Re: [RFC] sysfs_rename_link() and its usage

[Veaceslav Falico]

On Tue, Jan 14, 2014 at 10:21:35AM -0800, Greg KH wrote:
> On Tue, Jan 14, 2014 at 06:17:40PM +0100, Veaceslav Falico wrote:
> > Hi,
> >
> > I'm hitting a strange issue and/or I'm completely lost in sysfs internals.
> >
> > Consider having two net_device *a, *b; which are registered normally.
> > Now, to create a link from /sys/class/net/a->name/linkname to b, one should
> > use:
> >
> > sysfs_create_link(&(a->dev.kobj), &(b->dev.kobj), linkname);
> >
> > To remove it, even simpler:
> >
> > sysfs_remove_link(&(a->dev.kobj), linkname);
> >
> > This works like a charm. However, if I want to use (obviously, with the
> > symlink present):
> >
> > sysfs_rename_link(&(a->dev.kobj), &(b->dev.kobj), oldname, newname);
>
> You forgot the namespace option to this call, what kernel version are
> you using here?

It's git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next ,
3.13-rc6 with some networking patches on top of it.

And wrt namespace - there are two functions, one is sysfs_rename_link(),
which calls the second one - sysfs_rename_link_ns() with NULL namespace.

> > this fails with:
> >
> > "sysfs: ns invalid in 'a->name' for 'oldname'"
>
> Looks like the namespace for this link isn't valid.

Yep, though dunno why.

> > in
> >
> >  608 struct sysfs_dirent *sysfs_find_dirent(struct sysfs_dirent *parent_sd,
> > ...
> >  615         if (!!sysfs_ns_type(parent_sd) != !!ns) {
> >  616                 WARN(1, KERN_WARNING "sysfs: ns %s in '%s' for '%s'\n",
> >  617                         sysfs_ns_type(parent_sd) ? "required" : "invalid",
> >  618                         parent_sd->s_name, name);
> >  619                 return NULL;
> >  620         }
> >
> > Code path:
> > warn_slowpath_fmt+0x46/0x50
> > sysfs_get_dirent_ns+0x30/0x80
> > sysfs_find_dirent+0x84/0x110
> > sysfs_get_dirent_ns+0x3e/0x80
> > sysfs_rename_link_ns+0x54/0xd0
> >
> > I have no idea what this code means. Is there any reason for it to
> > fail (i.e. am I doing something wrong?) or I've hit a bug?
>
> What exactly are you trying to do here?  Care to provide a pointer to
> your code somewhere?

I've tried to handle the network device renames, ended up doing it with
sysfs_remove/add_link() calls. Patches:

http://patchwork.ozlabs.org/patch/310798/
http://patchwork.ozlabs.org/patch/310796/

If an upper device (say, vlan, bonding, bridge etc.) enslaves another
device, symlinks are created (given bridge0 and eth0):

/sys/class/net/eth0/upper_bridge0 -> ../bridge0
/sys/class/net/bridge0/lower_eth0 -> ../eth0

However, in case someone renames eth0/bridge0, these symlinks should also
be renamed, and sysfs_rename_link() should fit great here. But it fails.

> > I've tested the only user of it (bridge) - and it works fine, however it's
> > not using its own net_device's kobject but rather its own dir.
>
> The driver core also uses this function, and it works there, so I'd
> blame your code :)

Yeah, I'd also like to blame it, I'm afraid of touching anything
sysfs-related :).

> thanks,
>
> greg k-h
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