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

[Song Liu]

> On Jun 19, 2025, at 3:01 AM, Christian Brauner <brauner at kernel.org> wrote:
> 
> 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.
> <0001-kernfs-remove-iattr_mutex.patch>

This looks better indeed. 

I will drop 1/4 and include this patch. 

Thanks,
Song