linux-kernel - Re: [RFC V2 11/18] famfs_fuse: Basic famfs mount opts

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ttjh3gqk3fmykwrb7dg6xaqhkpxk7g773fkvuzvbdlefimpseg@l5ermgxixeen>
Date: Fri, 11 Jul 2025 10:28:20 -0500
From: John Groves <John@...ves.net>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Dan Williams <dan.j.williams@...el.com>, 
	Miklos Szeredi <miklos@...redi.hu>, Bernd Schubert <bschubert@....com>, 
	John Groves <jgroves@...ron.com>, Jonathan Corbet <corbet@....net>, 
	Vishal Verma <vishal.l.verma@...el.com>, Dave Jiang <dave.jiang@...el.com>, 
	Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, 
	Randy Dunlap <rdunlap@...radead.org>, Jeff Layton <jlayton@...nel.org>, 
	Kent Overstreet <kent.overstreet@...ux.dev>, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	nvdimm@...ts.linux.dev, linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	Amir Goldstein <amir73il@...il.com>, Jonathan Cameron <Jonathan.Cameron@...wei.com>, 
	Stefan Hajnoczi <shajnocz@...hat.com>, Joanne Koong <joannelkoong@...il.com>, 
	Josef Bacik <josef@...icpanda.com>, Aravind Ramesh <arramesh@...ron.com>, 
	Ajay Joshi <ajayjoshi@...ron.com>
Subject: Re: [RFC V2 11/18] famfs_fuse: Basic famfs mount opts

On 25/07/08 08:59PM, Darrick J. Wong wrote:
> On Thu, Jul 03, 2025 at 01:50:25PM -0500, John Groves wrote:
> > * -o shadow=<shadowpath>
> 
> What is a shadow?
> 
> > * -o daxdev=<daxdev>

Derp - OK, that's a stale commit message. Here is the one for the -next
version of this patch:

    famfs_fuse: Basic famfs mount opt: -o shadow=<shadowpath>

    The shadow path is a (usually tmpfs) file system area used by the famfs 
    user space to commuicate with the famfs fuse server. There is a minor 
    dilemma that the user space tools must be able to resolve from a mount 
    point path to a shadow path. The shadow path is exposed via /proc/mounts, 
    but otherwise not used by the kernel. User space gets the shadow path 
    from /proc/mounts...


> 
> And, uh, if there's a FUSE_GET_DAXDEV command, then what does this mount
> option do?  Pre-populate the first element of that set?
> 
> --D
> 

I took out -o daxdev, but had failed to update the commit msg.

The logic is this: The general model requires the FUSE_GET_DAXDEV message /
response, so passing in the primary daxdev as a -o arg creates two ways to
do the same thing.

The only initial heartburn about this was one could imagine a case where a
mount happens, but no I/O happens for a while so the mount could "succeed",
only to fail later if the primary daxdev could not be accessed.

But this can't happen with famfs, because the mount procedure includes 
creating "meta files" - .meta/.superblock and .meta/.log and accessing them
immediately. So it is guaranteed that FUSE_GET_DAXDEV will be sent right away,
and if it fails, the mount will be unwound.

Thanks Darrick!
John

<snip>