linux-kernel - Re: [PATCH v1 3/4] signal: support CLONE_PIDFD with pidfd_send

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Date:   Wed, 17 Apr 2019 17:36:32 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Oleg Nesterov <oleg@...hat.com>
CC:     torvalds@...ux-foundation.org, viro@...iv.linux.org.uk,
        jannh@...gle.com, dhowells@...hat.com, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, serge@...lyn.com, luto@...nel.org,
        arnd@...db.de, ebiederm@...ssion.com, keescook@...omium.org,
        tglx@...utronix.de, mtk.manpages@...il.com,
        akpm@...ux-foundation.org, cyphar@...har.com,
        joel@...lfernandes.org, dancol@...gle.com
Subject: Re: [PATCH v1 3/4] signal: support CLONE_PIDFD with pidfd_send_signal

On April 17, 2019 5:20:55 PM GMT+02:00, Oleg Nesterov <oleg@...hat.com> wrote:
>On 04/17, Christian Brauner wrote:
>>
>> On Wed, Apr 17, 2019 at 04:01:06PM +0200, Oleg Nesterov wrote:
>> > On 04/16, Christian Brauner wrote:
>> > >
>> > > @@ -3581,12 +3588,12 @@ SYSCALL_DEFINE4(pidfd_send_signal, int,
>pidfd, int, sig,
>> > >  	if (flags)
>> > >  		return -EINVAL;
>> > >
>> > > -	f = fdget_raw(pidfd);
>> > > +	f = fdget(pidfd);
>> >
>> > could you explain this change?
>> >
>> > I am just curious, I don't understand why should we disallow O_PATH
>and how
>> > this connects to this patch.
>>
>> Sending a signal through a pidfd is considered to be on a par with a
>> "write" to that pidfd.
>
>OK, but how this connects to "support pidfds" ?
>
>> Additionally, we use the fops associated with the fd to detect
>whether
>> it is actually a pidfd or not. This is not possible with O_PATH since
>> f_ops will be set to dummy fops.
>
>indeed... I didn't know this, thanks!
>
>But this means that pidfd_send_signal() will return -EBADF with or
>without
>this change; pidfd_to_pid() will return -EBADF even if fdget_raw()
>suceeds,
>right?
>
>To clarify, I am not arguing. I am trying to understand why exactly do
>we
>need this s/fdget_raw/fdget/ change and, why it doesn't come as a
>separate
>patch. Can you add a note into the changelog?

I should split this into a separate patch indeed.
Let me do that for v2.

Christian