libpod: fix /etc/hostname with --uts=host

when --uts=host is provided, the expectation is to use the hostname
from the host not the container name.

Closes: https://github.com/containers/podman/issues/20448

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

libpod/container.go

@@ -686,6 +686,23 @@ func (c *Container) Hostname() string {
 		return c.config.Spec.Hostname
 	}
 
+	// if the container is not running in a private UTS namespace,
+	// return the host's hostname.
+	privateUTS := false
+	if c.config.Spec.Linux != nil {
+		for _, ns := range c.config.Spec.Linux.Namespaces {
+			if ns.Type == spec.UTSNamespace {
+				privateUTS = true
+				break
+			}
+		}
+	}
+	if !privateUTS {
+		hostname, err := os.Hostname()
+		if err == nil {
+			return hostname
+		}
+	}
 	if len(c.ID()) < 11 {
 		return c.ID()
 	}

test/system/500-networking.bats

@@ -911,4 +911,15 @@ EOF
     run_podman network rm $net1
 }
 
+# Issue #20448 - /etc/hostname with --uts=host must show "uname -n"
+@test "podman --uts=host must use 'uname -n' for /etc/hostname" {
+    run_podman info --format '{{.Host.Hostname}}'
+    hostname="$output"
+    run_podman run --rm --uts=host $IMAGE cat /etc/hostname
+    assert "$output" = $hostname "/etc/hostname with --uts=host must be equal to 'uname -n'"
+
+    run_podman run --rm --net=host --uts=host $IMAGE cat /etc/hostname
+    assert "$output" = $hostname "/etc/hostname with --uts=host --net=host must be equal to 'uname -n'"
+}
+
 # vim: filetype=sh