linux-kernel - Re: [PATCH 01/18] vfs: add miscattr ops

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Message-ID: <YFypkbAy+BQ3yjME@zeniv-ca.linux.org.uk>
Date:   Thu, 25 Mar 2021 15:17:37 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Eric Biggers <ebiggers@...nel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        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 01/18] vfs: add miscattr ops

On Thu, Mar 25, 2021 at 03:52:28PM +0100, Miklos Szeredi wrote:
> On Tue, Mar 23, 2021 at 1:24 AM Eric Biggers <ebiggers@...nel.org> wrote:
> 
> > > +int vfs_miscattr_set(struct dentry *dentry, struct miscattr *ma)
> > > +{
> > > +     struct inode *inode = d_inode(dentry);
> > > +     struct miscattr old_ma = {};
> > > +     int err;
> > > +
> > > +     if (d_is_special(dentry))
> > > +             return -ENOTTY;
> > > +
> > > +     if (!inode->i_op->miscattr_set)
> > > +             return -ENOIOCTLCMD;
> > > +
> > > +     if (!inode_owner_or_capable(inode))
> > > +             return -EPERM;
> >
> > Shouldn't this be EACCES, not EPERM?
> 
> $ git diff master.. | grep -C1 inode_owner_or_capable | grep
> "^-.*\(EPERM\|EACCES\)" | cut -d- -f3 | sort | uniq -c
>      12 EACCES;
>       4 EPERM;
> 
> So EACCES would win if this was a democracy.  However:
> 
> "[EACCES]
> Permission denied. An attempt was made to access a file in a way
> forbidden by its file access permissions."
> 
> "[EPERM]
> Operation not permitted. An attempt was made to perform an operation
> limited to processes with appropriate privileges or to the owner of a
> file or other resource."
> 
> The EPERM description matches the semantics of
> inode_owner_or_capable() exactly.  It's a pretty clear choice.

Except that existing implementation (e.g. for ext2) gives -EACCES here...
OTOH, EPERM matches the behaviour of chown(2), as well as that of
*BSD chflags(2), which is the best match to functionality (setting and
clearing immutable/append-only/etc.)

So I'd probably go with EPERM, and watched for userland breakage;
if something *does* rely upon the historical EACCES here, we might
have to restore that.