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

[Al Viro <viro@zeniv.linux.org.uk>]

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@kernel.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.