Using rr with janet on Linux

Why

gdb [1] is great, but it's way too easy to move past a point of interest (e.g. "darn I used next when I meant step"), forget to observe something, or just not remember all of the details that you might have been observing.

rr lets you "go backwards" or, put differently, grants the power of "do over".

In more detail:

Missed a return value while observing execution? Use reverse-step or reverse-next to "go back" far enough and use finish to see the missed return value.
Choosing an initial breakpoint for investigation isn't as tough because one can reverse-continue to a subsequently set breakpoint that is "behind" where one is currently.
It's much easier to get a better idea concerning the qusetion "how did we end up here?".
Recorded content may be shareable with others -- see rr pack.

Recording and Replaying Steps

ensure rr is installed

$ which rr
/usr/bin/rr

check whether appropriate PMU support available [2]

$ sudo dmesg | grep PMU
[    0.690026] Performance Events: PEBS fmt3+, Skylake events, 32-deep LBR, full-width counters, Intel PMU driver.
[    0.690026] NMI watchdog: Enabled. Permanently consumes one hw-PMU counter.
[    5.590973] RAPL PMU: API unit is 2^-32 Joules, 5 fixed counters, 655360 ms ovfl timer
[    5.590976] RAPL PMU: hw unit of domain pp0-core 2^-14 Joules
[    5.590977] RAPL PMU: hw unit of domain package 2^-14 Joules
[    5.590978] RAPL PMU: hw unit of domain dram 2^-14 Joules
[    5.590979] RAPL PMU: hw unit of domain pp1-gpu 2^-14 Joules
[    5.590980] RAPL PMU: hw unit of domain psys 2^-14 Joules

check if an archaic invocation is needed

$ sysctl kernel.perf_event_paranoid
kernel.perf_event_paranoid = 4

we got 4 (>1), so we need to tweak something apparently (remember to revert later)

$ sudo sysctl kernel.perf_event_paranoid=1

build janet

$ cd ~/src/janet.debug
$ make

prepare and peek in trivial .janet file

$ echo "3 8" > test-parse.janet

$ cat test-parse.janet 
3 8

make recording

$ rr record ./build/janet test-parse.janet
rr: Saving execution to trace directory `/home/user/.local/share/rr/janet-0'.

replay recording -- gdb-like ui

start the replaying

$ rr replay
GNU gdb (Ubuntu 12.1-0ubuntu1~22.04) 12.1
Copyright (C) 2022 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from /home/user/.local/share/rr/janet-0/mmap_hardlink_4_janet...
Really redefine built-in command "restart"? (y or n) [answered Y; input not from terminal]
Remote debugging using 127.0.0.1:24436
Reading symbols from /lib64/ld-linux-x86-64.so.2...
Reading symbols from /usr/lib/debug/.build-id/61/ef896a699bb1c2e4e231642b2e1688b2f1a61e.debug...
BFD: warning: system-supplied DSO at 0x6fffd000 has a section extending past end of file
0x00007f68924162b0 in _start () from /lib64/ld-linux-x86-64.so.2

set a breakpoint (note: process / inferior is already "running", but paused)

(rr) b src/core/parse.c:janet_parser_consume
Breakpoint 1 at 0x5645f031cd69: file src/core/parse.c, line 701.

continue until the breakpoint (beginning of `janet_parser_consume`)

(rr) c
Continuing.

Breakpoint 1, janet_parser_consume (parser=0x5645f1bd5050, c=51 '3') at src/core/parse.c:701
701	    int consumed = 0;

see where we are (line 701)

(rr) l
696	}
697	
698	/* Public API */
699	
700	void janet_parser_consume(JanetParser *parser, uint8_t c) {
701	    int consumed = 0;
702	    janet_parser_checkdead(parser);
703	    if (c == '\r') {
704	        parser->line++;
705	        parser->column = 0;

take a step forward... (now line 702)

(rr) step
702	    janet_parser_checkdead(parser);

take a step back... (now line 701)

(rr) reverse-step

Breakpoint 1, janet_parser_consume (parser=0x55a77e648050, c=51 '3') at src/core/parse.c:701
701	    int consumed = 0;

...and another step back (now line 931)

(rr) reverse-step
cfun_parse_consume (argc=3, argv=0x55a77e638700) at src/core/parse.c:931
931	        janet_parser_consume(p, view.bytes[i]);

observe we've popped out backwards to where `janet_parser_consume` was called from

(rr) l
926	        view.len -= offset;
927	        view.bytes += offset;
928	    }
929	    int32_t i;
930	    for (i = 0; i < view.len; i++) {
931	        janet_parser_consume(p, view.bytes[i]);
932	        switch (janet_parser_status(p)) {
933	            case JANET_PARSE_ROOT:
934	            case JANET_PARSE_PENDING:
935	                break;

exit rr

(rr) quit
A debugging session is active.

	Inferior 1 [process 25112] will be detached.

Quit anyway? (y or n) y
Detaching from program: /home/user/.local/share/rr/janet-1/mmap_hardlink_4_janet, process 25112
[Inferior 1 (process 25112) detached]

cast protection spell

$ sudo sysctl kernel.perf_event_paranoid=4

Quotes

Debugging a program in reverse is the most natural way to debug software. The cause of the bug is somewhere in the past, and the developer starts off from the bug manifestation. What the developer is actually doing is trying to get to the source of the bug by going bit by bit to the origin of the bug.

via: https://johnnysswlab.com/rr-the-magic-of-recording-and-replay-debugging/

And rr has quickly become the number one tool I reach for when debugging complicated C++ code. rr only runs on Linux and I don't even use Linux as my day-to-day operating system! But rr provides such a great debugging experience, and gives me such a huge productivity boost, that I will reboot into Fedora just to use rr for all but the most trivial bugs.

via: https://fitzgeraldnick.com/2015/11/02/back-to-the-futurre.html

References

[1] lldb is nice too! Haven't tried, but apaprently someone has rr and lldb working together. [2] https://github.com/rr-debugger/rr/wiki/Will-rr-work-on-my-system

sogaiu/rr-with-janet.md