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Date:   Sat, 20 Mar 2021 20:39:09 +0300
From:   Alexey Dobriyan <adobriyan@gmail.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH] Document that PF_KTHREAD _is_ ABI
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 <CALCETrUPAvUOr8V5db0gu5RKVftKFwbBEkh6Aob57v+D-xdEig@mail.gmail.com>
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On Sat, Mar 20, 2021 at 10:23:12AM -0700, Andy Lutomirski wrote:
> > On Mar 20, 2021, at 9:31 AM, Alexey Dobriyan <adobriyan@gmail.com> wrote:
> >
> > ﻿PF_KTHREAD value is visible via field number 9 of /proc/*/stat
> >
> >    $ sudo cat /proc/2/stat
> >    2 (kthreadd) S 0 0 0 0 -1 2129984 0 ...
> >                  ^^^^^^^
> >
> > It is used by at least systemd to check for kernel-threadness:
> > https://github.com/systemd/systemd/blob/main/src/basic/process-util.c#L354
> > src/basic/process-util.c:is_kernel_thread()
> 
> Eww.
> 
> Could we fix it differently and more permanently by modifying the proc
> code to display the values systemd expects?

Right now there is no need to fix anything because 4 bits are available.
I put a comment so that PF_KTHREAD won't be moved accidently definitely
breaking systemd.

> > It means that the value can't be changed despite perceived notion that
> > task_struct flags are internal to kernel and can be shuffled at whim.
> >
> > Formally, _all_ struct task_struct::flags PF_* values are kernel ABI
> > which is a disaster.
> >
> > I hope we can mask everything but few flags and hope for the best :^)

> > +/*
> > + * PF_KTHREAD is part of kernel ABI, visible via value #9 in /proc/$pid/stat
> > + */
> > #define PF_KTHREAD        0x00200000    /* I am a kernel thread */

I think everything should be masked except PF_KTHREAD and maybe few
flags for which known users exist and then don't touch them.

Some flags are clearly internal like PF_MEMALLOC and PF_IDLE.

Some aren't -- PF_FORKNOEXEC. However it is silly for userspace to query it
because programs knows if it forked but didn't exec without external help.