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Message-ID: <nw5xnaile4phgaqxf77jhqlguscgicfwsfspgtxeuronrvwqai@xykuanoemqay>
Date: Tue, 4 Feb 2025 08:51:07 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Christian Brauner <christian@...uner.io>, 
	Shuah Khan <shuah@...nel.org>, "Liam R . Howlett" <Liam.Howlett@...cle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, pedro.falcato@...il.com, 
	linux-kselftest@...r.kernel.org, linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, 
	linux-api@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Oliver Sang <oliver.sang@...el.com>, John Hubbard <jhubbard@...dia.com>, Tejun Heo <tj@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Koutny <mkoutny@...e.com>, 
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v7 1/6] pidfd: add PIDFD_SELF* sentinels to refer to own
 thread/process

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@...cle.com>

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

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