cmd/compile: user symbols can be in reserved namespace

[Pr0Ger]

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

$ go version
go version go1.14 darwin/amd64

What did you do?

package main

import "debug/gosym"

func main()  {
	s1 := gosym.Sym{Name: "go.uber.org/zap/zapcore.(*CheckedEntry).Write"}

	println(s1.PackageName())
}

What did you expect to see?

go.uber.org/zap/zapcore

What did you see instead?

empty string

I see problem is right here but since I'm not aware how linker uses these prefixes I can't figure out how to properly fix this condition

go/src/debug/gosym/symtab.go

Lines 40 to 44 in 43c6ada

	// A prefix of "type." and "go." is a compiler-generated symbol that doesn't belong to any package.
	// See variable reservedimports in cmd/compile/internal/gc/subr.go
	if strings.HasPrefix(name, "go.") || strings.HasPrefix(name, "type.") {
	return ""
	}

[ianlancetaylor]

The debug/gosym package is intended to be used to read object files generated by the compiler. It's not intended to be used by creating your own instances of gosym.Sym.

Can you show us a complete program that demonstrates this problem when opening an object generated by the compiler? Please also include the sources for the object that you want to open. Thanks.

[Pr0Ger]

Yeah, sure (go.uber.org/zap is a public available library):

package main

import (
	"debug/gosym"
	"debug/macho"
	"fmt"
	"os"
	"strings"

	"go.uber.org/zap"
)

func main() {
	logger, _ := zap.NewDevelopment()
	logger.Debug("using logger just a little bit")

	executable, _ := os.Executable()
	exe, _ := macho.Open(executable)

	pclndat, _ := exe.Section("__gopclntab").Data()
	symTabRaw, _ := exe.Section("__gosymtab").Data()

	pcln := gosym.NewLineTable(pclndat, exe.Section("__text").Addr)
	symTab, _ := gosym.NewTable(symTabRaw, pcln)
	for _, funk := range symTab.Funcs {
		if strings.Contains(funk.Name, "go.uber.org") {
			fmt.Printf("%s -> %s\n", funk.Name, funk.PackageName())
		}
	}
}

And output (just a few lines):

❯ go run test.go
go: finding module for package go.uber.org/zap
go: downloading go.uber.org/zap v1.14.0
go: found go.uber.org/zap in go.uber.org/zap v1.14.0
2020-03-09T22:16:31.280+0300	DEBUG	test_package_name/test.go:15	using logger just a little bit
go.uber.org/zap/buffer.(*Buffer).AppendString ->
go.uber.org/zap/buffer.(*Buffer).AppendBool ->
...

[ianlancetaylor]

Thanks. If user packages can be in the compiler-reserved "go." namespace, that seems like a potential problem.

CC @griesemer @mdempsky @josharian @randall77

[mdempsky]

Bitten again by our ad hoc symbol mangling rules. :(

I think a short-term workaround could be debug/gosym only checks for "go." and "type." when it can't find a '/'. I don't think the compiler ever generates go.* or type.* symbols that contain a /.

This would still cause problems for top-level user package paths like "go.mystuff" (as opposed to "go.mystuff/dir"). Not sure how to best address that off hand.

Edit: Nevermind, I'm mistaken:

$ nm $(go tool -n compile) | grep 'go\..*/' | head -1
0000000000d9aca0 R go.itab.*cmd/compile/internal/gc.blockHeap,container/heap.Interface

[cuonglm]

Can we change go. to go.. instead for compiler generated symbols?

[cuonglm]

Can we change go. to go.. instead for compiler generated symbols?

Seems doable, I make a quick change to go.itab -> go..itab and it's compiled ok and test passed.

[odeke-em]

Some 2 years ago, there was a somewhat similar issue #25113 reported and fixed by @aarzilli.
We had clashes with userland variables that collided with compiler generated names such as statictmp_0, and we had to rename from statictmp -> .stmp, from @josharian's suggestion in CL https://go-review.googlesource.com/c/go/+/142497. Seems like a similar mangling scheme could help here, as @cuonglm suggested in #37762 (comment), thank you!

[gopherbot]

Change https://golang.org/cl/226282 mentions this issue: cmd/compile,link: use "go.." for compiler genereated types

[gopherbot]

Change https://golang.org/cl/226283 mentions this issue: debug/gosym: correct checking condition for compiler generated symbols