cmd/go: invalid -ldflags is silently ignored

[hyangah]

go version go1.15.5 linux/amd64

$ cat main.go
package main
var X = ""
func main() {	println(X)  }

$ go run -ldflags='-X main.X=Hello' main.go
Hello
$ go run -ldflags='"-X main.X=Hello"' main.go

$

The -ldflags of the second go run is in invalid format, but the go command silently ignores it and keeps working.

$ go run -ldflags='"-X main.X=Hello"' -x main.go
WORK=/tmp/go-build569857493
mkdir -p $WORK/b001/
...
mkdir -p $WORK/b001/exe/
cd .
$GOROOT/pkg/tool/linux_amd64/link -o $WORK/b001/exe/main -importcfg $WORK/b001/importcfg.link -s -w -buildmode=exe -buildid=HYVxxilVrLgdctP3U_YF/h7WWNLZkY13bQM5oqdBD/IUwm0pzLWLHdmcshsYuR/HYVxxilVrLgdctP3U_YF -extld=clang $WORK/b001/_pkg_.a
$WORK/b001/exe/main

See above link command. That's missing -X main.X=Hello

[bcmills]

From what I can tell, that flag is here:

go/src/cmd/go/internal/load/flag.go

Line 17 in 64fae25

BuildLdflags PerPackageFlag // -ldflags

That triggers the parsing case here:

go/src/cmd/go/internal/load/flag.go

Lines 52 to 63 in 64fae25

	if !strings.HasPrefix(v, "-") {
	i := strings.Index(v, "=")
	if i < 0 {
	return fmt.Errorf("missing =<value> in <pattern>=<value>")
	}
	if i == 0 {
	return fmt.Errorf("missing <pattern> in <pattern>=<value>")
	}
	pattern := strings.TrimSpace(v[:i])
	match = MatchPackage(pattern, cwd)
	v = v[i+1:]
	}

...which succeeds because it finds =, and then attempts to match "-X main.X against a package path (which obviously fails to match anything because it's not a syntactically valid package path).

Before we call MatchPackage on the parsed pattern, we should first verify that it is a syntactically valid package pattern.

CC @jayconrod @matloob