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Introduction

PANDA supports whole system deterministic record and replay in whole system mode on the i386, x86_64, and arm targets. We hope to add more soon; for example, partial SPARC support exists but is not yet reliable.

Background

Deterministic record and replay is a technique for capturing the non-deterministic inputs to a system -- that is, the things that would cause a system to behave differently if it were re-started from the same point with the same inputs. This includes things like network packets, hard drive reads, mouse and keyboard input, etc.

Our implementation of record and replay focuses on reproducing code execution. That is, the non-deterministic inputs we record are changes made to the CPU state and memory -- DMA, interrupts, in instructions, and so on. Unlike many record and replay implementations, we do not record the inputs to devices; this means that one cannot "go live" during a recording, but it greatly simplifies the implementation. To get an idea of what is recorded, imagine drawing a line around the CPU and RAM; things going from the outside world to the CPU and RAM, crossing this line, must be recorded.

Record and replay is extremely useful because it enables many sophisticated analysis that are too slow to run in real-time; for example, trying to do taint flow analysis makes the guest system so slow that it cannot make network connections (because remote systems time out before the guest can process the packets and respond). By creating a recording, which has fairly modest overhead, and performing analyses on the replayed execution, one can do analyses that simply aren't possible to do live.

Usage

Record and replay is controlled with four QEMU monitor commands: begin_record, end_record, begin_replay, end_replay:

  • begin_record <name>

    Starts a recording session, saved as <name>. Note that there is currently no safeguard to prevent overwriting previous recordings, so be careful to choose a unique name.

    The recording log consists of two parts: the snapshot, which is named <name>-rr-snp, and the recording log, which is named <name>-rr-nondet.log.

  • end_record

    Ends an active recording session. The guest will be paused, but can be resumed and another recording can be made once the guest is resumed.

  • begin_replay <name>

    Begin a replay of the session named <name>. Note that QEMU not must be halted for this to work.

  • end_replay

    End an active replay. Normally the replay will just finish on its own, but if you want to end it early you can use this command.

    Quitting QEMU (either by pressing ^C or typing "quit" in the monitor) will also end the replay.

Alternatively, one can start replays from the command line using the -replay <name> option.

Of course, just running a replay isn't very useful by itself, so you will probably want to run the replay with some plugins enabled that perform some analysis on the replayed execution. See docs/PANDA.md for more details.

Migrating Recordings from Older Versions of PANDA

Prior to commit 9139261d70, snapshot data was stored in the QCOW itself. To enable sharing of record/replay logs without sharing the QCOW, this has changed so that the snapshot is stored in a separate file, named <name>-rr-snp.

Of course, this means that recordings made before commit 9139261d70 cannot be directly used with the current version of PANDA. Luckily, it's easy to convert existing snapshots to the new format. Assuming your recording is named foo, stored in hd.qcow2, and was created using QEMU architecture $ARCH with memory size $MEM the command is:

echo -e 'migrate exec:cat>foo-rr-snp\nq" | qemu-system-$ARCH -m $MEM -hda hd.qcow2 -monitor stdio -vnc :0 -S -loadvm foo-rr-snp

Naturally, you should adjust foo, $ARCH, $MEM, and hd.qcow2 to match your particular situation. Once the command finishes, foo-rr-snp should exist in the current directory. Assuming that foo-rr-nondet.log is also in the current directory, you can then run the replay with:

qemu-system-$ARCH -m $MEM -vnc :0 -replay foo

Note that the QCOW is no longer required.

Sharing Recordings

To make it easier to share record/replay logs, PANDA has two scripts, rrpack.py and rrunpack.py, that bundle up and compress a recording. These can be found in the scripts directory. To pack up a recording, just use:

scripts/rrpack.py <name>

This will bundle up <name>-rr-snp and <name>-rr-nondet.log and put them into PANDA's packed record/replay format in a file named <name>.rr. This file can be unpacked and verified using:

scripts/rrunpack.py <name>.rr

A central repository for sharing record/replay logs is available at the PANDA Share website.