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builtin: appending a two max-len []struct{} results in a slice with negative len #7550

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UserAB1236872 opened this issue Mar 15, 2014 · 5 comments
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@UserAB1236872
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@UserAB1236872 UserAB1236872 commented Mar 15, 2014

What does 'go version' print?

go version 1.2.1 windows/amd64

What steps reproduce the problem?
Run this minimal program:

package main

import "fmt"

func main() {
    length := (1 << 63) - 1
    a := make([]struct{}, length/(1<<32))
    b := make([]struct{}, length/(1<<32))
    a = append(a, b...)
    fmt.Println(len(a), cap(a))
}

Note that this *cannot* be reproduced on the Playground. Even if you substitute the
value of length with the 32-bit MaxInt (since the Playground seems to use a size of 4
instead of 8 for ints), you'll correctly get a panic from this operation.

What happened?

It will print -2 7638104968020361214 (that is, the slice has len -2 and cap
7638104968020361214).

What should have happened instead?

Probably a panic of some sort.

Please provide any additional information below.

This seems to only happen with slices of type struct{}. I tried to reproduce the error
with a []int, and it always panicked in some way. One other odd node, if instead of doing

a = append(a,b...)

you do

a = append(a,struct{}{})

It will correctly panic with the error:

panic: runtime error:  runtime: allocation size out of range

One confusing thing is that the panic the Playground gives for these operations is:

panic: runtime error: growslice: cap out of range

So that may be an additional bug, or may be expected.
@UserAB1236872
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@UserAB1236872 UserAB1236872 commented Mar 15, 2014

Comment 1:

Oops, sorry, I copied a modification of the program, the lines should be
length := (1 << 63) - 1
a := make([]struct{}, length)
b := make([]struct{}, length)
a = append(a, b...)
fmt.Println(len(a), cap(a))
The division was when I was trying to check it with a []int

@ianlancetaylor
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@ianlancetaylor ianlancetaylor commented Mar 16, 2014

Comment 2:

For clarity, for this program:
package main
import "fmt"
func main() {
        length := (1 << 63) - 1
        a := make([]struct{}, length)
        b := make([]struct{}, length)
        a = append(a, b...)
        fmt.Println(len(a), cap(a))
}
"go run" prints "-2 -2".  When built with gccgo, it crashes with a "append: slice
overflow" panic.

Labels changed: added repo-main, release-go1.3.

@davecheney
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@davecheney davecheney commented Apr 2, 2014

Comment 3:

FWIW, 32bit platforms print the expected error message.

@davecheney
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@davecheney davecheney commented Apr 2, 2014

Comment 4:

https://golang.org/cl/83520043 is a start of a fix, it needs tightening up

Owner changed to @davecheney.

Status changed to Started.

@davecheney
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@davecheney davecheney commented Apr 3, 2014

Comment 5:

This issue was closed by revision 9121e7e.

Status changed to Fixed.

@rsc rsc added this to the Go1.3 milestone Apr 14, 2015
@rsc rsc removed the release-go1.3 label Apr 14, 2015
@golang golang locked and limited conversation to collaborators Jun 25, 2016
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5 participants