[release-branch.go1.17] os/exec: return clear error for missing cmd.Path

Following up on CL 403694, there is a bit of confusion about
when Path is and isn't set, along with now the exported Err field.
Catch the case where Path and Err (and lookPathErr) are all unset
and give a helpful error.

Updates #52574
Followup after #43724.

Fixes #53056
Fixes CVE-2022-30580

Change-Id: I03205172aef3801c3194f5098bdb93290c02b1b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/403759
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
(cherry picked from commit 960ffa98ce73ef2c2060c84c7ac28d37a83f345e)
Reviewed-on: https://go-review.googlesource.com/c/go/+/408578
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

2 files changed, 11 insertions, 0 deletions

diff --git a/src/os/exec/exec.go b/src/os/exec/exec.go
index 0c49575511..505de58e84 100644
--- a/src/os/exec/exec.go
+++ b/src/os/exec/exec.go
@@ -374,6 +374,9 @@ func lookExtensions(path, dir string) (string, error) {
 // The Wait method will return the exit code and release associated resources
 // once the command exits.
 func (c *Cmd) Start() error {
+	if c.Path == "" && c.lookPathErr == nil {
+		c.lookPathErr = errors.New("exec: no command")
+	}
 	if c.lookPathErr != nil {
 		c.closeDescriptors(c.closeAfterStart)
 		c.closeDescriptors(c.closeAfterWait)
diff --git a/src/os/exec/exec_test.go b/src/os/exec/exec_test.go
index d854e0de84..a951be718d 100644
--- a/src/os/exec/exec_test.go
+++ b/src/os/exec/exec_test.go
@@ -1156,3 +1156,11 @@ func TestChildCriticalEnv(t *testing.T) {
 		t.Error("no SYSTEMROOT found")
 	}
 }
+
+func TestNoPath(t *testing.T) {
+	err := new(exec.Cmd).Start()
+	want := "exec: no command"
+	if err == nil || err.Error() != want {
+		t.Errorf("new(Cmd).Start() = %v, want %q", err, want)
+	}
+}