linux-kernel - Re: [PATCH] vfs: Don't reject unknown parameters

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Message-ID: <32253.1576604947@warthog.procyon.org.uk>
Date:   Tue, 17 Dec 2019 17:49:07 +0000
From:   David Howells <dhowells@...hat.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     dhowells@...hat.com, Al Viro <viro@...iv.linux.org.uk>,
        Laura Abbott <labbott@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ilya Dryomov <idryomov@...il.com>,
        Jeremi Piotrowski <jeremi.piotrowski@...il.com>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Phillip Lougher <phillip@...ashfs.org.uk>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vfs: Don't reject unknown parameters

Miklos Szeredi <miklos@...redi.hu> wrote:

> > So you could bloody well just leave recognition (and handling) of "source"
> > to the caller, leaving you with just this:
> >
> >         if (strcmp(param->key, "source") == 0)
> >                 return -ENOPARAM;
> >         /* Just log an error for backwards compatibility */
> >         errorf(fc, "%s: Unknown parameter '%s'", fc->fs_type->name, param->key);
> >         return 0;
> 
> Which is fine for the old mount(2) interface.
> 
> But we have a brand new API as well; do we really need to carry these
> backward compatibility issues forward?  I mean checking if a
> param/flag is supported or not *is* useful and lacking that check is
> the source of numerous headaches in legacy interfaces (just take the
> open(2) example and the introduction of O_TMPFILE).

The problem with what you're suggesting is that you can't then make
/sbin/mount to use the new syscalls because that would change userspace
behaviour - unless you either teach /sbin/mount which filesystems discard
which errors from unrecognised options or pass a flag to the kernel to shift
into or out of 'strict' mode.

David