Re: [PATCH v2 1/6] Add a new flags-accepting interface for anonymous inodes

From: Daniel Colascione <dancol@google.com>
To: Stephen Smalley <sds@tycho.nsa.gov>
Cc: Tim Murray <timmurray@google.com>,
	Nosh Minwalla <nosh@google.com>, Nick Kralevich <nnk@google.com>,
	Lokesh Gidra <lokeshgidra@google.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	selinux@vger.kernel.org
Subject: Re: [PATCH v2 1/6] Add a new flags-accepting interface for anonymous inodes
Date: Wed, 12 Feb 2020 09:23:09 -0800	[thread overview]
Message-ID: <CAKOZuet1vcDwkqoJgqmDjg7pjLGfvh11ZdUZpoyoXXWdz9Y4CQ@mail.gmail.com> (raw)
In-Reply-To: <88ea16bd-38be-b4f9-dfb3-e0626f5b6aaf@tycho.nsa.gov>

Thanks again for the review.

On Wed, Feb 12, 2020 at 8:36 AM Stephen Smalley <sds@tycho.nsa.gov> wrote:
>
> On 2/11/20 5:55 PM, Daniel Colascione wrote:
> > Add functions forwarding from the old names to the new ones so we
> > don't need to change any callers.
> >
> > Signed-off-by: Daniel Colascione <dancol@google.com>
>
> (please add linux-fsdevel, viro to cc on future versions of this patch
> since this is a VFS change)
>
> > ---
> >   fs/anon_inodes.c            | 62 ++++++++++++++++++++++---------------
> >   include/linux/anon_inodes.h | 27 +++++++++++++---
> >   2 files changed, 59 insertions(+), 30 deletions(-)
> >
> > diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
> > index 89714308c25b..caa36019afca 100644
> > --- a/fs/anon_inodes.c
> > +++ b/fs/anon_inodes.c
> > @@ -56,60 +56,71 @@ static struct file_system_type anon_inode_fs_type = {
> >   };
> >
> >   /**
> > - * anon_inode_getfile - creates a new file instance by hooking it up to an
> > - *                      anonymous inode, and a dentry that describe the "class"
> > - *                      of the file
> > + * anon_inode_getfile2 - creates a new file instance by hooking it up to
> > + *                       an anonymous inode, and a dentry that describe
> > + *                       the "class" of the file
>
> Not going to bikeshed on names but anon_inode_getfile_flags or _secure
> or something would be more descriptive.

_flags is fine, but I think _secure is overfitting.

> >    *
> >    * @name:    [in]    name of the "class" of the new file
> >    * @fops:    [in]    file operations for the new file
> >    * @priv:    [in]    private data for the new file (will be file's private_data)
> > - * @flags:   [in]    flags
> > + * @flags:   [in]    flags for the file
> > + * @anon_inode_flags: [in] flags for anon_inode*
>
> Do we really envision ever needing more than one new flag here?  If not,
> then making it a bool secure parameter or encoding it as an
> unused/ignored flag bit in the existing flags argument would seem
> preferable.

A bool and a flag is the same as far as the machine is concerned with
respect to argument passing, and I find the flag much more descriptive
than a bare "true" or a "false" scattered at call sites. Besides, a
flags argument could lead to less churn later.

> In some cases, we actually want the "anon inode" to inherit the security
> context of a related inode (e.g. ioctls on /dev/kvm can create anon
> inodes representing VMs, vCPUs, etc and further ioctls are performed on
> those inodes), in which case we may need the caller to pass in the
> related inode as well.

See my other reply on this subject. Passing an optional related inode
seems like a decent approach here.

> >    *
> > - * Creates a new file by hooking it on a single inode. This is useful for files
> > + * Creates a new file by hooking it on an unspecified inode. This is useful for files
> >    * that do not need to have a full-fledged inode in order to operate correctly.
> >    * All the files created with anon_inode_getfile() will share a single inode,
> >    * hence saving memory and avoiding code duplication for the file/inode/dentry
> >    * setup.  Returns the newly created file* or an error pointer.
> > + *
> > + * anon_inode_flags must be zero.
> >    */
> > -struct file *anon_inode_getfile(const char *name,
> > -                             const struct file_operations *fops,
> > -                             void *priv, int flags)
> > +struct file *anon_inode_getfile2(const char *name,
> > +                              const struct file_operations *fops,
> > +                              void *priv, int flags, int anon_inode_flags)
> >   {
> > +     struct inode *inode;
> >       struct file *file;
> >
> > -     if (IS_ERR(anon_inode_inode))
> > -             return ERR_PTR(-ENODEV);
> > -
> > -     if (fops->owner && !try_module_get(fops->owner))
> > -             return ERR_PTR(-ENOENT);
> > +     if (anon_inode_flags)
> > +             return ERR_PTR(-EINVAL);
>
> Not sure this is how it is normally done (i.e. one patch to just
> introduce an extended interface but disallow all use of it, then a
> separate patch to introduce the first use).  Would recommend combining;
> otherwise reviewers can't see how it will be used without looking at both.

All things being equal, finer-grained patches are better: they allow
for easier bisection. But I don't feel strongly one way or the other
here, so let's see what other reviewers say.