MYST_RETAIN_SYMBOLS env var to retain symbols

[anakrish]

Normally, when run under a debugger, Mystikos writes out each loaded shared library
to /tmp/myst/ so that the debugger can load symbols from these libraries.
At the end of the execution these files are deleted.

rr debugger allows record and replay debugging.
You record a failure once, then debug the recording, deterministically, as
many times as you want. The same execution is replayed every time.
rr also provides efficient reverse execution under gdb. Set breakpoints and
data watchpoints and quickly reverse-execute to where they were hit.

At a very high-level, rr works by capuring all kernel interactions,
including thread scheduling, and then re-applying all the interactions during playback.
The playback is not a real-execution, and does not produce most side effects
(e.g writing files, opening sockets etc).

Thus, during playback, the shared libraries necessay for the debugger are not written out.
To make rr work for Mystikos, the symbol files need to be retained during execution.
That is the purpose of the MYST_RETAIN_SYMBOLS flag.

---

Prerequisites:

• Install rr from https://github.com/rr-debugger/rr/releases
• Read up https://rr-project.org/
• Try out rr on a simple hellworld program outside mystikos
  $ rr record helloworld
  $ rr replay
  rr will suggest the necessary configuration settings that need to be made.
  Do as suggested.

Usage:

For a test, doing make TARGET=linux would print out the actual Mystikos command.
Execute MYST_RETAIN_SYMBOLS=1 rr record <mystikos command> to create a recording.

E.g:
$ MYST_RETAIN_SYMBOLS=1 rr record myst exec-linux rootfs /bin/hello red green blue
rr: Saving execution to trace directory `/home/user/.local/share/rr/myst-35'.
Hello world!
I received: argv[0]={/bin/hello}, argv[1]={red}, argv[2]={green}, argv[3]={blue}
=== passed test (/bin/hello)

To replay and debug, do rr replay -d /path/to/myst-gdb
E.g:
$ rr replay -d ../../../build/bin/myst-gdb
GNU gdb (Ubuntu 11.1-0ubuntu2) 11.1
0x00007fe413b720d0 in _start () from /lib64/ld-linux-x86-64.so.2
(rr)

Most GDB commands are supported; but keep in mind that this is a replay, not an actual
execution. It is like watching a video where you can easily skip forward and backward,
but not actually waching something actually happen.

Put a breakpoint:
(rr) b main
Breakpoint 2 at 0x10000024e32: file host.c, line 527.
(rr) c
Continuing.

Breakpoint 2, main (argc=7, argv=0x7ffda41c9688, envp=0x7ffda41c96c8) at host.c:527
527 int ec = _main(argc, argv, envp);
(rr) c
Continuing.
oegdb: Loaded enclave module /home/anand/msft/mystikos/build/lib/libmystkernel.so
oegdb: analyzing symbols for module /home/anand/msft/mystikos/build/lib/libmystkernel.so
oegdb: Loaded enclave module ./.mystX68Tlv/libmystcrt
oegdb: analyzing symbols for module ./.mystX68Tlv/libmystcrt
oegdb: Loaded enclave module ./.mystX68Tlv/hello
oegdb: analyzing symbols for module ./.mystX68Tlv/hello

Breakpoint 2, main (argc=4, argv=0x7fe412cf2010) at hello.c:10
10 assert(argc == 4);
(rr)

Step over, debug as before.
(rr) n
11 assert(strcmp(argv[0], "/bin/hello") == 0);
(rr) n
12 assert(strcmp(argv[1], "red") == 0);
(rr) n
13 assert(strcmp(argv[2], "green") == 0);
(rr) n
14 assert(strcmp(argv[3], "blue") == 0);

Reverse execution works, but seems flaky. Needs more investigation.

Signed-off-by: Anand Krishnamoorthi anakrish@microsoft.com

[vtikoo]

LGTM. For my understanding, do the replays not themselves invoke the load and unload symbol paths?

[anakrish]

LGTM. For my understanding, do the replays not themselves invoke the load and unload symbol paths?

I've tried to explain this in the PR description. Can you let me know if that is not clear so that I can reword it if needed?

[vtikoo]

LGTM. For my understanding, do the replays not themselves invoke the load and unload symbol paths?

I've tried to explain this in the PR description. Can you let me know if that is not clear so that I can reword it if needed?

Makes sense, the symbol loading and unloading syscalls are not performed during replay.

[anakrish]

@vtikoo Feel free to merge this PR if it is useful.

[vtikoo]

@anakrish yes would be great to get this merged. Its complaining about rebase conflicts, could you take a look?

[anakrish]

I don't have the original branch and I'm no longer set up to work on Mystikos.
Since the changes are really small, you could submit them via a new PR.

[anakrish]

Changes commited by #1449