linux-kernel - Re: [patch 11/14] vfs: move executable checking into ->permission()

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Date:	Wed, 21 May 2008 21:09:36 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	trond.myklebust@....uio.no
CC:	miklos@...redi.hu, linux-fsdevel@...r.kernel.org,
	hch@...radead.org, viro@...IV.linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 11/14] vfs: move executable checking into ->permission()

> > +/**
> > + * exec_permission - check for general execute permission on file
> > + * @inode:	inode to check access rights for
> > + * @mask:	right to check for
> > + *
> > + * Exec permission on a regular file is denied if none of the execute
> > + * bits are set.
> > + *
> > + * This needs to be called by filesystems which define a
> > + * ->permission() method, and don't call generic_permission().
> > + */
> > +int exec_permission(struct inode *inode, int mask)
> > +{
> > +	if ((mask & MAY_EXEC) && S_ISREG(inode->i_mode) &&
> > +	    !(inode->i_mode & S_IXUGO))
> > +		return -EACCES;
> > +
> > +	return 0;
> > +}
> 
> Hmm... What if !(mask & MAY_EXEC)? AFAICS, the above will return 0.
> A better approach would be to use something like
> 
> 	if (!(mask & MAY_EXEC))
> 		return -EACCES;
> 	if (S_ISREG(inode->i_mode) && !(inode->i_mode & S_IXUGO))
> 		return -EACCES;
> 	return 0;

No, we don't want to deny read or write (that's up to the filesystem
how it handles it), just want to deny execute if no x bits are set in
the mode.

I didn't really pay attention to individual filesystems, including
NFS, just mechanically moved the check from permission() to
->permission().  So it's possible that the code could be further
optimized in some cases.

Miklos
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