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

[Pedro Falcato <pedro.falcato@gmail.com>]

On Fri, Oct 25, 2024 at 6:41 PM John Hubbard <jhubbard@nvidia.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@oracle.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