ip: Exit exec in child process if setup fails

If we forked, returning from the function will make the calling code to
continue in both the child and parent process. Make cmd_exec exit if
setup failed and it forked already.

An example of issues this causes, where a failure in setup causes
multiple unnecessary tries:

```
$ ip netns
ef
ab
$ ip -all netns exec ls

netns: ef
setting the network namespace "ef" failed: Operation not permitted

netns: ab
setting the network namespace "ab" failed: Operation not permitted

netns: ab
setting the network namespace "ab" failed: Operation not permitted
```

Signed-off-by: Yedaya Katsman <yedaya.ka@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

1 files changed, 6 insertions, 1 deletions

diff --git a/lib/exec.c b/lib/exec.c
index 9b1c8f4a1..893937550 100644
--- a/lib/exec.c
+++ b/lib/exec.c
@@ -36,8 +36,13 @@ int cmd_exec(const char *cmd, char **argv, bool do_fork,
 		}
 	}
 
-	if (setup && setup(arg))
+	if (setup && setup(arg)) {
+		if (do_fork) {
+			/* In child, nothing to do */
+			_exit(1);
+		}
 		return -1;
+	}
 
 	if (execvp(cmd, argv)  < 0)
 		fprintf(stderr, "exec of \"%s\" failed: %s\n",