interp: mis-interpretation of global map initializer

[eliasnaur]

$ cat precomp.go
package main

import (
	"crypto/sha256"
	"fmt"
	"os"
)

var (
	tag       = sha256.Sum256(nil)
	globalMap = map[string][32]byte{
		"": tag,
	}
)

func main() {
	localMap := map[string][32]byte{
		"": tag,
	}
	if localMap[""] != globalMap[""] {
		fmt.Printf("global %.32x\nlocal %.32x\n", globalMap, localMap)
		os.Exit(1)
	}
}
$ go run precomp.go
$ tinygo run precomp.go
global map[:0000000000000000000000000000000000000000000000000000000000000000]
local map[:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855]
failed to run compiled binary /tmp/nix-shell.HWhNg6/tinygo1488071277/main: exit status 1

This breaks anything relying on https://github.com/btcsuite/btcd/blob/67b8efd3ba53b60ff0eba5d79babe2c3d82f6c54/chaincfg/chainhash/hash.go#L50.

[dgryski]

Interestingly, switching the runtime hash to tsip allows this code to pass. I wonder if it's because interp decides not to run the tsip code for some reason.

~/go/src/github.com/dgryski/bug/interp $ for hash in fnv leveldb tsip; do echo $hash; tinygo run -tags=runtime_memhash_${hash} main.go; done
fnv
global map[:0000000000000000000000000000000000000000000000000000000000000000]
local map[:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855]
failed to run compiled binary /var/folders/5b/_hr1d1fd3qn4t9vtfx5p61wm0000gp/T/tinygo3541483627/main: exit status 1
leveldb
global map[:0000000000000000000000000000000000000000000000000000000000000000]
local map[:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855]
failed to run compiled binary /var/folders/5b/_hr1d1fd3qn4t9vtfx5p61wm0000gp/T/tinygo1726170860/main: exit status 1
tsip
~/go/src/github.com/dgryski/bug/interp $

[eliasnaur]

Can you say where to look into a fix, however hacky? This is a miscompilation of an external package that is impractical to change.

[dgryski]

When building with -tags=runtime_memhash_tsip I see the following:

                    load: <2468+4> -> 1819895653
                    call: math/bits.RotateLeft32(1819895653, 5, 0)
                        trunc: 5 32
                        call <1447> <!> <!> <!>
                        ret <!>
                    !! revert because of error: interp: unsupported instruction (to be emitted at runtime)

So now at least we know why switching the hash function works.

[dgryski]

Here's a simpler reproducer:

package main

import (
	"math/bits"
)

func sum(b []byte) byte {
	return bits.RotateLeft8(1, len(b))
}

var (
	tag       = sum([]byte{1})
	globalMap = map[string]byte{
		"": tag,
	}
)

func main() {
	localMap := map[string]byte{
		"": tag,
	}
	if localMap[""] != globalMap[""] {
		println("global", globalMap[""], localMap[""])
		panic("mismatch")
	}

	println("value:", localMap[""])
}

So what's happening here is that sum() function is unable to be interpreted at interp time and so is pushed off to runtime, but we're not holding off the construction of the map that depends on that value.

So while we should probably add the rotate instructions to interp, the underlying issue is that the map initialization doens't seem to be reverted when one of the values used during construction is.

[dgryski]

@niaow @aykevl I haven't done any work debugging interp, so any pointers here would be helpful.

[dkegel-fastly]

@eliasnaur You could disable interp as a hacky workaround...?