From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 59AED25614; Sat, 27 Jan 2024 14:33:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1706365987; cv=none; b=nXNYxIeyPVm0mnCyooTUQ4Drx50X3M4JcbeqleaYzGpFGRF/gConckrDYxwN+TtYfHx9/JjdAOuyj5G8JFDli34uwHp29xL98JutE9gwAlNMLGDQCYyLXNfI00OtPXqxl6oLgtSnVYBimiNMUnTmTgntY7a5PjIckX2gItRlMTo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1706365987; c=relaxed/simple; bh=7yiYZJ1S3J1xZ/XaStLzzEMFrh8kHMHMD/9wg1lDgSQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=TlBLDtI1CCj6fGXvPLqCkMh3TiZ75pmYP8hxnj0+i1SdsZ5/nNnA9nMMewkyASpDZarqV/86V1RNFmbMOSJPnLxBVgnpRl3epND/g6ok1i/GyEzq+HQC/MjkS1KFK/GEyZS3U7BGMbtGMNCP909R79Nehis24CYywv53pgmeQb4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PUGLURXm; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="PUGLURXm" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3F9EFC433C7; Sat, 27 Jan 2024 14:33:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1706365986; bh=7yiYZJ1S3J1xZ/XaStLzzEMFrh8kHMHMD/9wg1lDgSQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=PUGLURXmWsWtWrp67aRiQ61WnjKxdqUb76keJ8ogLYkTWpmDK1bGdFdWlxdSMR7Ux P1Rj2ByY50mNoUGE1QEpYEFpUS5345753C8cCspO3i19s635qsjxSJZLddGdNVZzqn DgEmi0xzntYp95oUI27tm2tYgqooZTst4U675aPFEd/9kMMcvi2nRZ6vUNC6jlZIMA lrudB4JMBJCb4jMBsd07HKg6NkFW8TANMBD2BfqUwqv9gvCCBdFLpn1A8Fwg/bwsiA ecfNe+W7q5o7EFab6ObFeKqYAucfEu4D/OB4d445mMTlWJwQWnsTEjoy4W5+FOPVE0 vkq2w4G0yJNaQ== Date: Sat, 27 Jan 2024 15:33:02 +0100 From: Christian Brauner To: Oleg Nesterov Cc: Tycho Andersen , linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, Tycho Andersen , "Eric W. Biederman" Subject: Re: [PATCH v3 1/3] pidfd: allow pidfd_open() on non-thread-group leaders Message-ID: <20240127-anheben-unfehlbar-52b320b211aa@brauner> References: <20240123153452.170866-1-tycho@tycho.pizza> <20240123153452.170866-2-tycho@tycho.pizza> <20240123195608.GB9978@redhat.com> <20240125140830.GA5513@redhat.com> <20240127105410.GA13787@redhat.com> Precedence: bulk X-Mailing-List: linux-api@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20240127105410.GA13787@redhat.com> On Sat, Jan 27, 2024 at 11:54:32AM +0100, Oleg Nesterov wrote: > Hi Tycho, > > On 01/26, Tycho Andersen wrote: > > > > On Thu, Jan 25, 2024 at 03:08:31PM +0100, Oleg Nesterov wrote: > > > What do you think? > > > > Thank you, it passes all my tests. > > Great, thanks! > > OK, I'll make v2 on top of the recent > "pidfd: cleanup the usage of __pidfd_prepare's flags" > > but we need to finish our discussion with Christian about the > usage of O_EXCL. Just write the patch exactly like you want. If it's as a little a detail as the uapi flag value we can just always change that when applying. There's no reason to introduce artificial delays because of my preference here.. > > As for clone(CLONE_PIDFD | CLONE_THREAD), this is trivial but > I think this needs another discussion too, lets do this later. > > > > + /* unnecessary if do_notify_parent() was already called, > > > + we can do better */ > > > + do_notify_pidfd(tsk); > > > > "do better" here could be something like, > > > > [...snip...] > > No, no, please see below. > > For the moment, please forget about PIDFD_THREAD, lets discuss > the current behaviour. > > > but even with that, there's other calls in the tree to > > do_notify_parent() that might double notify. > > Yes, and we can't avoid this. Well, perhaps do_notify_parent() > can do something like > > if (ptrace_reparented()) > do_notify_pidfd(); > > so that only the "final" do_notify_parent() does do_notify_pidfd() > but this needs another discussion and in fact I don't think this > would be right or make much sense. Lets forget this for now. > > Now. Even without PIDFD_THREAD, I think it makes sense to change > do_notify_parent() to do > > if (thread_group_empty(tsk)) > do_notify_pidfd(tsk); > > thread_group_empty(tsk) can only be true if tsk is a group leader > and it is the last thread. And this is exactly what pidfd_poll() > currently needs. > > In fact I'd even prefer to do this in a separate patch for the > documentation purposes. > > Now, PIDFD_THREAD can just add > > if (!thread_group_empty(tsk)) > do_notify_pidfd(tsk); > > right after "tsk->exit_state = EXIT_ZOMBIE", that is all. Sounds good. > > This also preserves the do_notify_pidfd/__wake_up_parent ordering. > Not that I think this is important, just for consistency. > > > This brings up another interesting behavior that I noticed while > > testing this, if you do a poll() on pidfd, followed quickly by a > > pidfd_getfd() on the same thread you just got an event on, you can > > sometimes get an EBADF from __pidfd_fget() instead of the more > > expected ESRCH higher up the stack. > > exit_notify() is called after exit_files(). pidfd_getfd() returns > ESRCH if the exiting thread completes release_task(), otherwise it > returns EBADF because ->files == NULL. This too doesn't really > depend on PIDFD_THREAD. > > > I wonder if it makes sense to abuse ->f_flags to add a PIDFD_NOTIFIED? > > Then we can refuse further pidfd syscall operations in a sane way, and > > But how? We only have "struct pid *", how can we find all files > "attached" to this pid? We can't. There's some use-cases that would make this desirable but that would mean we would need another data structure to track this. I once toyed with a patch for this but never got so excited about it to actually finish it.