Re: [PATCH v4 2/4] pidfd: add PIDFD_SELF_* sentinels to refer to own thread/process

[Shakeel Butt <shakeel.butt@linux.dev>]

On Thu, Oct 17, 2024 at 10:05:50PM GMT, Lorenzo Stoakes wrote:
> It is useful to be able to utilise the pidfd mechanism to reference the
> current thread or process (from a userland point of view - thread group
> leader from the kernel's point of view).
> 
> Therefore introduce PIDFD_SELF_THREAD to refer to the current thread, and
> PIDFD_SELF_THREAD_GROUP to refer to the current thread group leader.
> 
> For convenience and to avoid confusion from userland's perspective we alias
> these:
> 
> * PIDFD_SELF is an alias for PIDFD_SELF_THREAD - This is nearly always what
>   the user will want to use, as they would find it surprising if for
>   instance fd's were unshared()'d and they wanted to invoke pidfd_getfd()
>   and that failed.
> 
> * PIDFD_SELF_PROCESS is an alias for PIDFD_SELF_THREAD_GROUP - Most users
>   have no concept of thread groups or what a thread group leader is, and
>   from userland's perspective and nomenclature this is what userland
>   considers to be a process.

Should users use PIDFD_SELF_PROCESS in process_madvise() for self
madvise() (once the support is added)?

> 
[...]
>  
> +static struct pid *pidfd_get_pid_self(unsigned int pidfd, unsigned int *flags)
> +{
> +	bool is_thread = pidfd == PIDFD_SELF_THREAD;
> +	enum pid_type type = is_thread ? PIDTYPE_PID : PIDTYPE_TGID;
> +	struct pid *pid = *task_pid_ptr(current, type);
> +
> +	/* The caller expects an elevated reference count. */
> +	get_pid(pid);

Do you want this helper to work for scenarios where pid is used across
context? Otherwise can't we get rid of this get and later put for self?

> +	return pid;
> +}
> +

Overall looks good to me.

Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>