Re: [RFC 0/3] Put vdso in ramfs-like filesystem (vdsofs)

From: "H. Peter Anvin" <hpa@zytor.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Dmitry Safonov <0x7f454c46@gmail.com>,
	Dmitry Safonov <dsafonov@virtuozzo.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>,
	Oleg Nesterov <oleg@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [RFC 0/3] Put vdso in ramfs-like filesystem (vdsofs)
Date: Tue, 20 Sep 2016 18:07:21 -0700	[thread overview]
Message-ID: <6de39359-511e-1ce9-9201-85bd3425a05d@zytor.com> (raw)
In-Reply-To: <CALCETrVGouJVpCNdioDGt551twHpcZEqvr-GiMrEW1=FozA7VQ@mail.gmail.com>

On 09/20/16 17:54, Andy Lutomirski wrote:
>  - If vvar is in the same inode, then that inode won't be a valid ELF
> image, because the ELF header won't be in the right place.

So the vvar ought to move into an actual ELF segment, which is probably
The Right Thing anyway.

>  - vvar is highly magical.  IMO letting it get mapped with VM_MAYWRITE
> is asking for trouble, as anything that writes it will COW it, leading
> to strange malfunctions.
> 
>  - vvar can, and has, had IO pages in it.  This means that the actual
> cache types can vary page-to-page in the vvar area, which is not
> something that ordinary files do.

Neither of these are any different than many devices, or various files
in procfs.

> My personal preference is to let them both be real struct file *
> objects (possibly shared between all processes of the same vdso ABI)
> but to prevent user code from ever creating an fd referring to one of
> these files.

Why?  It would help people doing weird things like process snapshotting
or bimodal execution enormously.  We want to share an inode, obviously;
the pointer is another issue.

> Also, if we let the users get an fd pointing to the vdso, then we're
> more or less committing to never having contents in the vdso text that
> vary per-process.  Are we okay with that.

This might be a reason to put these objects in procfs rather than sysfs,
but I have to admit that this seems *extremely* far fetched to me.
Obviously they vary per process in the sense that there are already
several to choose from.  In the case of process-unique vdsos there would
be a large number of them, of course.

	-hpa