Reproduction:

1. Make /etc/subuid a symlink
2. In your favourite shell run: newuidmap with any valid args, e.g. newuidmap $$ ...

The process will exit with 1 and no message printed.

strace shows the cause of the issue:

$ strace --failed-only -e trace=openat -- newuidmap $$
openat(AT_FDCWD, "/etc/subuid", O_RDONLY|O_NOCTTY|O_NONBLOCK|O_NOFOLLOW|O_CLOEXEC) = -1 ELOOP (Too many levels of symbolic links)
+++ exited with 1 +++

This is a bit painful to debug, I think it would be helpful if the program could print the message from the error, e.g.

newuidmap: cannot open /etc/subuid: Too many levels of symbolic links

Here's a naive patch that achieves this (a better solution would be using the standard logging process for this repo, but I didn't look too deep to see what those are)

diff --git i/src/newuidmap.c w/src/newuidmap.c
index 8deb06f0..e62649d0 100644
--- i/src/newuidmap.c
+++ w/src/newuidmap.c
@@ -139,6 +139,9 @@ int main(int argc, char **argv)
 	}
 
 	if (!sub_uid_open(O_RDONLY)) {
+		fprintf (stderr,
+		         _("%s: cannot open %s: %s\n"),
+		         Prog, sub_uid_dbname (), strerror (errno));
 		return EXIT_FAILURE;
 	}

My real-world use case: was running rootless docker (specifically the one packaged here: https://aur.archlinux.org/packages/docker-rootless-extras) with /etc/subuid as a symlink, and was left debugging from a log:

        <-- SNIP -->
        Mar 18 20:17:04 desktop dockerd-rootless.sh[14113]: + exec rootlesskit --state-dir=/run/user/1000/dockerd-rootless --net=slirp4netns --mtu=65520 --slirp4netns-sandbox=auto --slirp4netns-seccomp=auto --disable-host-loopback --port-driver=builtin --copy-up=/etc --copy-up=/run --propag>
        Mar 18 20:17:04 desktop dockerd-rootless.sh[14113]: [rootlesskit:parent] error: failed to setup UID/GID map: newuidmap 14124 [0 1000 1 1 165536 65536] failed: : exit status 1
        <-- SNIP -->

Debugging with the message, i.e. newuidmap 14124 [0 1000 1 1 165536 65536] failed: newuidmap: cannot open /etc/subuid: Too many levels of symbolic links: exit status 1 would've saved me some time