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Minor improvements to rr post

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Jade Lovelace 2022-03-15 17:02:30 -07:00
parent 1966ec526f
commit f6444034c3
1 changed files with 17 additions and 3 deletions
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20
content/posts/debugging-rr-children/index.md
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@ -47,7 +47,12 @@ We want to figure out where the crasher is crashing.
## Using rr to do it the easy way
Let's use `rr` to do this more easily. First, record a run:
For those unfamiliar, [`rr`](https://rr-project.org) is a time travel debugger
for Linux: it records a run of a program and then can deterministically replay
it (including in reverse!) as many times as you want. It's mostly replaced
running things directly in `gdb` for me.
Let's use it to find the fault in the child process. First, record a run:
```
» rr record ./caller
@ -67,7 +72,8 @@ PID PPID EXIT CMD
```
Next, use either `--onfork=<PID>` or `--onprocess=<PID>` to get a debugger on
the problem process:
the problem process. Since we believe it's failing after the process is both
`fork`'d and `exec`'d into the new program, `--onprocess` is appropriate.
```
» rr replay --onprocess=1432674
@ -82,6 +88,7 @@ Reading symbols from /lib64/ld-linux-x86-64.so.2...
BFD: warning: system-supplied DSO at 0x6fffd000 has a section extending past end of file
0x00007f01f1ffb930 in _start () from /lib64/ld-linux-x86-64.so.2
=> 0x00007f01f1ffb930 <_start+0>: 48 89 e7 mov %rsp,%rdi
(rr) continue
Continuing.
[caller] spawned pid 1432674
@ -90,14 +97,17 @@ Continuing.
Program received signal SIGABRT, Aborted.
0x00007f01f1e0734c in __pthread_kill_implementation () from /usr/lib/libc.so.6
=> 0x00007f01f1e0734c <__pthread_kill_implementation+284>: 89 c5 mov %eax,%ebp
(rr) backtrace
#0 0x00007f01f1e0734c in __pthread_kill_implementation () from /usr/lib/libc.so.6
#1 0x00007f01f1dba4b8 in raise () from /usr/lib/libc.so.6
#2 0x00007f01f1da4534 in abort () from /usr/lib/libc.so.6
#3 0x000055757da48161 in main () at crasher.c:6
(rr) frame 3
#3 0x000055757da48161 in main () at crasher.c:6
6 abort();
(rr) list
1 #include <stdio.h>
2 #include <stdlib.h>
@ -106,6 +116,7 @@ Program received signal SIGABRT, Aborted.
5 printf("[crasher] about to crash\n");
6 abort();
7 }
(rr)
```
@ -114,7 +125,7 @@ startup-quietly on` into `~/.config/gdb/gdbearlyinit` to get that.
## Annotating output with event numbers
Another neat feature I found in rr is the ability to annotate output with the
Another neat feature I found in `rr` is the ability to annotate output with the
PIDs and event numbers with `--mark-stdio`, which can be useful if you have a
program that is doing a bunch of things before the event of interest.
@ -147,6 +158,7 @@ Reading symbols from /lib64/ld-linux-x86-64.so.2...
BFD: warning: system-supplied DSO at 0x6fffd000 has a section extending past end of file
0x0000000070000002 in syscall_traced ()
=> 0x0000000070000002: c3 ret
(rr) bt
#0 0x0000000070000002 in syscall_traced ()
#1 0x00007f01f1fd0430 in _raw_syscall ()
@ -155,9 +167,11 @@ BFD: warning: system-supplied DSO at 0x6fffd000 has a section extending past end
#13 0x00007f01f1dfe543 in __GI__IO_file_overflow () from /usr/lib/libc.so.6
#14 0x00007f01f1df36fa in puts () from /usr/lib/libc.so.6
#15 0x000055757da4815c in main () at crasher.c:5
(rr) frame 15
#15 0x000055757da4815c in main () at crasher.c:5
5 printf("[crasher] about to crash\n");
(rr) list
1 #include <stdio.h>
2 #include <stdlib.h>