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[PULL,v3,4/8] fuzz: add an instrumentation filter

Message ID 20210901115004.34768-5-alxndr@bu.edu (mailing list archive)
State New, archived
Headers show
Series [PULL,v3,1/8] fuzz: fix sparse memory access in the DMA callback | expand

Commit Message

Alexander Bulekov Sept. 1, 2021, 11:50 a.m. UTC
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>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
---
 configure                                     | 28 +++++++++++++++----
 .../oss-fuzz/instrumentation-filter-template  | 15 ++++++++++
 2 files changed, 37 insertions(+), 6 deletions(-)
 create mode 100644 scripts/oss-fuzz/instrumentation-filter-template
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Patch

diff --git a/configure b/configure
index 9a79a004d7..dcdbe3f068 100755
--- a/configure
+++ b/configure
@@ -4198,13 +4198,21 @@  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
 
@@ -4884,6 +4892,14 @@  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:*