os/exec: Command does not pass absolute path to argv[0]

[APTy]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.7.1 darwin/amd64

What operating system and processor architecture are you using (go env)?

darwin amd64

What did you do?

Run sshd in exec.Command() fails due to an incorrect absolute path.
https://play.golang.org/p/z1UMtdSY2N

What did you expect to see?

Successful execution of the command

Server listening on 0.0.0.0 port 8022.
Server listening on :: port 8022.

What did you see instead?

A message indicating that the command was not executed as it should be.

sshd re-exec requires execution with an absolute path

Discussion

exec.Command does in fact expand the absolute path of a command and save it to cmd.Path; however, it does not update the value of cmd.Args[0], which remains the original string name.

Even though the executed program is run correctly from its full system path, programs like sshd may make application-level checks against argv[0]. In these cases, we see the failure case as above.

Note that this may be intentional, but the docs for Command state that the path is resolved to a complete name, which makes the behavior above a little unexpected.

If name contains no path separators, Command uses LookPath to resolve the path to a complete name if possible. Otherwise it uses name directly.

[ianlancetaylor]

If you write exec.Command("ls") then I think the os/exec package should invoke /bin/ls passing "ls" as argv[0]. The user of os/exec should be able to control the argv[0] passed to the program. For your case I think you should be calling exec.LookPath("ssh") to get the value to pass to exec.Command.

If there is a bug here, I think it is only in the documentation.

[APTy]

Cool, just wanted to make sure that was indeed the intended behavior - it does make sense. In this case I agree a documentation fix will suffice. Thanks!

[mattn]

FYI (as you know), It's possible to pass different argument into path and argv[0] for execve. So that exec.LookPath just find command name from your $PATH.

For linux, you can get absolute path with readlink("/proc/self/exe", path, sizeof(path)-1).
For windows, GetModuleFileName. For macOS, _NSGetExecutablePath.

[gopherbot]

CL https://golang.org/cl/31710 mentions this issue.