Add regression test for the session-straggler sweep; resolve TODO entry

test_session_cleanup.cpp exercises kill_via_cgroup() directly against two
plain forked processes (one setsid()-ing away from the other before it
exits), confirming a reparented straggler is actually reaped -- reproducing
the real escape shape (no pid namespace support at all) through a full
mount/bwrap session isn't possible from the CLI on a single run, since
--unshare-pid is a config-file-only setting, not a flag.

Also resolves TODO.md's SIGINT/SIGTERM entry and extends the relevant
CLAUDE.md sections (bwrap.{h,cpp}, session_cgroup.{h,cpp}, kill_session.{h,cpp})
with the fix's rationale and its known residual limitation (a kernel with
neither cgroup v2 nor pid namespace support still can't be reached
automatically).

CLAUDE.md

@@ -405,6 +405,26 @@ Source layout (all under `src/`):
     matters. No production code changed for this; it's purely an artifact
     of forking network primitives from within a Catch2-instrumented
     process.
+  - `tests/integration/test_session_cleanup.cpp` (`[integration][root]`,
+    no `[net]`) — regression test for `run_bwrap()`'s own automatic
+    post-exit straggler sweep (`bwrap.cpp`/`session_cgroup.h`'s own
+    "Resolved" entry above). Deliberately exercises `kill_via_cgroup()`
+    (`kill_session.h`) directly against a real cgroup with two plain forked
+    processes (one standing in for the tracked bwrap pid, `setsid()`-ing
+    away a second before exiting) rather than through the full mount/bwrap
+    pipeline — reproducing the actual escape shape this fix targets (no pid
+    namespace support at all) through a real sandboxed session isn't
+    possible from the CLI on a single run (`--unshare-pid` is a
+    config-file-only `NamespaceConfig` field, not a flag), whereas the
+    mechanism actually under test — cgroup membership surviving
+    reparenting, and `kill_via_cgroup()` reaping it — needs no
+    container/image/bwrap involvement at all. Hit the exact same
+    Catch2-fatal-signal-handler-inheritance issue `test_root_networking.cpp`'s
+    own tap-relay test already found (the straggler process, forked from
+    this same Catch2-instrumented process, would otherwise catch its own
+    expected shutdown `SIGTERM` via the inherited handler and report a
+    spurious failure) — fixed the same way, resetting `SIGTERM` to
+    `SIG_DFL` right before forking the straggler.
   - `tests/support/fixtures.{h,cpp}` — `find_busybox_fixture()` (searches
     `images/busybox.tar` relative to cwd, this project's own established
     manual-testing convention; `nullopt` if absent, so `[net]` tests
@@ -543,7 +563,14 @@ Source layout (all under `src/`):
   see below) the instant the real `bwrap` pid is known, then calls
   `release_session_lock()` once `run_process_foreground()` returns (covering
   every exit path — normal, nonzero, or a forwarded-signal exit — since that call
-  always blocks until the child has actually exited).
+  always blocks until the child has actually exited). Between that return and
+  `release_session_lock()`/`remove_session_cgroup()`, `run_bwrap()` also
+  unconditionally sweeps the session's own cgroup (`session_cgroup_pids()`,
+  `kill_via_cgroup()` — `session_cgroup.h`/`kill_session.h`) for any process
+  still left in it, force-stopping it before cleanup proceeds — see
+  `session_cgroup.h`'s own "Resolved" entry for the full detail on why (a
+  daemonized/reparented straggler could otherwise outlive the session
+  regardless of how it ended).
   `build_bwrap_args()` no longer passes `--clearenv`/`--setenv` to `bwrap` itself;
   instead, `build_sandbox_env()` builds the sandboxed command's exact environment
   (`PATH`, `HOME`, `PWD` — hardcoded to `"/"`, matching `--chdir`'s own value; note
@@ -1700,16 +1727,69 @@ Source layout (all under `src/`):
   testing causes on the two environments this project actually runs on).
   `remove_session_cgroup()` (called from the same post-`run_process_foreground()`
   spot `release_session_lock()` already is) only succeeds once the cgroup is
-  empty — a straggler process still alive at normal exit (e.g. a daemonized
-  process that outlived the session's own main command, a pre-existing
-  exposure independent of this feature) leaves it in place with a warning, not
-  a fatal error. `session_cgroup_pids()` reads `cgroup.procs` — this is the
+  empty. `session_cgroup_pids()` reads `cgroup.procs` — this is the
   actual answer to "gather every process running inside the container":
   unlike anything derived from `/proc` parent-pid chains or pid namespaces,
   cgroup membership reliably includes every process the session ever started.
   `session_cgroup_supports_kill()`/`kill_session_cgroup()` wrap the
   `cgroup.kill` knob (Linux 5.14+): writing `"1"` to it atomically `SIGKILL`s
   every process currently in the cgroup in one step.
+
+  **Resolved: a daemonized/escaped straggler could survive the session
+  ending, regardless of how it ended.** Reported by the user (Ctrl-C on a
+  foreground session could leave processes running if they'd created a new
+  session of their own — `forward_signal_to_foreground_child()`, `process.cpp`,
+  only ever forwards the signal to the single tracked bwrap pid) — and, on
+  reflection, a *normal* exit (or `-D/--daemonize`) had the exact same gap,
+  since nothing ever swept the session's own cgroup automatically in any of
+  those cases; only an explicit, separate `--kill` did. Fixed not in the
+  signal handler itself (genuinely awkward: `kill_session()`'s own
+  cgroup-first strategy selection does blocking polling/`waitpid()`s, unsafe
+  from a signal handler, and re-deriving the session there via
+  `list_sessions()` would see `run_bwrap()`'s own still-open `SessionLock` fd
+  as "still running", since `flock()` ownership is per open file
+  description, not per process) but in `run_bwrap()` (`bwrap.cpp`) itself:
+  `kill_via_cgroup()` (previously `kill_session.cpp`-local) was exported
+  (`kill_session.h`) and is now called directly, right after
+  `run_process_foreground()` returns and before `remove_session_cgroup()`
+  runs, whenever `session_cgroup_pids()` shows anything still left —
+  unconditionally, regardless of *why* `run_process_foreground()` just
+  returned (normal exit, or bwrap forwarding a caught SIGINT/SIGTERM). Since
+  a dead process is removed from its own cgroup automatically, bwrap's own
+  pid is already gone from `cgroup.procs` by that point, so this sweep only
+  ever finds genuine leftover processes, never bwrap itself. `-D/--daemonize`
+  needed no special-casing at all: it re-enters this exact same `run_bwrap()`
+  call from within its own already-forked/`setsid()`'d child, so the sweep
+  runs there too, for free — there's only ever the one call site. The grace
+  period used here (`straggler_grace_period_seconds`, `bwrap.cpp`, a
+  file-local constant) is deliberately much shorter than `kill_session()`'s
+  own manual `--kill` default (3s vs. 10s): this runs on *every*
+  `run_bwrap()` return, so the overwhelmingly common zero-stragglers case
+  must stay instant (it does — `session_cgroup_pids()` returning empty
+  short-circuits `kill_via_cgroup()`'s own `poll_until()` immediately, no
+  delay at all), while a genuine straggler still gets a brief chance to exit
+  gracefully before being force-killed. **Verified on this dev machine, root,
+  via the scoped `doas` rule**: a new `[integration][root]` regression test
+  (`tests/integration/test_session_cleanup.cpp`) confirms `kill_via_cgroup()`
+  actually reaps a process that forks, `setsid()`s away, and outlives its own
+  parent — deliberately exercised directly against a real cgroup with two
+  plain forked processes rather than through the full mount/bwrap pipeline,
+  since the actual escape shape under test (a kernel with no pid namespace
+  support at all, so a daemonizing process reparents completely outside any
+  namespace) isn't something a single `-r/--run` invocation can force via the
+  CLI — `--unshare-pid` is a config-file-only `NamespaceConfig` field, not a
+  flag. On a kernel that *does* support pid namespaces (this dev machine
+  included), the default case already gets equivalent protection for free
+  straight from the kernel — killing a pid namespace's own pid 1, whether via
+  a normal exit or a forced kill, collapses the whole namespace regardless of
+  this fix — so this sweep's real-world benefit is concentrated on kernels
+  like the real target device's own, which has neither pid namespace nor (as
+  of this writing) confirmed cgroup delegation; on-device re-verification of
+  both is still needed (see `TODO.md`). **Known residual limitation,
+  unchanged**: a kernel with *neither* cgroup v2 nor pid namespace support
+  still has no automatic way to reach a reparented straggler — the same
+  fundamental gap `kill_via_tracked_pid()` (the weakest of `--kill`'s own
+  three strategies, below) already represents.
 - `sandbox_process.{h,cpp}` — process-tree/namespace-resolution utilities
   shared by `exec_session.{h,cpp}` and `kill_session.{h,cpp}` (see both
   below); pulled into their own file (rather than staying private to
@@ -1919,6 +1999,11 @@ Source layout (all under `src/`):
      not the original snapshot, since a process could have forked a new child
      after the graceful sweep but before dying). The **only** mechanism that
      reliably reaches every process regardless of pid namespace support.
+     Exported (moved out of this file's own anonymous namespace, declared in
+     `kill_session.h`) since `run_bwrap()` (`bwrap.cpp`) reuses it directly
+     for its own automatic post-exit straggler sweep — see `session_cgroup.h`'s
+     own "Resolved" entry above for why that caller calls this directly
+     rather than going through `kill_session(pid)` itself.
      **Critical correctness point, caught during design review before this
      shipped**: the "is it stopped yet" poll must gate on the *cgroup being
      empty*, not `list_sessions()`'s running flag — that flag only reflects

TODO.md

@@ -1,35 +1,63 @@
 # TODO
 
-## Ctrl-C/SIGTERM on a foreground session doesn't reach everything --kill does
+## Resolved: Ctrl-C/SIGTERM (or a normal exit) could leave session stragglers running
 
-Reported by the user while device-testing the signal-exit-logging fix (not
-yet reproduced/confirmed with a specific repro, just the architectural gap):
-`forward_signal_to_foreground_child()` (`process.cpp`, installed as the
-SIGINT/SIGTERM handler around `run_process_foreground()`'s own `waitpid()`)
-does a single, simple `kill(g_foreground_child_pid, sig)` -- only the one
-pid `run_bwrap()` originally tracked (bwrap's own outer process). If
-anything inside the sandbox daemonizes/double-forks and calls `setsid()`
+Reported by the user while device-testing the signal-exit-logging fix, then
+fixed on request: `forward_signal_to_foreground_child()` (`process.cpp`,
+installed as the SIGINT/SIGTERM handler around `run_process_foreground()`'s
+own `waitpid()`) only ever does a single, simple `kill(g_foreground_child_pid,
+sig)` -- just the one pid `run_bwrap()` tracks (bwrap's own outer process).
+If anything inside the sandbox daemonizes/double-forks and calls `setsid()`
 (escaping into a new session, the same shape `--kill`'s own investigation
 found for a real `php-fpm --daemonize` + `exec caddy` init script -- see
-`kill_session.{h,cpp}`'s own three-strategy design in `CLAUDE.md`), it
-escapes this simple forward entirely and is left running after Ctrl-C ends
-the foreground session, even though `--kill` against the exact same session
-would reach it (via `kill_via_cgroup()`, or `kill_via_pid_namespace()` where
-supported).
+`kill_session.{h,cpp}`'s own three-strategy design in `CLAUDE.md`), that
+signal forward alone could never reach it -- and, on reflection, a *normal*
+exit (or `-D/--daemonize`) had exactly the same gap, since nothing swept the
+session's own leftovers in any of those cases either.
 
-**Likely fix, not yet designed in detail**: reuse `kill_session()`'s own
-strategy selection (cgroup-first, falling back to pid-namespace or the
-tracked-pid-only approach per host capability) from
-`forward_signal_to_foreground_child()` too, instead of a bare `kill()` --
-the session's own cgroup (`session_cgroup.h`) and tracked pid are already
-known by the time `run_process_foreground()` is running, so the pieces
-`kill_session()` needs should already be available without major
-restructuring. Signal-safety needs care: `kill_session()` as it exists
-today does blocking work (polling, `waitpid()`s) that isn't safe to call
-directly from a signal handler -- likely needs the handler to just set a
-flag/write to a self-pipe and have the actual `kill_session()`-style sweep
-happen back in `run_process_foreground()`'s own normal control flow once
-`waitpid()` returns, rather than doing it inside the handler itself.
+**Fix, per the user's own suggested direction**: rather than teaching the
+signal handler itself to do `kill_session()`-style work (genuinely awkward --
+`kill_session()` does blocking polling/`waitpid()`s, unsafe to call directly
+from a signal handler, and re-derives the session via `list_sessions()`,
+which would see `run_bwrap()`'s own still-open `SessionLock` fd as "still
+running" since `flock()` ownership is per open file description, not per
+process), `kill_via_cgroup()` (`kill_session.cpp`'s own cgroup-based
+strategy) was exported and is now called directly from `run_bwrap()` itself,
+right after `run_process_foreground()` returns and before the session
+cgroup is torn down -- unconditionally, regardless of *why* it returned
+(normal exit, a forwarded SIGINT/SIGTERM), so a container that daemonizes
+and exits successfully can no longer leave a background process running
+either. `-D/--daemonize` needed no special-casing at all: it re-enters this
+exact same `run_bwrap()` call from within its own already-forked/`setsid()`'d
+child, so the sweep runs there too, for free. See `bwrap.{h,cpp}`'s and
+`kill_session.{h,cpp}`'s own entries in `CLAUDE.md` for the full detail.
+
+**Verified on this dev machine, root, via the scoped `doas` rule**: a new
+`[integration][root]` regression test
+(`tests/integration/test_session_cleanup.cpp`) confirms `kill_via_cgroup()`
+actually reaps a process that forks, `setsid()`s away, and outlives its own
+parent's exit -- the exact escape shape this fix targets -- and a live
+`-r/--run` session backgrounding a long-running process and exiting
+normally left no leftover process and no leftover cgroup directory
+afterward. This dev machine's own kernel supports pid namespaces, though,
+so the *default* case (`--unshare-pid` requested) already gets this for
+free from the kernel itself (killing a pid namespace's own pid 1 -- whether
+via a normal exit or a forced kill -- collapses the whole namespace
+regardless of this fix); the scenario this fix specifically targets --
+`--unshare-pid` unavailable or disabled, so a daemonizing process reparents
+completely outside any namespace -- couldn't be exercised end-to-end
+locally (no CLI-level way to force that off for a single run; it's a
+config-file-only `NamespaceConfig` field), and needs on-device
+confirmation, same as everything else in this project that depends on the
+real target's own kernel capabilities. Whether the real device's cgroup v2
+is actually writable as root (the case that matters there) also hasn't
+been specifically re-verified yet.
+
+**Known residual limitation**: this only helps when `create_session_cgroup()`
+actually succeeded (cgroup v2 mounted and writable). A kernel with *neither*
+cgroup v2 nor pid namespace support still has no way to reach a reparented
+straggler automatically -- the same fundamental gap `kill_via_tracked_pid()`
+(the weakest of `--kill`'s own three strategies) already represents.
 
 ## Resolved: config/state paths sometimes resolved relative to cwd
 

meson.build

@@ -31,6 +31,7 @@ if get_option('enable_tests')
         'tests/integration/test_config_bwrap_chain.cpp',
         'tests/integration/test_rootless_run.cpp',
         'tests/integration/test_root_networking.cpp',
+        'tests/integration/test_session_cleanup.cpp',
     ]
 endif
 

tests/integration/test_session_cleanup.cpp

@@ -0,0 +1,134 @@
+// Copyright (C) 2026 Viorel Munteanu
+//
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 2 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this program; if not, write to the Free Software Foundation, Inc.,
+// 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+// [integration][root]: regression test for run_bwrap()'s own automatic
+// post-exit straggler sweep (bwrap.cpp) -- exercises the exported
+// kill_via_cgroup() (kill_session.h) directly against a real cgroup, rather
+// than going through the full mount/bwrap pipeline. That's a deliberate
+// choice, not a shortcut: the actual bug this targets is a process that
+// daemonizes/double-forks and setsid()'s away, reparenting outside the
+// tracked process's own tree (e.g. no pid namespace support at all, the
+// real target device's own kernel) -- reproducing that specific escape
+// through a real bwrap sandbox would need to fake the kernel's own pid
+// namespace absence (not controllable from the CLI at all -- it's a
+// config-file-only NamespaceConfig field, see bwrap.h), whereas the actual
+// mechanism under test (cgroup membership surviving reparenting, and
+// kill_via_cgroup() reaping it) is fully exercised with two plain forked
+// processes and no container/image/bwrap involved at all.
+
+#include <unistd.h>
+
+#include <csignal>
+#include <string>
+
+#include <sys/wait.h>
+
+#include <catch2/catch_test_macros.hpp>
+
+#include "kill_session.h"
+#include "session_cgroup.h"
+
+TEST_CASE("session cgroup sweep: kill_via_cgroup reaps a reparented straggler", "[integration][root]") {
+    if (geteuid() != 0) {
+        SKIP("requires root");
+    }
+
+    const std::string container_name = "selftest-cgroup-sweep";
+
+    // sync_pipe makes the child wait until it's actually been moved into the
+    // session cgroup before it forks its own straggler -- otherwise the
+    // straggler could be forked (and thus inherit cgroup membership) before
+    // create_session_cgroup() below has run, racing the exact same way
+    // run_bwrap()'s own on_start callback races against bwrap forking its
+    // own children.
+    int sync_pipe[2];
+    REQUIRE(pipe(sync_pipe) == 0);
+
+    pid_t tracked_pid = fork();
+    REQUIRE(tracked_pid >= 0);
+    if (tracked_pid == 0) {
+        close(sync_pipe[1]);
+        char buf = 0;
+        ssize_t unused = read(sync_pipe[0], &buf, 1);
+        (void)unused;
+        close(sync_pipe[0]);
+
+        // Reset to the default disposition before forking: the straggler
+        // below is expected to receive a real SIGTERM later (from
+        // kill_via_cgroup()) and terminate via it -- Catch2 installs its own
+        // fatal-signal handler around a running TEST_CASE, which a forked
+        // child inherits (only the disposition *at fork time* matters), so
+        // without this reset the straggler's own ordinary shutdown signal
+        // gets caught by that inherited handler instead, producing a
+        // spurious "FAILED ... due to a fatal error condition: SIGTERM"
+        // report interleaved into this test's real output -- the exact same
+        // lesson test_root_networking.cpp's tap-relay test already learned
+        // for its own forked relay child. Confirmed cosmetic only when hit:
+        // the assertions below still ran and passed correctly either way.
+        signal(SIGTERM, SIG_DFL);
+
+        // Mirrors a container process that daemonizes and exits successfully
+        // (or is signaled and dies) while a backgrounded child of its own
+        // keeps running, detached into a new session -- the exact shape
+        // that used to escape bwrap's own pid tree with nothing left to
+        // reap it.
+        pid_t straggler = fork();
+        if (straggler == 0) {
+            setsid();
+            pause();
+            _exit(0);
+        }
+        _exit(0);
+    }
+    close(sync_pipe[0]);
+
+    // Mirrors create_session_cgroup() being called from run_bwrap()'s own
+    // on_start callback, the instant the tracked pid is known -- before it's
+    // had any real chance to fork further.
+    auto cgroup = create_session_cgroup(container_name, tracked_pid);
+    if (!cgroup) {
+        char go = 1;
+        ssize_t unused = write(sync_pipe[1], &go, 1);
+        (void)unused;
+        close(sync_pipe[1]);
+        int status = 0;
+        waitpid(tracked_pid, &status, 0);
+        SKIP("could not create a session cgroup on this host (see session_cgroup.h)");
+    }
+
+    char go = 1;
+    REQUIRE(write(sync_pipe[1], &go, 1) == 1);
+    close(sync_pipe[1]);
+
+    int status = 0;
+    // Mirrors run_process_foreground()'s own waitpid() on the tracked pid --
+    // by the time this returns, `tracked_pid` is already gone from
+    // cgroup.procs (a dead process is removed from its cgroup
+    // automatically), leaving only the straggler behind, exactly like a
+    // real bwrap exit would.
+    waitpid(tracked_pid, &status, 0);
+
+    auto before = session_cgroup_pids(*cgroup);
+    // Confirms the straggler really did escape into the cgroup in the first
+    // place -- otherwise the kill_via_cgroup() check below would trivially
+    // "pass" against an already-empty cgroup and prove nothing.
+    REQUIRE_FALSE(before.empty());
+
+    CHECK(kill_via_cgroup(*cgroup, /*grace_period_seconds=*/2));
+    CHECK(session_cgroup_pids(*cgroup).empty());
+
+    remove_session_cgroup(*cgroup);
+}