linux-kernel - Re: [PATCH 1/2] pid: add pidfd_get

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Date:   Fri, 8 Oct 2021 10:47:36 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Christian Brauner <christian.brauner@...ntu.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Matthew Bobrowski <repnop@...gle.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Jan Kara <jack@...e.cz>, Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH 1/2] pid: add pidfd_get_task() helper

On 04.10.21 14:50, Christian Brauner wrote:
> The number of system calls making use of pidfds is constantly
> increasing. Some of those new system calls duplicate the code to turn a
> pidfd into task_struct it refers to. Give them a simple helper for this.
> 
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Suren Baghdasaryan <surenb@...gle.com>
> Cc: Matthew Bobrowski <repnop@...gle.com>
> Cc: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Minchan Kim <minchan@...nel.org>
> Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
> ---
>   include/linux/pid.h |  1 +
>   kernel/pid.c        | 34 ++++++++++++++++++++++++++++++++++
>   2 files changed, 35 insertions(+)
> 
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index af308e15f174..343abf22092e 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -78,6 +78,7 @@ struct file;
>   
>   extern struct pid *pidfd_pid(const struct file *file);
>   struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags);
> +struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags);
>   int pidfd_create(struct pid *pid, unsigned int flags);
>   
>   static inline struct pid *get_pid(struct pid *pid)
> diff --git a/kernel/pid.c b/kernel/pid.c
> index efe87db44683..2ffbb87b2ce8 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -539,6 +539,40 @@ struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags)
>   	return pid;
>   }
>   
> +/**
> + * pidfd_get_task() - Get the task associated with a pidfd
> + *
> + * @pidfd: pidfd for which to get the task
> + * @flags: flags associated with this pidfd
> + *
> + * Return the task associated with the given pidfd.
> + * Currently, the process identified by @pidfd is always a thread-group leader.
> + * This restriction currently exists for all aspects of pidfds including pidfd
> + * creation (CLONE_PIDFD cannot be used with CLONE_THREAD) and pidfd polling
> + * (only supports thread group leaders).
> + *
> + * Return: On success, the task_struct associated with the pidfd.
> + *	   On error, a negative errno number will be returned.

Nice doc.

You might want to document what callers of this function are expected to 
do to clean up.


> + */
> +struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
> +{
> +	unsigned int f_flags;
> +	struct pid *pid;
> +	struct task_struct *task;
> +
> +	pid = pidfd_get_pid(pidfd, &f_flags);
> +	if (IS_ERR(pid))
> +		return ERR_CAST(pid);
> +
> +	task = get_pid_task(pid, PIDTYPE_TGID);
> +	put_pid(pid);

The code to be replaced always does the put_pid() after the 
put_task_struct(). Is this new ordering safe? (didn't check)

> +	if (!task)
> +		return ERR_PTR(-ESRCH);
> +
> +	*flags = f_flags;
> +	return task;
> +}
> +
>   /**
>    * pidfd_create() - Create a new pid file descriptor.
>    *
> 

I'd have squashed this into the second patch, makes it a lot easier to 
review and it's only a MM cleanup at this point.

-- 
Thanks,

David / dhildenb