Add user: support to compose services

ComposeService::user/group (compose_file.{h,cpp}) parse a "user[:group]"
key, split on the first ':' the same way an image's own declared USER is
split (oci_image.cpp). start_compose_services() passes both straight
through to run_mounted_container()'s existing user/group parameters, which
already fall back to the image's own declared user when unset -- the same
default -r/--run itself has when --user isn't given.

Verified end to end on the real target machine (root, via the scoped doas
rule), checked via `ps -eo pid,ppid,uid,cmd` (not -x/--exec, see below):
the actual sandboxed command runs as the resolved uid/gid, matching plain
-r --user's own already-working behavior.

Also recorded in TODO.md: verifying this surfaced a real but unrelated
bug in resolve_namespace_pid() (sandbox_process.cpp), which -x/--exec's
own default-identity resolution uses -- it stops at bwrap's own pid-1
namespace supervisor instead of walking one level deeper to the real
(correctly priv-dropped) target, so `-x/--exec <pid> -- id` with no
explicit --user misreports root for a session that's actually running as
a non-root user the whole time. Not a regression from this change and not
fixed here.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gv3s5jckJKzh6JkMoi2Akz

CLAUDE.md

@@ -2678,9 +2678,14 @@ Source layout (all under `src/`):
      since Compose's own `ports:` syntax has none, so a later `-p`
      resolution picks the service's own sole `extern` network, the same
      "unqualified `-p`" default an ordinary CLI `-p` already has. An
-     explicit `--user`/`--group` still isn't wired up for compose services
+     explicit `--user`/`--group`, wired the same way: `user: "user[:group]"`
-     — every service runs as whatever user its own image declares, same as
+     (`ComposeService::user`/`group`, `compose_file.h` — split on the
-     `-r/--run`'s own default.
+     *first* `':'`, the exact same way an image's own declared `USER` is
+     split, `oci_image.cpp`'s own `read_oci_image_config()`) is passed
+     straight through to `run_mounted_container()`'s own `user`/`group`
+     parameters; when unset, that function already falls back to the
+     image's own declared user, the exact same default `-r/--run` has when
+     `--user` isn't given.
   5. `compose_state_file_path()`/`record_compose_services()` — write the
      compose file's own real, absolute path as a header line (needed
      because `compose_state_file_path()`'s own *filename* only encodes a

TODO.md

@@ -130,3 +130,32 @@ project's own state should stay root-only or move too. Not urgent -- this
 project's networking is already root-only in every other respect (bridges,
 iptables, etc.), so this is a narrower, lower-priority hardening pass for
 later, not a blocker for anything currently in progress.
+
+## `-x/--exec`/`--kill`'s `resolve_namespace_pid()` can stop one level too shallow when `--user` used priv-drop
+
+Found while verifying compose's new `user:` support (root, via the scoped
+`doas` rule): a session started with `--user` (root, `--unshare-pid` in
+effect, the default) has a real process tree of `outer bwrap` -> `bwrap's
+own pid-1 supervisor for the new pid namespace` (never itself execs into
+anything -- it's what keeps the namespace alive, since the kernel requires
+some process to be pid 1) -> the actual target, forked by that supervisor,
+which execs `slocker-lite-priv-drop` and then the real command. Confirmed
+via `ps -eo pid,ppid,uid,cmd` that the real command genuinely runs as the
+resolved uid/gid (priv-drop itself is correct, not the bug) -- but
+`resolve_namespace_pid()` (`sandbox_process.{h,cpp}`), used by both
+`-x/--exec`'s own default-identity resolution and `--kill`, stops at the
+pid-1 supervisor (its namespaces already differ from the true outer pid,
+satisfying that function's own single-hop comparison) instead of walking
+one level deeper to the real target. Concretely: `-x/--exec <pid> -- id`
+with no explicit `--user` on a session that resolved a non-root user via
+priv-drop incorrectly reports `root` (reads the supervisor's own
+still-root `/proc/.../status`), even though the session's real command is
+genuinely running as the resolved non-root user the whole time.
+
+**Not urgent**: `-x/--exec --user <explicit>` and `--kill` are unaffected
+in practice (`--kill`'s own cgroup/pid-namespace strategies already reach
+every process regardless of tree depth; only `-x/--exec`'s *default*
+identity guess is off). Needs a real fix to `resolve_namespace_pid()` (or
+a documented, narrower workaround) to correctly walk past bwrap's own
+pid-1 supervisor when `--unshare-pid` is in effect, rather than assuming
+the first namespace-differing child is the real target.

src/compose_file.cpp

@@ -518,6 +518,22 @@ std::optional<ComposeFile> load_compose_file(const std::filesystem::path& path)
             service.container_name = std::string(scalar_value(*container_name_node));
         }
 
+        if (const yaml_node_t* user_node = find_in_mapping(document, *value_node, "user")) {
+            if (user_node->type != YAML_SCALAR_NODE || scalar_value(*user_node).empty()) {
+                spdlog::error("compose file {}: service '{}' user must be a non-empty string", path.string(),
+                              service_name);
+                return std::nullopt;
+            }
+            std::string user_spec(scalar_value(*user_node));
+            auto colon = user_spec.find(':');
+            if (colon == std::string::npos) {
+                service.user = user_spec;
+            } else {
+                service.user = user_spec.substr(0, colon);
+                service.group = user_spec.substr(colon + 1);
+            }
+        }
+
         if (const yaml_node_t* command_node = find_in_mapping(document, *value_node, "command")) {
             auto command = parse_command(document, *command_node, service_name);
             if (!command) {

src/compose_file.h

@@ -52,6 +52,17 @@ struct ComposeService {
     std::string name;  // the mapping key under `services`
     std::string image;
     std::optional<std::string> container_name;
+    // Parsed from `user: "user[:group]"`, split on the *first* ':' exactly
+    // the same way an image's own declared `USER` is split
+    // (oci_image.cpp's own read_oci_image_config()). Both unset when the
+    // compose file gives no `user:` key at all -- matching `-r/--run`'s
+    // own default when `--user` isn't given, the image's own declared user
+    // is used instead (falling back further to root only if the image
+    // declares none either), handled automatically by
+    // run_mounted_container() (commands.h) since it already implements
+    // that exact fallback for `-r/--run` itself.
+    std::optional<std::string> user;
+    std::optional<std::string> group;
     // Always normalized to an argv list: Compose's own list form is used
     // as-is; its scalar shell-string form is wrapped as {"sh", "-c",
     // <string>}, matching Compose's "run through the image's shell"

src/compose_orchestrator.cpp

@@ -289,7 +289,7 @@ std::vector<ComposeStartedService> start_compose_services(const ComposeFile& com
         // any other service, which is why the only way out of this block
         // below is an explicit _exit(), never `continue`.
         int exit_code = run_mounted_container(image_tar, mounted, container_name, service.command, use_nsenter,
-                                               std::nullopt, std::nullopt, dns_hostname, volume_specs,
+                                               service.user, service.group, dns_hostname, volume_specs,
                                                service.environment_specs, true, network_specs, port_forward_specs,
                                                false, config);
         _exit(exit_code < 0 ? 1 : exit_code);

src/compose_orchestrator.h

@@ -154,9 +154,11 @@ struct ComposeStartedService {
 // never a network: prefix (Compose's own ports: syntax has no such
 // qualifier), so a later -p resolution picks the service's own sole
 // `extern` network, the same "unqualified -p" default an ordinary CLI
-// `-p` already has. An explicit --user/--group isn't wired up for compose
+// `-p` already has. `service.user`/`service.group` (ComposeService, parsed
-// services yet, though -- every service still runs as whatever user its
+// from a `user: "user[:group]"` key) are passed straight through to
-// own image declares, same as -r/--run's own default.
+// run_mounted_container()'s own `user`/`group` parameters -- when unset,
+// that function already falls back to the image's own declared user, the
+// exact same default `-r/--run` itself has when `--user` isn't given.
 //
 // Returns one ComposeStartedService per service that actually started --
 // fewer than compose.services on any partial failure (already logged

tests/unit/test_compose_file.cpp

@@ -125,6 +125,50 @@ TEST_CASE("compose file: container_name -- optional, and duplicates across servi
     CHECK_FALSE(load_compose_file(colliding_with_name).has_value());
 }
 
+TEST_CASE("compose file: user -- optional, with and without a group, split the same way as an image's own USER",
+          "[unit]") {
+    ScratchXdgDirs scratch;
+
+    auto unset = write_compose(scratch.path(),
+                                "services:\n"
+                                "  web:\n"
+                                "    image: busybox:latest\n");
+    auto loaded_unset = load_compose_file(unset);
+    REQUIRE(loaded_unset.has_value());
+    CHECK_FALSE(loaded_unset->services[0].user.has_value());
+    CHECK_FALSE(loaded_unset->services[0].group.has_value());
+
+    auto user_only = write_compose(scratch.path(),
+                                    "services:\n"
+                                    "  web:\n"
+                                    "    image: busybox:latest\n"
+                                    "    user: \"1000\"\n");
+    auto loaded_user_only = load_compose_file(user_only);
+    REQUIRE(loaded_user_only.has_value());
+    REQUIRE(loaded_user_only->services[0].user.has_value());
+    CHECK(*loaded_user_only->services[0].user == "1000");
+    CHECK_FALSE(loaded_user_only->services[0].group.has_value());
+
+    auto user_and_group = write_compose(scratch.path(),
+                                         "services:\n"
+                                         "  web:\n"
+                                         "    image: busybox:latest\n"
+                                         "    user: \"1000:1000\"\n");
+    auto loaded_both = load_compose_file(user_and_group);
+    REQUIRE(loaded_both.has_value());
+    REQUIRE(loaded_both->services[0].user.has_value());
+    CHECK(*loaded_both->services[0].user == "1000");
+    REQUIRE(loaded_both->services[0].group.has_value());
+    CHECK(*loaded_both->services[0].group == "1000");
+
+    auto empty_user = write_compose(scratch.path(),
+                                     "services:\n"
+                                     "  web:\n"
+                                     "    image: busybox:latest\n"
+                                     "    user: \"\"\n");
+    CHECK_FALSE(load_compose_file(empty_user).has_value());
+}
+
 TEST_CASE("compose file: command -- list form used as-is, scalar form wrapped in sh -c", "[unit]") {
     ScratchXdgDirs scratch;