Re: [PATCH v5 4/4] selftests: Add tests for creating pidns init via setns

[Christian Brauner <brauner@kernel.org>]

On Mon, Mar 16, 2026 at 12:34:18PM +0100, Pavel Tikhomirov wrote:
> 
> 
> On 3/13/26 12:09, Pavel Tikhomirov wrote:
> > First testcase "pidns_init_via_setns" checks that a process can become
> > Pid 1 (init) in a new Pid namespace created via unshare() and joined via
> > setns().
> > 
> > Second testcase "pidns_init_via_setns_set_tid" checks that during this
> > process we can use clone3() + set_tid and set the pid in both the new
> > and old pid namespaces (owned by different user namespaces).
> > 
> > Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
> > ---
> >  .../selftests/pid_namespace/.gitignore        |   1 +
> >  .../testing/selftests/pid_namespace/Makefile  |   2 +-
> >  .../pid_namespace/pidns_init_via_setns.c      | 238 ++++++++++++++++++
> >  3 files changed, 240 insertions(+), 1 deletion(-)
> >  create mode 100644 tools/testing/selftests/pid_namespace/pidns_init_via_setns.c
> > 
> > diff --git a/tools/testing/selftests/pid_namespace/.gitignore b/tools/testing/selftests/pid_namespace/.gitignore
> > index 5118f0f3edf4..c647c6eb3367 100644
> > --- a/tools/testing/selftests/pid_namespace/.gitignore
> > +++ b/tools/testing/selftests/pid_namespace/.gitignore
> > @@ -1,2 +1,3 @@
> >  pid_max
> > +pidns_init_via_setns
> >  regression_enomem
> > diff --git a/tools/testing/selftests/pid_namespace/Makefile b/tools/testing/selftests/pid_namespace/Makefile
> > index b972f55d07ae..b01a924ac04b 100644
> > --- a/tools/testing/selftests/pid_namespace/Makefile
> > +++ b/tools/testing/selftests/pid_namespace/Makefile
> > @@ -1,7 +1,7 @@
> >  # SPDX-License-Identifier: GPL-2.0
> >  CFLAGS += -g $(KHDR_INCLUDES)
> >  
> > -TEST_GEN_PROGS = regression_enomem pid_max
> > +TEST_GEN_PROGS = regression_enomem pid_max pidns_init_via_setns
> >  
> >  LOCAL_HDRS += $(selfdir)/pidfd/pidfd.h
> >  
> > diff --git a/tools/testing/selftests/pid_namespace/pidns_init_via_setns.c b/tools/testing/selftests/pid_namespace/pidns_init_via_setns.c
> > new file mode 100644
> > index 000000000000..7e4c610291d3
> > --- /dev/null
> > +++ b/tools/testing/selftests/pid_namespace/pidns_init_via_setns.c
> > @@ -0,0 +1,238 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +#define _GNU_SOURCE
> > +#include <fcntl.h>
> > +#include <sched.h>
> > +#include <stdio.h>
> > +#include <sys/types.h>
> > +#include <unistd.h>
> > +
> > +#include "kselftest_harness.h"
> > +#include "../pidfd/pidfd.h"
> > +
> > +/*
> > + * Test that a process can become PID 1 (init) in a new PID namespace
> > + * created via unshare() and joined via setns().
> > + *
> > + * Flow:
> > + *  1. Parent creates a pipe for synchronization.
> > + *  2. Parent forks a child.
> > + *  3. Parent calls unshare(CLONE_NEWPID) to create a new PID namespace.
> > + *  4. Parent signals the child via the pipe.
> > + *  5. Child opens parent's /proc/<ppid>/ns/pid_for_children and calls
> > + *     setns(fd, CLONE_NEWPID) to join the new namespace.
> > + *  6. Child forks a grandchild.
> > + *  7. Grandchild verifies getpid() == 1.
> > + */
> > +TEST(pidns_init_via_setns)
> > +{
> > +	pid_t child, parent_pid;
> > +	int pipe_fd[2];
> > +	char buf;
> > +
> > +	parent_pid = getpid();
> > +
> > +	ASSERT_EQ(0, pipe(pipe_fd));
> > +
> > +	child = fork();
> > +	ASSERT_GE(child, 0);
> > +
> > +	if (child == 0) {
> > +		char path[256];
> > +		int nsfd;
> > +		pid_t grandchild;
> > +
> > +		close(pipe_fd[1]);
> > +
> > +		/* Wait for parent to complete unshare */
> > +		ASSERT_EQ(1, read_nointr(pipe_fd[0], &buf, 1));
> > +		close(pipe_fd[0]);
> > +
> > +		snprintf(path, sizeof(path),
> > +			 "/proc/%d/ns/pid_for_children", parent_pid);
> > +		nsfd = open(path, O_RDONLY);
> > +		ASSERT_GE(nsfd, 0);
> > +
> > +		ASSERT_EQ(0, setns(nsfd, CLONE_NEWPID));
> > +		close(nsfd);
> > +
> > +		grandchild = fork();
> > +		ASSERT_GE(grandchild, 0);
> > +
> > +		if (grandchild == 0) {
> > +			/* Should be init (PID 1) in the new namespace */
> > +			if (getpid() != 1)
> > +				_exit(1);
> > +			_exit(0);
> > +		}
> > +
> > +		ASSERT_EQ(0, wait_for_pid(grandchild));
> > +		_exit(0);
> > +	}
> > +
> > +	close(pipe_fd[0]);
> > +
> > +	if (geteuid())
> > +		ASSERT_EQ(0, unshare(CLONE_NEWUSER));
> 
> I think it's better to move this hack above to the very top of the test.
> 
> Was found by https://sashiko.dev/#/patchset/20260313111014.2068913-1-ptikhomirov%40virtuozzo.com

Please resend and include a new entry for pid namespaces as a patch.
Make me a maintainer and yourself a reviewer, please.