linux-kernel - Re: [PATCH 00/18] new API for FS_IOC_[GS]ETFLAGS/FS_IOC

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Date:   Wed, 3 Feb 2021 14:28:02 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        Andreas Dilger <adilger@...ger.ca>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Christoph Hellwig <hch@....de>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Dave Kleikamp <shaggy@...nel.org>,
        David Sterba <dsterba@...e.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Jan Kara <jack@...e.cz>,
        Joel Becker <jlbec@...lplan.org>,
        Matthew Garrett <matthew.garrett@...ula.com>,
        Mike Marshall <hubcap@...ibond.com>,
        Richard Weinberger <richard@....at>,
        Ryusuke Konishi <konishi.ryusuke@...il.com>,
        Theodore Ts'o <tytso@....edu>, Tyler Hicks <code@...icks.com>
Subject: Re: [PATCH 00/18] new API for FS_IOC_[GS]ETFLAGS/FS_IOC_FS[GS]ETXATTR

On Wed, Feb 03, 2021 at 03:13:29PM +0100, Miklos Szeredi wrote:
> On Wed, Feb 3, 2021 at 2:58 PM Matthew Wilcox <willy@...radead.org> wrote:
> 
> > Network filesystems frequently need to use the credentials attached to
> > a struct file in order to communicate with the server.  There's no point
> > fighting this reality.
> 
> IDGI.  Credentials can be taken from the file and from the task.  In
> this case every filesystem except cifs looks at task creds. Why are
> network filesystem special in this respect?

I don't necessarily mean 'struct cred'.  I mean "the authentication
that the client has performed to the server".  Which is not a per-task
thing, it's stored in the struct file, which is why we have things like

        int (*write_begin)(struct file *, struct address_space *mapping,
                                loff_t pos, unsigned len, unsigned flags,
                                struct page **pagep, void **fsdata);

disk filesystems ignore the struct file argument, but network filesystems
very much use it.