Re: [PATCH 1/2] pid: make sub-init creation retryable

[Andrei Vagin]

On Fri, Feb 27, 2026 at 4:04 AM Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>
> Currently we allow only one attempt to create init in a new namespace.
> If the first fork() fails after alloc_pid() succeeds, free_pid() clears
> PIDNS_ADDING and thus disables further PID allocations.
>
> Nowadays this looks like an unnecessary limitation. The original reason
> to handle "case PIDNS_ADDING" in free_pid() is gone, most probably after
> commit 69879c01a0c3 ("proc: Remove the now unnecessary internal mount of
> proc").
>
> Change free_pid() to keep ns->pid_allocated == PIDNS_ADDING, and change
> alloc_pid() to reset the cursor early, right after taking pidmap_lock.
>
> Test-case:
>
>         #define _GNU_SOURCE
>         #include <linux/sched.h>
>         #include <sys/syscall.h>
>         #include <sys/wait.h>
>         #include <assert.h>
>         #include <sched.h>
>         #include <errno.h>
>
>         int main(void)
>         {
>                 struct clone_args args = {
>                         .exit_signal = SIGCHLD,
>                         .flags  = CLONE_PIDFD,
>                         .pidfd  = 0,
>                 };
>                 unsigned long pidfd;
>                 int pid;
>
>                 assert(unshare(CLONE_NEWPID) == 0);
>
>                 pid = syscall(__NR_clone3, &args, sizeof(args));
>                 assert(pid == -1 && errno == EFAULT);
>
>                 args.pidfd = (unsigned long)&pidfd;
>                 pid = syscall(__NR_clone3, &args, sizeof(args));
>                 if (pid)
>                         assert(pid > 0 && wait(NULL) == pid);
>                 else
>                         assert(getpid() == 1);
>
>                 return 0;
>         }
>
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>

Acked-by: Andrei Vagin <avagin@xxxxxxxxx>

Thanks,
Andrei