Re: [PATCH 1/2] liveupdate: Reference count outgoing FLB data

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On 05-28 17:41, David Matlack wrote:
> Increment the outgoing FLB refcount in liveupdate_flb_get_outgoing() so
> that the FLB structure cannot be freed while the caller is actively
> using it. Add an additional liveupdate_flb_put_outgoing() function so
> the caller can explicitly indicate when it is done using the outgoing
> FLB.
> 
> During a Live Update, the kernel may need to fetch the outgoing FLB
> outside of the scope of a file handler's preserve() and unpreserve()
> callbacks. In that situation there is no way for the caller to protect
> itself against the outgoing FLB from being freed while it is using it.
> Incrementing the reference count in liveupdate_flb_get_outgoing()
> ensures it cannot be freed.
> 
> This change also aligns the outgoing FLB lifecycle management with the
> incoming FLB, since the latter uses the same get/put semantics.
> 
> Fixes: cab056f2aae7 ("liveupdate: luo_flb: introduce File-Lifecycle-Bound global state")
> Assisted-by: Gemini:gemini-3-pro-preview
> Signed-off-by: David Matlack <dmatlack@google.com>

Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com> 

> ---
>  include/linux/liveupdate.h  |  5 +++++
>  kernel/liveupdate/luo_flb.c | 10 +++++++---
>  2 files changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/liveupdate.h b/include/linux/liveupdate.h
> index 88722e5caf02..c344bf987b63 100644
> --- a/include/linux/liveupdate.h
> +++ b/include/linux/liveupdate.h
> @@ -243,6 +243,7 @@ int liveupdate_flb_get_incoming(struct liveupdate_flb *flb, void **objp);
>  void liveupdate_flb_put_incoming(struct liveupdate_flb *flb);
>  
>  int liveupdate_flb_get_outgoing(struct liveupdate_flb *flb, void **objp);
> +void liveupdate_flb_put_outgoing(struct liveupdate_flb *flb);
>  
>  #else /* CONFIG_LIVEUPDATE */
>  
> @@ -292,5 +293,9 @@ static inline int liveupdate_flb_get_outgoing(struct liveupdate_flb *flb,
>  	return -EOPNOTSUPP;
>  }
>  
> +static inline void liveupdate_flb_put_outgoing(struct liveupdate_flb *flb)
> +{
> +}
> +
>  #endif /* CONFIG_LIVEUPDATE */
>  #endif /* _LINUX_LIVEUPDATE_H */
> diff --git a/kernel/liveupdate/luo_flb.c b/kernel/liveupdate/luo_flb.c
> index 8f5c5dd01cd0..7ddef552ff6b 100644
> --- a/kernel/liveupdate/luo_flb.c
> +++ b/kernel/liveupdate/luo_flb.c
> @@ -135,7 +135,7 @@ static int luo_flb_file_preserve_one(struct liveupdate_flb *flb)
>  	return 0;
>  }
>  
> -static void luo_flb_file_unpreserve_one(struct liveupdate_flb *flb)
> +void liveupdate_flb_put_outgoing(struct liveupdate_flb *flb)
>  {
>  	struct luo_flb_private *private = luo_flb_get_private(flb);
>  
> @@ -266,7 +266,7 @@ int luo_flb_file_preserve(struct liveupdate_file_handler *fh)
>  
>  exit_err:
>  	list_for_each_entry_continue_reverse(iter, flb_list, list)
> -		luo_flb_file_unpreserve_one(iter->flb);
> +		liveupdate_flb_put_outgoing(iter->flb);
>  	up_read(&luo_register_rwlock);
>  
>  	return err;
> @@ -291,7 +291,7 @@ void luo_flb_file_unpreserve(struct liveupdate_file_handler *fh)
>  
>  	guard(rwsem_read)(&luo_register_rwlock);
>  	list_for_each_entry_reverse(iter, flb_list, list)
> -		luo_flb_file_unpreserve_one(iter->flb);
> +		liveupdate_flb_put_outgoing(iter->flb);
>  }
>  
>  /**
> @@ -546,6 +546,10 @@ int liveupdate_flb_get_outgoing(struct liveupdate_flb *flb, void **objp)
>  		return -EOPNOTSUPP;
>  
>  	guard(mutex)(&private->outgoing.lock);
> +	if (!private->outgoing.obj)
> +		return -ENOENT;
> +
> +	refcount_inc(&private->outgoing.count);
>  	*objp = private->outgoing.obj;
>  
>  	return 0;
> -- 
> 2.54.0.823.g6e5bcc1fc9-goog
>