- /dev/null
+++ b/scripts/oss-fuzz/instrumentation-filter-template
@@ -0,0 +1,15 @@
+# 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/*
+src:*/net/*
+
+# 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



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