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runtime: print register state for all crashes (user panics, ...) #50294

aktau opened this issue Dec 21, 2021 · 0 comments

runtime: print register state for all crashes (user panics, ...) #50294

aktau opened this issue Dec 21, 2021 · 0 comments


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@aktau aktau commented Dec 21, 2021

What version of Go are you using (go version)?

$ go version
go version devel go1.18-2d1d548 Tue Dec 21 03:55:43 2021 +0000 linux/amd64

Does this issue reproduce with the latest release?


What did you do?


It crashes (due to nil dereference) as follows:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x454746]

goroutine 1 [running]:
main.Square(0x4046d9?, 0x60?, 0x0?)
	/tmp/sandbox1838486988/prog.go:7 +0x6
main.Passthrough(0x458a40?, 0xc0000021a0?)
	/tmp/sandbox1838486988/prog.go:12 +0x1c
	/tmp/sandbox1838486988/prog.go:14 +0x25

Program exited.

What did you expect to see?

Context: internal ABI specification

I expected to be able to see at least one of the input arguments to the crashing function Square() as they are still live. At the point of the crash, the RAX register used to pass the first parameter has been clobbered, but its state gives valuable input about what the original value of the first parameter was. The second parameter can be found in RBX untouched.

The disassembly is as follows (truncated, annotated):

0000000000454b40 <main.Square>:
  454b40:       48 0f af c3             imul   %rbx,%rax
  454b44:       31 c9                   xor    %ecx,%ecx
  454b46:       48 03 01                add    (%rcx),%rax ; CRASH by nil dereference
  454b49:       c3                      ret    

0000000000454b60 <main.Passthrough>:
  454b60:       49 3b 66 10             cmp    0x10(%r14),%rsp
  454b64:       76 20                   jbe    454b86 <main.Passthrough+0x26>
  454b66:       48 83 ec 20             sub    $0x20,%rsp
  454b6a:       48 89 6c 24 18          mov    %rbp,0x18(%rsp)
  454b6f:       48 8d 6c 24 18          lea    0x18(%rsp),%rbp
  454b74:       48 89 d9                mov    %rbx,%rcx
  454b77:       e8 c4 ff ff ff          call   454b40 <main.Square>
  454b7c:       48 8b 6c 24 18          mov    0x18(%rsp),%rbp
  454b81:       48 83 c4 20             add    $0x20,%rsp
  454b85:       c3                      ret    
  454b86:       48 89 44 24 08          mov    %rax,0x8(%rsp)
  454b8b:       48 89 5c 24 10          mov    %rbx,0x10(%rsp)
  454b90:       e8 8b cd ff ff          call   451920 <runtime.morestack_noctxt.abi0>
  454b95:       48 8b 44 24 08          mov    0x8(%rsp),%rax
  454b9a:       48 8b 5c 24 10          mov    0x10(%rsp),%rbx
  454b9f:       90                      nop
  454ba0:       eb be                   jmp    454b60 <main.Passthrough>

0000000000454bc0 <main.main>:
  454bc0:       49 3b 66 10             cmp    0x10(%r14),%rsp
  454bc4:       76 29                   jbe    454bef <main.main+0x2f>
  454bc6:       48 83 ec 18             sub    $0x18,%rsp
  454bca:       48 89 6c 24 10          mov    %rbp,0x10(%rsp)
  454bcf:       48 8d 6c 24 10          lea    0x10(%rsp),%rbp
  454bd4:       b8 09 00 00 00          mov    $0x9,%eax
  454bd9:       48 89 c3                mov    %rax,%rbx
  454bdc:       0f 1f 40 00             nopl   0x0(%rax)
  454be0:       e8 7b ff ff ff          call   454b60 <main.Passthrough>
  454be5:       48 8b 6c 24 10          mov    0x10(%rsp),%rbp
  454bea:       48 83 c4 18             add    $0x18,%rsp
  454bee:       c3                      ret    
  454bef:       e8 2c cd ff ff          call   451920 <runtime.morestack_noctxt.abi0>
  454bf4:       eb ca                   jmp    454bc0 <main.main>

The same thing happens when using an actual panic to force a crash, but at least there we can see that the registers are being clobbered (annotated):

0000000000454b40 <main.Square>:
  454b40:       49 3b 66 10             cmp    0x10(%r14),%rsp
  454b44:       76 22                   jbe    454b68 <main.Square+0x28>
  454b46:       48 83 ec 18             sub    $0x18,%rsp
  454b4a:       48 89 6c 24 10          mov    %rbp,0x10(%rsp)
  454b4f:       48 8d 6c 24 10          lea    0x10(%rsp),%rbp
  454b54:       48 8d 05 e5 44 00 00    lea    0x44e5(%rip),%rax        # 459040 <type.*+0x4040> ; REGISTER CLOBBERED
  454b5b:       48 8d 1d 26 16 02 00    lea    0x21626(%rip),%rbx        # 476188 <runtime.mainPC+0x8> ; REGISTER CLOBBERED
  454b62:       e8 59 63 fd ff          call   42aec0 <runtime.gopanic> ; PANIC
  454b67:       90                      nop
  454b68:       e8 b3 cd ff ff          call   451920 <runtime.morestack_noctxt.abi0>
  454b6d:       eb d1                   jmp    454b40 <main.Square>

What did you see instead?

I expected that the parameter values in the stack trace would be as accurate as they could be, or at least more accurate than they are now. Now they contain values that aren't in any way related to the code that was running. The signal handler (likely) clobbers the registers state, which is why I believe we see values like 0x4046d9?, 0x60?, 0x0?. We can see the same values if we call panic() manually.

I've been told it would be a more involved project to improve the stack printer in such a way that it could take into account (better) the liveness, clobber state et cetera to get the "best possible estimate" for the parameter value. But a cheaper solution that would serve well in the interim would be to just dump the register state (all registers) when handling a crash. Then, a crash investigator can combine this information with an objdump to figure out whatever possible about the state of the crashing stack.

An argument raised was that dumping register state would be scary to users. Some counterpoints:

  • Go already dumps registers when there is a fatal error in the runtime. I don't see why user panics should be different.
  • When Go 1.17 introduced the regabi, the debugging experience for stack traces was weakened by the values being incorrect fairly often. It used to be that register state wasn't quite so necessary because previously to Go 1.17 the parameters were generally correct. This change would improve the situation a bit again.
  • The expected user of a stack trace is a developer, who may benefit from some insight into the computing stack. Even if the developer who first looks at the crash does not have the required experience to extract useful information, they may reroute that to an expert who can use it. If the register state is not printed as is currently the case, no-one can help.
  • C++ crash handlers at the place I work always print the register state (but no arguments), and anecdotally (for me) it has been useful in the past.

cc @cherrymui @aclements who I've talked to about this.

@cherrymui cherrymui added this to the Backlog milestone Dec 21, 2021
@ianlancetaylor ianlancetaylor changed the title runtime/crash: print register state for all crashes (user panics, ...) runtime: print register state for all crashes (user panics, ...) Dec 21, 2021
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