scripts/oss-fuzz: Add crash trace minimization script

Once we find a crash, we can convert it into a QTest trace. Usually this
trace will contain many operations that are unneeded to reproduce the
crash. This script tries to minimize the crashing trace, by removing
operations and trimming QTest bufwrite(write addr len data...) commands.

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Message-Id: <20201023150746.107063-12-alxndr@bu.edu>
Signed-off-by: Thomas Huth <thuth@redhat.com>

scripts/oss-fuzz/minimize_qtest_trace.py

@@ -0,0 +1,157 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+
+"""
+This takes a crashing qtest trace and tries to remove superflous operations
+"""
+
+import sys
+import os
+import subprocess
+import time
+import struct
+
+QEMU_ARGS = None
+QEMU_PATH = None
+TIMEOUT = 5
+CRASH_TOKEN = None
+
+write_suffix_lookup = {"b": (1, "B"),
+                       "w": (2, "H"),
+                       "l": (4, "L"),
+                       "q": (8, "Q")}
+
+def usage():
+    sys.exit("""\
+Usage: QEMU_PATH="/path/to/qemu" QEMU_ARGS="args" {} input_trace output_trace
+By default, will try to use the second-to-last line in the output to identify
+whether the crash occred. Optionally, manually set a string that idenitifes the
+crash by setting CRASH_TOKEN=
+""".format((sys.argv[0])))
+
+def check_if_trace_crashes(trace, path):
+    global CRASH_TOKEN
+    with open(path, "w") as tracefile:
+        tracefile.write("".join(trace))
+
+    rc = subprocess.Popen("timeout -s 9 {timeout}s {qemu_path} {qemu_args} 2>&1\
+    < {trace_path}".format(timeout=TIMEOUT,
+                           qemu_path=QEMU_PATH,
+                           qemu_args=QEMU_ARGS,
+                           trace_path=path),
+                          shell=True,
+                          stdin=subprocess.PIPE,
+                          stdout=subprocess.PIPE)
+    stdo = rc.communicate()[0]
+    output = stdo.decode('unicode_escape')
+    if rc.returncode == 137:    # Timed Out
+        return False
+    if len(output.splitlines()) < 2:
+        return False
+
+    if CRASH_TOKEN is None:
+        CRASH_TOKEN = output.splitlines()[-2]
+
+    return CRASH_TOKEN in output
+
+
+def minimize_trace(inpath, outpath):
+    global TIMEOUT
+    with open(inpath) as f:
+        trace = f.readlines()
+    start = time.time()
+    if not check_if_trace_crashes(trace, outpath):
+        sys.exit("The input qtest trace didn't cause a crash...")
+    end = time.time()
+    print("Crashed in {} seconds".format(end-start))
+    TIMEOUT = (end-start)*5
+    print("Setting the timeout for {} seconds".format(TIMEOUT))
+    print("Identifying Crashes by this string: {}".format(CRASH_TOKEN))
+
+    i = 0
+    newtrace = trace[:]
+    # For each line
+    while i < len(newtrace):
+        # 1.) Try to remove it completely and reproduce the crash. If it works,
+        # we're done.
+        prior = newtrace[i]
+        print("Trying to remove {}".format(newtrace[i]))
+        # Try to remove the line completely
+        newtrace[i] = ""
+        if check_if_trace_crashes(newtrace, outpath):
+            i += 1
+            continue
+        newtrace[i] = prior
+
+        # 2.) Try to replace write{bwlq} commands with a write addr, len
+        # command. Since this can require swapping endianness, try both LE and
+        # BE options. We do this, so we can "trim" the writes in (3)
+        if (newtrace[i].startswith("write") and not
+            newtrace[i].startswith("write ")):
+            suffix = newtrace[i].split()[0][-1]
+            assert(suffix in write_suffix_lookup)
+            addr = int(newtrace[i].split()[1], 16)
+            value = int(newtrace[i].split()[2], 16)
+            for endianness in ['<', '>']:
+                data = struct.pack("{end}{size}".format(end=endianness,
+                                   size=write_suffix_lookup[suffix][1]),
+                                   value)
+                newtrace[i] = "write {addr} {size} 0x{data}\n".format(
+                    addr=hex(addr),
+                    size=hex(write_suffix_lookup[suffix][0]),
+                    data=data.hex())
+                if(check_if_trace_crashes(newtrace, outpath)):
+                    break
+            else:
+                newtrace[i] = prior
+
+        # 3.) If it is a qtest write command: write addr len data, try to split
+        # it into two separate write commands. If splitting the write down the
+        # middle does not work, try to move the pivot "left" and retry, until
+        # there is no space left. The idea is to prune unneccessary bytes from
+        # long writes, while accommodating arbitrary MemoryRegion access sizes
+        # and alignments.
+        if newtrace[i].startswith("write "):
+            addr = int(newtrace[i].split()[1], 16)
+            length = int(newtrace[i].split()[2], 16)
+            data = newtrace[i].split()[3][2:]
+            if length > 1:
+                leftlength = int(length/2)
+                rightlength = length - leftlength
+                newtrace.insert(i+1, "")
+                while leftlength > 0:
+                    newtrace[i] = "write {addr} {size} 0x{data}\n".format(
+                            addr=hex(addr),
+                            size=hex(leftlength),
+                            data=data[:leftlength*2])
+                    newtrace[i+1] = "write {addr} {size} 0x{data}\n".format(
+                            addr=hex(addr+leftlength),
+                            size=hex(rightlength),
+                            data=data[leftlength*2:])
+                    if check_if_trace_crashes(newtrace, outpath):
+                        break
+                    else:
+                        leftlength -= 1
+                        rightlength += 1
+                if check_if_trace_crashes(newtrace, outpath):
+                    i -= 1
+                else:
+                    newtrace[i] = prior
+                    del newtrace[i+1]
+        i += 1
+    check_if_trace_crashes(newtrace, outpath)
+
+
+if __name__ == '__main__':
+    if len(sys.argv) < 3:
+        usage()
+
+    QEMU_PATH = os.getenv("QEMU_PATH")
+    QEMU_ARGS = os.getenv("QEMU_ARGS")
+    if QEMU_PATH is None or QEMU_ARGS is None:
+        usage()
+    # if "accel" not in QEMU_ARGS:
+    #     QEMU_ARGS += " -accel qtest"
+    CRASH_TOKEN = os.getenv("CRASH_TOKEN")
+    QEMU_ARGS += " -qtest stdio -monitor none -serial none "
+    minimize_trace(sys.argv[1], sys.argv[2])