linux-kernel - Re: [PATCH v5 2/5] pidfd: add PIDFD_SELF_* sentinels to refer to own thread/process

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Message-ID: <CAKbZUD0fxczjSJo34MnWRNT4M6HTfWN0DRXr9CFe_+cKJW_mog@mail.gmail.com>
Date: Fri, 25 Oct 2024 19:38:59 +0100
From: Pedro Falcato <pedro.falcato@...il.com>
To: John Hubbard <jhubbard@...dia.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 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>, 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>
Subject: Re: [PATCH v5 2/5] pidfd: add PIDFD_SELF_* sentinels to refer to own thread/process

On Fri, Oct 25, 2024 at 6:41 PM John Hubbard <jhubbard@...dia.com> wrote:
>
> On 10/25/24 5:50 AM, Pedro Falcato wrote:
> > On Fri, Oct 25, 2024 at 10:41 AM Lorenzo Stoakes
> > <lorenzo.stoakes@...cle.com> wrote:
> ...
> >> +static inline int pidfd_is_self_sentinel(pid_t pid)
> >> +{
> >> +       return pid == PIDFD_SELF_THREAD || pid == PIDFD_SELF_THREAD_GROUP;
> >> +}
> >
> > Do we want this in the uapi header? Even if this is useful, it might
> > come with several drawbacks such as breaking scripts that parse kernel
> > headers (and a quick git grep suggests we do have static inlines in
> > headers, but in rather obscure ones) and breaking C89:
> >
>
> Let's please not say "C89" anymore, we've moved on! :)
>
> The notes in [1], which is now nearly 2.5 years old, discuss the move to
> C11, and specifically how to handle the inline keyword.

That seems to only apply to the kernel internally, uapi headers are
included from userspace too (-std=c89 -pedantic doesn't know what a
gnu extension is). And uapi headers _generally_ keep to defining
constants and structs, nothing more.
I don't know what the guidelines for uapi headers are nowadays, but we
generally want to not break userspace.

>
> I think it's quite clear at this point, that we should not hold up new
> work, based on concerns about handling the inline keyword, nor about
> C89.

Right, but the correct solution is probably to move
pidfd_is_self_sentinel to some other place, since it's not even
supposed to be used by userspace (it's semantically useless to
userspace, and it's only two users are in the kernel, kernel/pid.c and
exit.c).

-- 
Pedro