Just a minor correction:
go tool objdump showed that *golang does not touch the pointer value before
overwriting it*.
The situation does occur nevertheless because obj.data may be nil [0x0,0,0]
initially, and length/capacity are assigned before the data pointer is set:
obj.data : [0x0,0,0]
GR1> { obj.data := newData [Step X] }
obj.data : [0x0,4,4]
GR2> { f(…, src:[0x0,4,4],…) }
GR1> { obj.data:= newData [Step X+N] }
obj.data : [0x2349231,4,4]
On Wednesday, August 16, 2017 at 12:03:03 PM UTC+8, Björn Karge wrote:
>
> Jakob, thanks for your suggestion - it lifted me out of the too narrow
> context :-)
>
> The reason seems to be that src is already corrupt when arriving at f()
> because of a data race (between go routines GR1, GR2 below), copying a
> slice during (non-atomic) assignment.
> The null pointer is probably placed by go in the course of the slice
> assignment in order to fail fast and avoid buffer overruns during data
> races.
>
> obj.data : [0x1c28374,2,3]
> GR1> { obj.data := newData [Step X] }
> obj.data : [0x0,2,3]
> GR2> { f(…, src:[0x0,2,3],…) }
> GR1> { obj.data:= newData [Step X+N] }
> obj.data : [0x2349231,4,4]
>
> Cheers!
>
> Björn
>
>
> On Wednesday, August 16, 2017 at 12:41:47 AM UTC+8, Björn Karge wrote:
>>
>> Hello!
>>
>> I got this panic trace which I am trying to get my head around:
>>
>> panic: runtime error: invalid memory address or nil pointer dereference
>> [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x45c3d2]
>> message=‘’ •••
>> goroutine 6786902223 [running]:
>> panic(0xc03040, 0x116d2f0)
>> golang/go/src/runtime/panic.go:531 +0x1cf fp=0xc47d406d90
>> sp=0xc47d406cf8
>> runtime.panicmem()
>> golang/go/src/runtime/panic.go:63 +0x5e fp=0xc47d406db0
>> sp=0xc47d406d90
>> runtime.sigpanic()
>> golang/go/src/runtime/signal_unix.go:290 +0x29f fp=0xc47d406e00
>> sp=0xc47d406db0
>> runtime.memmove(0xc44f8e4500, 0x0, 0x18)
>> golang/go/src/runtime/memmove_amd64.s:178 +0x682 fp=0xc47d406e08
>> sp=0xc47d406e00 our/source/code.glob..func5.1(0x0, 0x3, 0x3, 0xc43104d380,
>> 0xc436f05900, 0x73, 0xc47bd2e8a0, 0x24, 0xc437f1d616, 0x6, ...)
>> /our/source/code.go:123 +0xa2 fp=0xc47d406f58 sp=0xc47d406e08
>> runtime.goexit()
>> golang/go/src/runtime/asm_amd64.s:2197 +0x1 fp=0xc47d406f60
>> sp=0xc47d406f58
>> created by our/source/code.glob..func5
>> /our/source/code.go:132 +0x150
>>
>> Here's the stripped-down source code fragment (our/source/code.go with
>> relevant line numbers added)
>>
>> var f = func(a A, b, c string, src []int64, d string, e, f string, g []byte)
>> chan error {
>> ec := make(chan error, somepkg.Count())
>>
>> go func(a A) {
>> dest := make([]int64, len(src))
>> 123: copy(dest, src)
>>
>>
>> defer close(ec)
>> ...
>>
>> defer something()
>> ...
>>
>> 132:}(a)
>>
>> return ec
>> }
>>
>>
>> At first, this looks like an OOM as it seems to be triggered by a copy to a
>> freshly allocated array (dest := make([]int64, len(src)) + copy(dest, src)).
>> But at closer inspection memmove(0xc44f8e4500, 0x0, 0x18) seems to indicate
>> that the *source* is 0 (the second parameter if we trust memmove_amd64.s).
>> In that case, how could the length (0x18) have been determined in the first
>> place?
>>
>> Put differently, how can *ptr* of a slice (as in
>> https://blog.golang.org/go-slices-usage-and-internals) be a null pointer
>> when the slice has a length? Does this indicate a missed check of malloc (or
>> however the equivalent is called in the go runtime env)? Or am I
>> misunderstanding the trace?
>>
>>
>>
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