pasta: Do not configure ID mappings when invoked with --netns-only

Add a `bool` argument `config_idmaps` to `pasta_start_ns()` that guards
the logic to configure user and group ID mappings. It is set to `false`
when `netns_only` is `true`. Fixes bug 216.

Link: https://bugs.passt.top/show_bug.cgi?id=216
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Dwayne B. Bent <dbb@dbb.dev>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

3 files changed, 17 insertions, 12 deletions

diff --git a/conf.c b/conf.c
index 0fcba5c..2223604 100644
--- a/conf.c
+++ b/conf.c
@@ -1945,7 +1945,7 @@ void conf(struct ctx *c, int argc, char **argv)
 		if (*netns) {
 			pasta_open_ns(c, netns);
 		} else {
-			pasta_start_ns(c, uid, gid,
+			pasta_start_ns(c, uid, gid, !netns_only,
 				       argc - optind, argv + optind);
 		}
 	}
diff --git a/pasta.c b/pasta.c
index 4e7ee54..5aa56b7 100644
--- a/pasta.c
+++ b/pasta.c
@@ -236,10 +236,11 @@ static int pasta_spawn_cmd(void *arg)
  * @c:		Execution context
  * @uid:	UID we're running as in the init namespace
  * @gid:	GID we're running as in the init namespace
+ * @config_idmaps:	Whether to configure user mappings
  * @argc:	Number of arguments for spawned command
  * @argv:	Command to spawn and arguments
  */
-void pasta_start_ns(struct ctx *c, uid_t uid, gid_t gid,
+void pasta_start_ns(struct ctx *c, uid_t uid, gid_t gid, bool config_idmaps,
 		    int argc, char *argv[])
 {
 	char ns_fn_stack[NS_FN_STACK_SIZE]
@@ -249,7 +250,6 @@ void pasta_start_ns(struct ctx *c, uid_t uid, gid_t gid,
 		.argv = argv,
 		.c = c,
 	};
-	char uidmap[BUFSIZ], gidmap[BUFSIZ];
 	char *sh_argv[] = { NULL, NULL };
 	char sh_arg0[PATH_MAX + 1];
 	sigset_t set;
@@ -259,16 +259,20 @@ void pasta_start_ns(struct ctx *c, uid_t uid, gid_t gid,
 		c->quiet = 1;
 
 	/* Configure user and group mappings */
-	if (snprintf_check(uidmap, BUFSIZ, "0 %u 1", uid))
-		die_perror("Can't build uidmap");
+	if (config_idmaps) {
+		char uidmap[BUFSIZ], gidmap[BUFSIZ];
 
-	if (snprintf_check(gidmap, BUFSIZ, "0 %u 1", gid))
-		die_perror("Can't build gidmap");
+		if (snprintf_check(uidmap, BUFSIZ, "0 %u 1", uid))
+			die_perror("Can't build uidmap");
 
-	if (write_file("/proc/self/uid_map", uidmap) ||
-	    write_file("/proc/self/setgroups", "deny") ||
-	    write_file("/proc/self/gid_map", gidmap)) {
-		warn("Couldn't configure user mappings");
+		if (snprintf_check(gidmap, BUFSIZ, "0 %u 1", gid))
+			die_perror("Can't build gidmap");
+
+		if (write_file("/proc/self/uid_map", uidmap) ||
+		    write_file("/proc/self/setgroups", "deny") ||
+		    write_file("/proc/self/gid_map", gidmap)) {
+			warn("Couldn't configure user mappings");
+		}
 	}
 
 	if (argc == 0) {
diff --git a/pasta.h b/pasta.h
index 07e04b3..edd7747 100644
--- a/pasta.h
+++ b/pasta.h
@@ -6,12 +6,13 @@
 #ifndef PASTA_H
 #define PASTA_H
 
+#include <stdbool.h>
 #include <unistd.h>
 
 extern int pasta_child_pid;
 
 void pasta_open_ns(struct ctx *c, const char *netns);
-void pasta_start_ns(struct ctx *c, uid_t uid, gid_t gid,
+void pasta_start_ns(struct ctx *c, uid_t uid, gid_t gid, bool config_idmaps,
 		    int argc, char *argv[]);
 void pasta_ns_conf(struct ctx *c);
 void pasta_child_handler(int signal);