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[PULL 18/31] scripts/oss-fuzz: Add script to reorder a generic-fuzzer tr

From: Thomas Huth
Subject: [PULL 18/31] scripts/oss-fuzz: Add script to reorder a generic-fuzzer trace
Date: Mon, 26 Oct 2020 11:06:19 +0100

From: Alexander Bulekov <alxndr@bu.edu>

The generic-fuzzer uses hooks to fulfill DMA requests just-in-time.
This means that if we try to use QTEST_LOG=1 to build a reproducer, the
DMA writes will be logged _after_ the in/out/read/write that triggered
the DMA read. To work work around this, the generic-fuzzer annotates
these just-in time DMA fulfilments with a tag that we can use to
discern them. This script simply iterates over a raw qtest
trace (including log messages, errors, timestamps etc), filters it and
re-orders it so that DMA fulfillments are placed directly _before_ the
qtest command that will cause the DMA access.

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Message-Id: <20201023150746.107063-11-alxndr@bu.edu>
Signed-off-by: Thomas Huth <thuth@redhat.com>
 .../oss-fuzz/reorder_fuzzer_qtest_trace.py    | 103 ++++++++++++++++++
 1 file changed, 103 insertions(+)
 create mode 100755 scripts/oss-fuzz/reorder_fuzzer_qtest_trace.py

diff --git a/scripts/oss-fuzz/reorder_fuzzer_qtest_trace.py 
new file mode 100755
index 0000000000..890e1def85
--- /dev/null
+++ b/scripts/oss-fuzz/reorder_fuzzer_qtest_trace.py
@@ -0,0 +1,103 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+Use this to convert qtest log info from a generic fuzzer input into a qtest
+trace that you can feed into a standard qemu-system process. Example usage:
+QEMU_FUZZ_ARGS="-machine q35,accel=qtest" QEMU_FUZZ_OBJECTS="*" \
+        ./i386-softmmu/qemu-fuzz-i386 --fuzz-target=generic-pci-fuzz
+# .. Finds some crash
+QEMU_FUZZ_ARGS="-machine q35,accel=qtest" QEMU_FUZZ_OBJECTS="*" \
+        ./i386-softmmu/qemu-fuzz-i386 --fuzz-target=generic-pci-fuzz
+        /path/to/crash 2> qtest_log_output
+scripts/oss-fuzz/reorder_fuzzer_qtest_trace.py qtest_log_output > qtest_trace
+./i386-softmmu/qemu-fuzz-i386 -machine q35,accel=qtest \
+        -qtest stdin < qtest_trace
+### Details ###
+Some fuzzer make use of hooks that allow us to populate some memory range, just
+before a DMA read from that range. This means that the fuzzer can produce
+activity that looks like:
+    [start] read from mmio addr
+    [end]   read from mmio addr
+    [start] write to pio addr
+        [start] fill a DMA buffer just in time
+        [end]   fill a DMA buffer just in time
+        [start] fill a DMA buffer just in time
+        [end]   fill a DMA buffer just in time
+    [end]   write to pio addr
+    [start] read from mmio addr
+    [end]   read from mmio addr
+We annotate these "nested" DMA writes, so with QTEST_LOG=1 the QTest trace
+might look something like:
+[R +0.028431] readw 0x10000
+[R +0.028434] outl 0xc000 0xbeef  # Triggers a DMA read from 0xbeef and 0xbf00
+[DMA][R +0.034639] write 0xbeef 0x2 0xAAAA
+[DMA][R +0.034639] write 0xbf00 0x2 0xBBBB
+[R +0.028431] readw 0xfc000
+This script would reorder the above trace so it becomes:
+readw 0x10000
+write 0xbeef 0x2 0xAAAA
+write 0xbf00 0x2 0xBBBB
+outl 0xc000 0xbeef
+readw 0xfc000
+I.e. by the time, 0xc000 tries to read from DMA, those DMA buffers have already
+been set up, removing the need for the DMA hooks. We can simply provide this
+reordered trace via -qtest stdio to reproduce the input
+Note: this won't work for traces where the device tries to read from the same
+DMA region twice in between MMIO/PIO commands. E.g:
+    [R +0.028434] outl 0xc000 0xbeef
+    [DMA][R +0.034639] write 0xbeef 0x2 0xAAAA
+    [DMA][R +0.034639] write 0xbeef 0x2 0xBBBB
+The fuzzer will annotate suspected double-fetches with [DOUBLE-FETCH]. This
+script looks for these tags and warns the users that the resulting trace might
+not reproduce the bug.
+import sys
+__author__     = "Alexander Bulekov <alxndr@bu.edu>"
+__copyright__  = "Copyright (C) 2020, Red Hat, Inc."
+__license__    = "GPL version 2 or (at your option) any later version"
+__maintainer__ = "Alexander Bulekov"
+__email__      = "alxndr@bu.edu"
+def usage():
+    sys.exit("Usage: {} /path/to/qtest_log_output".format((sys.argv[0])))
+def main(filename):
+    with open(filename, "r") as f:
+        trace = f.readlines()
+    # Leave only lines that look like logged qtest commands
+    trace[:] = [x.strip() for x in trace if "[R +" in x
+                or "[S +" in x and "CLOSED" not in x]
+    for i in range(len(trace)):
+        if i+1 < len(trace):
+            if "[DMA]" in trace[i+1]:
+                if "[DOUBLE-FETCH]" in trace[i+1]:
+                    sys.stderr.write("Warning: Likely double fetch on line"
+                                     "{}.\n There will likely be problems "
+                                     "reproducing behavior with the "
+                                     "resulting qtest trace\n\n".format(i+1))
+                trace[i], trace[i+1] = trace[i+1], trace[i]
+    for line in trace:
+        print(line.split("]")[-1].strip())
+if __name__ == '__main__':
+    if len(sys.argv) == 1:
+        usage()
+    main(sys.argv[1])

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