Re: [PATCH v7 1/6] pidfd: add PIDFD_SELF* sentinels to refer to own thread/process

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

On Thu, Jan 30, 2025 at 08:40:26PM +0000, 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.
> 
> We adjust pidfd_get_task() and the pidfd_send_signal() system call with
> specific handling for this, implementing this functionality for
> process_madvise(), process_mrelease() (albeit, using it here wouldn't
> really make sense) and pidfd_send_signal().
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

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