Re: [PATCH v2 1/2] vfs: block chmod of symlinks

[Al Viro]

On Wed, Sep 16, 2020 at 07:25:53AM +0100, Christoph Hellwig wrote:
> On Tue, Sep 15, 2020 at 08:22:54PM -0400, Rich Felker wrote:
> > It was discovered while implementing userspace emulation of fchmodat
> > AT_SYMLINK_NOFOLLOW (using O_PATH and procfs magic symlinks; otherwise
> > it's not possible to target symlinks with chmod operations) that some
> > filesystems erroneously allow access mode of symlinks to be changed,
> > but return failure with EOPNOTSUPP (see glibc issue #14578 and commit
> > a492b1e5ef). This inconsistency is non-conforming and wrong, and the
> > consensus seems to be that it was unintentional to allow link modes to
> > be changed in the first place.
> > 
> > Signed-off-by: Rich Felker <dalias@xxxxxxxx>
> > ---
> >  fs/open.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/fs/open.c b/fs/open.c
> > index 9af548fb841b..cdb7964aaa6e 100644
> > --- a/fs/open.c
> > +++ b/fs/open.c
> > @@ -570,6 +570,12 @@ int chmod_common(const struct path *path, umode_t mode)
> >  	struct iattr newattrs;
> >  	int error;
> >  
> > +	/* Block chmod from getting to fs layer. Ideally the fs would either
> > +	 * allow it or fail with EOPNOTSUPP, but some are buggy and return
> > +	 * an error but change the mode, which is non-conforming and wrong. */
> > +	if (S_ISLNK(inode->i_mode))
> > +		return -EOPNOTSUPP;
> 
> Our usualy place for this would be setattr_prepare.  Also the comment
> style is off, and I don't think we should talk about buggy file systems
> here, but a policy to not allow the chmod.  I also suspect the right
> error value is EINVAL - EOPNOTSUPP isn't really used in normal posix
> file system interfaces.

Er...   Wasn't that an ACL-related crap?  XFS calling posix_acl_chmod()
after it has committed to i_mode change, propagating the error to
caller of ->notify_change(), IIRC...

Put it another way, why do we want
        if (!inode->i_op->set_acl)
                return -EOPNOTSUPP;
in posix_acl_chmod(), when we have
        if (!IS_POSIXACL(inode))
                return 0;
right next to it?  If nothing else, make that
	if (!IS_POSIXACL(inode) || !inode->i_op->get_acl)
		return 0;	// piss off - nothing to adjust here