Konstantin, your description is correct. The code is trying to load a
function pointer out of an itab, but the pointer to the itab is nil.
I think this is actually a bug. If you have an interface with more than
pagesize/ptrsize methods in it, this code might not panic when it should.
On Monday, November 13, 2017 at 4:46:59 AM UTC-8, Alexander Kapshuk wrote:
>
> On Mon, Nov 13, 2017 at 1:42 PM, Konstantin Khomoutov <kos...@bswap.ru
> <javascript:>> wrote:
>
>> While debugging a program which had a panic due to an attempt to call a
>> method on a value of an interface typeš, I came across the behaviour I
>> find strange, and would like to get help understanding what happens.
>>
>> The behaviour is exhibited by this simple program:
>>
>> -------------------------------8<--------------------------------
>> 1 package main
>> 2
>> 3 import (
>> 4 "fmt"
>> 5 "os"
>> 6 )
>> 7
>> 8 func main() {
>> 9 var fi os.FileInfo
>> 10 s := fi.Name()
>> 11 fmt.Println(s)
>> 12 }
>> -------------------------------8<--------------------------------
>>
>> When built by Go 1.8.3 on Linux/amd64 and run on that same system
>> it expectedly panics at line 10.
>>
>>
>> What puzzles me, is that the address it panics is not 0x0 (which I would
>> expect from an x86/amd64 H/W platform to stand for nil) but 0x38:
>>
>> -------------------------------8<--------------------------------
>> $ go run foo.go
>> panic: runtime error: invalid memory address or nil pointer dereference
>> [signal SIGSEGV: segmentation violation code=0x1 addr=0x38 pc=0x47d148]
>>
>> goroutine 1 [running]:
>> main.main()
>> /home/user/foo.go:10 +0x28
>> exit status 2
>> -------------------------------8<--------------------------------
>>
>>
>> If I run `go tool objdump` on the generated binary, I get this
>> (instruction codes removed for brewity):
>>
>> -------------------------------8<--------------------------------
>> TEXT main.main(SB) /home/user/foo.go
>> foo.go:8 0x47d120 FS MOVQ FS:0xfffffff8, CX
>> foo.go:8 0x47d129 CMPQ 0x10(CX), SP
>> foo.go:8 0x47d12d JBE 0x47d1d3
>> foo.go:8 0x47d133 SUBQ $0x58, SP
>> foo.go:8 0x47d137 MOVQ BP, 0x50(SP)
>> foo.go:8 0x47d13c LEAQ 0x50(SP), BP
>> foo.go:10 0x47d141 MOVQ $0x38, AX
>> foo.go:10 0x47d148 MOVQ 0(AX), AX
>> foo.go:10 0x47d14b MOVQ $0x0, 0(SP)
>> foo.go:10 0x47d153 CALL AX
>> foo.go:10 0x47d155 MOVQ 0x10(SP), AX
>> foo.go:10 0x47d15a MOVQ 0x8(SP), CX
>> foo.go:11 0x47d15f MOVQ CX, 0x30(SP)
>> foo.go:11 0x47d164 MOVQ AX, 0x38(SP)
>> foo.go:11 0x47d169 MOVQ $0x0, 0x40(SP)
>> foo.go:11 0x47d172 MOVQ $0x0, 0x48(SP)
>> foo.go:11 0x47d17b LEAQ 0xf3de(IP), AX
>> foo.go:11 0x47d182 MOVQ AX, 0(SP)
>> foo.go:11 0x47d186 LEAQ 0x30(SP), AX
>> foo.go:11 0x47d18b MOVQ AX, 0x8(SP)
>> foo.go:11 0x47d190 CALL runtime.convT2E(SB)
>> foo.go:11 0x47d195 MOVQ 0x10(SP), AX
>> foo.go:11 0x47d19a MOVQ 0x18(SP), CX
>> foo.go:11 0x47d19f MOVQ AX, 0x40(SP)
>> foo.go:11 0x47d1a4 MOVQ CX, 0x48(SP)
>> foo.go:11 0x47d1a9 LEAQ 0x40(SP), AX
>> foo.go:11 0x47d1ae MOVQ AX, 0(SP)
>> foo.go:11 0x47d1b2 MOVQ $0x1, 0x8(SP)
>> foo.go:11 0x47d1bb MOVQ $0x1, 0x10(SP)
>> foo.go:11 0x47d1c4 CALL fmt.Println(SB)
>> foo.go:12 0x47d1c9 MOVQ 0x50(SP), BP
>> foo.go:12 0x47d1ce ADDQ $0x58, SP
>> foo.go:12 0x47d1d2 RET
>> foo.go:8 0x47d1d3 CALL runtime.morestack_noctxt(SB)
>> foo.go:8 0x47d1d8 JMP main.main(SB)
>> -------------------------------8<--------------------------------
>>
>> So, for the call at line 10 we have
>>
>> MOVQ $0x38, AX
>> MOVQ 0(AX), AX
>>
>> which I translate as "load the quad word 0x38 into the register AX
>> and then load the quad word located at offset 0 in the memory at
>> the address located in the register AX, into that same register".
>>
>> That second instruction fails (since IIRC Linux maps a special
>> sentinel page at address 0x0 to catch problems like this one).
>>
>>
>> I fail to comprehend why 0x38 appears to be a constant (some magic
>> number). Looks like this is an offset of something. Recalling [1],
>> I found out Go 1.8.3 defines an Itab as
>>
>> type itab struct {
>> inter *interfacetype
>> _type *_type
>> link *itab
>> bad int32
>> inhash int32 // has this itab been added to hash?
>> fun [1]uintptr // variable sized
>> }
>>
>> 0x38 is 56, and 56/sizeof(quad word) = 7, so the only further guess
>> I can make is that 0x38 is the offset of the 3rd element of the "fun"
>> field in an Itab.
>>
>> Am I correct?
>> If not, what does that 0x38 stand for?
>>
>> 1. https://research.swtch.com/interfaces
>>
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>
> Incidentally, this code in question does panic at addr=0x0 when run from
> within the Go Playground.
>
>
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