Re: [PATCH 00/17] clean up readlinks

From: Miklos Szeredi
Date: Tue Sep 27 2016 - 05:39:10 EST

On Tue, Sep 27, 2016 at 5:10 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Sep 12, 2016 at 09:29:02PM +0200, Miklos Szeredi wrote:
>> The first patch is actually a bug fix, but I put it into this bunch for
>> simplicity...
>> The rest are really cleanups as well as minor bugfixes that are byproducts
>> of the cleanups.
>> This series builds on the fact that i_op.readlink is already set to
>> generic_readlink() in 43/50 of the cases. And of those 7 only 4 are doing
>> something special. So more than 90% of readlinks are/could actually just
>> call back into get_link.
>> The interesting cases are:
>> - AFS, which has readlink but not get_link
>> - proc, that allow jumping while following symlinks
>> The first is handled by setting IOP_NOFOLLOW on the inode by the fs.
>> The second one is handled by introducing is_following_link() which returns
>> a bool depending on whether current->nameidata is NULL or not. If it
>> returns false ->get_link() should behave as ->readlink() did. Otherwise it
>> should behave as id did previously.
>> Builds and boots. Can even read symlinks.
> I have no problem with "let's get rid of generic_readlink" - not that
> it bought us much, but sure, if you want to have decision made based upon
> the combination of flags, let's do it. Just make NULL ->readlink + non-NULL
> ->get_link() mean generic_readlink(), and we are done.

Indeed. Except it really should be the other way round:

- .get_link always returning the symlink body
- only proc setting .jump_link to do its thing
- RIP .readlink

But that's an extra branch in the symlink following. I was worried
about that and hence gone for the unification of the two.

> Overloading ->get_link() for procfs-style ones is just plain wrong,
> though. Your current->nameidata != NULL thing is bloody brittle - what
> happens if some code triggers those readlinks when called by something
> during pathname resolution? Sure, right now existing callers won't.
> But it doesn't take much to grow such a place _and_ have the implications
> go unnoticed for quite a while.

Yeah. We can do your above suggestion, it's certainly less brittle.
But I think it's rather confusing, having ->get_link normally do
readlink, except for proc, where readlink is done by ->readlink.