Re: [PATCH V6 06/10] fork: add helpers to clone a process for kernel use

Eric W. Biederman
2021-12-17T18:53:19Z

Mike Christie <michael.christie@oracle.com> writes:

> The vhost layer is creating kthreads to execute IO and management
> operations. These threads need to share a mm with a userspace thread,
> inherit cgroups, and we would like to have the thread accounted for
> under the userspace thread's rlimit nproc value so a user can't overwhelm
> the system with threads when creating VMs.
>
> We have helpers for cgroups and mm but not for the rlimit nproc and in
> the future we will probably want helpers for things like namespaces. For
> those two items and to allow future sharing/inheritance, this patch adds
> two helpers, user_worker_create and user_worker_start that allow callers
> to create threads that copy or inherit the caller's attributes like mm,
> cgroups, namespaces, etc, and are accounted for under the callers rlimits
> nproc value similar to if the caller did a clone() in userspace. However,
> instead of returning to userspace the thread is usable in the kernel for
> modules like vhost or layers like io_uring.

If you are making this a general API it would be good to wrap the called
function the way kthread_create does so that the code in the function
can just return and let the wrapper call do_exit for it, especially if
you are going to have modular users.

There is a lot of deep magic in what happens if a thread created with
kernel_thread returns. It makes sense to expose that magic to the 1 or
2 callers that use kernel_thread directly. It does not make sense to
expose to anything higher up and in creating a nice API you are doing
that.

Currently I have just removed all of the modular users of do_exit
and in the process of removing do_exit itself so I am a little more
sensitive to this than I would ordinarily be. But I think my comment
stands even without my changes you conflict with.

Eric

> [added flag validation code from Christian Brauner's SIG_IGN patch]
> Signed-off-by: Mike Christie <michael.christie@oracle.com>
> Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
> include/linux/sched/task.h | 5 +++
> kernel/fork.c | 72 ++++++++++++++++++++++++++++++++++++++
> 2 files changed, 77 insertions(+)
>
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index f8a658700075..ecb21c0d95ce 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -95,6 +95,11 @@ struct mm_struct *copy_init_mm(void);
> extern pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags);
> extern long kernel_wait4(pid_t, int __user *, int, struct rusage *);
> int kernel_wait(pid_t pid, int *stat);
> +struct task_struct *user_worker_create(int (*fn)(void *), void *arg, int node,
> + unsigned long clone_flags,
> + u32 worker_flags);
> +__printf(2, 3)
> +void user_worker_start(struct task_struct *tsk, const char namefmt[], ...);
>
> extern void free_task(struct task_struct *tsk);
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index c9152596a285..e72239ae1e08 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2543,6 +2543,78 @@ struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node)
> return copy_process(NULL, 0, node, &args);
> }
>
> +static bool user_worker_flags_valid(struct kernel_clone_args *kargs)
> +{
> + /* Verify that no unknown flags are passed along. */
> + if (kargs->worker_flags & ~(USER_WORKER_IO | USER_WORKER |
> + USER_WORKER_NO_FILES | USER_WORKER_SIG_IGN))
> + return false;
> +
> + /*
> + * If we're ignoring all signals don't allow sharing struct sighand and
> + * don't bother clearing signal handlers.
> + */
> + if ((kargs->flags & (CLONE_SIGHAND | CLONE_CLEAR_SIGHAND)) &&
> + (kargs->worker_flags & USER_WORKER_SIG_IGN))
> + return false;
> +
> + return true;
> +}
> +
> +/**
> + * user_worker_create - create a copy of a process to be used by the kernel
> + * @fn: thread stack
> + * @arg: data to be passed to fn
> + * @node: numa node to allocate task from
> + * @clone_flags: CLONE flags
> + * @worker_flags: USER_WORKER flags
> + *
> + * This returns a created task, or an error pointer. The returned task is
> + * inactive, and the caller must fire it up through user_worker_start(). If
> + * this is an PF_IO_WORKER all singals but KILL and STOP are blocked.
> + */
> +struct task_struct *user_worker_create(int (*fn)(void *), void *arg, int node,
> + unsigned long clone_flags,
> + u32 worker_flags)
> +{
> + struct kernel_clone_args args = {
> + .flags = ((lower_32_bits(clone_flags) | CLONE_VM |
> + CLONE_UNTRACED) & ~CSIGNAL),
> + .exit_signal = (lower_32_bits(clone_flags) & CSIGNAL),
> + .stack = (unsigned long)fn,
> + .stack_size = (unsigned long)arg,
> + .worker_flags = USER_WORKER | worker_flags,
> + };
> +
> + if (!user_worker_flags_valid(&args))
> + return ERR_PTR(-EINVAL);
> +
> + return copy_process(NULL, 0, node, &args);
> +}
> +EXPORT_SYMBOL_GPL(user_worker_create);
> +
> +/**
> + * user_worker_start - Start a task created with user_worker_create
> + * @tsk: task to wake up
> + * @namefmt: printf-style format string for the thread name
> + * @arg: arguments for @namefmt
> + */
> +void user_worker_start(struct task_struct *tsk, const char namefmt[], ...)
> +{
> + char name[TASK_COMM_LEN];
> + va_list args;
> +
> + WARN_ON(!(tsk->flags & PF_USER_WORKER));
> +
> + va_start(args, namefmt);
> + vsnprintf(name, sizeof(name), namefmt, args);
> + set_task_comm(tsk, name);
> + va_end(args);
> +
> + wake_up_new_task(tsk);
> +}
> +EXPORT_SYMBOL_GPL(user_worker_start);
> +
> /*
> * Ok, this is the main fork-routine.
> *