[PATCH bpf-next 1/4] kernfs: Add __kernfs_xattr_get for RCU protected access

[Christian Brauner]

On Wed, Jun 18, 2025 at 04:37:36PM -0700, Song Liu wrote:
> Existing kernfs_xattr_get() locks iattr_mutex, so it cannot be used in
> RCU critical sections. Introduce __kernfs_xattr_get(), which reads xattr
> under RCU read lock. This can be used by BPF programs to access cgroupfs
> xattrs.
> 
> Signed-off-by: Song Liu <song at kernel.org>
> ---
>  fs/kernfs/inode.c      | 14 ++++++++++++++
>  include/linux/kernfs.h |  2 ++
>  2 files changed, 16 insertions(+)
> 
> diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
> index b83054da68b3..0ca231d2012c 100644
> --- a/fs/kernfs/inode.c
> +++ b/fs/kernfs/inode.c
> @@ -302,6 +302,20 @@ int kernfs_xattr_get(struct kernfs_node *kn, const char *name,
>  	return simple_xattr_get(&attrs->xattrs, name, value, size);
>  }
>  
> +int __kernfs_xattr_get(struct kernfs_node *kn, const char *name,
> +		       void *value, size_t size)
> +{
> +	struct kernfs_iattrs *attrs;
> +
> +	WARN_ON_ONCE(!rcu_read_lock_held());
> +
> +	attrs = rcu_dereference(kn->iattr);
> +	if (!attrs)
> +		return -ENODATA;

Hm, that looks a bit silly. Which isn't your fault. I'm looking at the
kernfs code that does the xattr allocations and I think that's the
origin of the silliness. It uses a single global mutex for all kernfs
users thus serializing all allocations for kernfs->iattr. That seems
crazy but maybe I'm missing a good reason.

I'm appending a patch to remove that mutex. @Greg, @Tejun, can you take
a look whether that makes sense to you. Then I can take that patch and
you can build yours on top of the series and I'll pick it all up in one
go.

You should then just use READ_ONCE(kn->iattr) or the
kernfs_iattrs_noalloc(kn) helper in your kfunc.