Re: [PATCH 03/14] liveupdate: luo_file: Add internal APIs for file preservation

[Pranjal Shrivastava <praan@google.com>]

On Tue, Feb 03, 2026 at 10:09:37PM +0000, Samiullah Khawaja wrote:
> From: Pasha Tatashin <pasha.tatashin@soleen.com>
> 
> The core liveupdate mechanism allows userspace to preserve file
> descriptors. However, kernel subsystems often manage struct file
> objects directly and need to participate in the preservation process
> programmatically without relying solely on userspace interaction.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> ---
>  include/linux/liveupdate.h       | 21 ++++++++++
>  kernel/liveupdate/luo_file.c     | 71 ++++++++++++++++++++++++++++++++
>  kernel/liveupdate/luo_internal.h | 16 +++++++
>  3 files changed, 108 insertions(+)
> 
> diff --git a/include/linux/liveupdate.h b/include/linux/liveupdate.h
> index fe82a6c3005f..8e47504ba01e 100644
> --- a/include/linux/liveupdate.h
> +++ b/include/linux/liveupdate.h
> @@ -23,6 +23,7 @@ struct file;
>  /**
>   * struct liveupdate_file_op_args - Arguments for file operation callbacks.
>   * @handler:          The file handler being called.
> + * @session:          The session this file belongs to.
>   * @retrieved:        The retrieve status for the 'can_finish / finish'
>   *                    operation.
>   * @file:             The file object. For retrieve: [OUT] The callback sets
> @@ -40,6 +41,7 @@ struct file;
>   */
>  struct liveupdate_file_op_args {
>  	struct liveupdate_file_handler *handler;
> +	struct liveupdate_session *session;
>  	bool retrieved;

Nit: I don't think this is on the latest tree. I see `int retrieved` [1]
in the latest tree. I guess we'd need to rebase it on the latest?

https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/include/linux/liveupdate.h#n46

>  	struct file *file;
>  	u64 serialized_data;
> @@ -234,6 +236,13 @@ int liveupdate_unregister_flb(struct liveupdate_file_handler *fh,
>  
>  int liveupdate_flb_get_incoming(struct liveupdate_flb *flb, void **objp);
>  int liveupdate_flb_get_outgoing(struct liveupdate_flb *flb, void **objp);
> +/* kernel can internally retrieve files */
> +int liveupdate_get_file_incoming(struct liveupdate_session *s, u64 token,
> +				 struct file **filep);
> +
> +/* Get a token for an outgoing file, or -ENOENT if file is not preserved */
> +int liveupdate_get_token_outgoing(struct liveupdate_session *s,
> +				  struct file *file, u64 *tokenp);
>  
>  #else /* CONFIG_LIVEUPDATE */
>  
> @@ -281,5 +290,17 @@ static inline int liveupdate_flb_get_outgoing(struct liveupdate_flb *flb,
>  	return -EOPNOTSUPP;
>  }
>  
> +static inline int liveupdate_get_file_incoming(struct liveupdate_session *s,
> +					       u64 token, struct file **filep)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
> +static inline int liveupdate_get_token_outgoing(struct liveupdate_session *s,
> +						struct file *file, u64 *tokenp)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
>  #endif /* CONFIG_LIVEUPDATE */
>  #endif /* _LINUX_LIVEUPDATE_H */
> diff --git a/kernel/liveupdate/luo_file.c b/kernel/liveupdate/luo_file.c
> index 32759e846bc9..7ac591542059 100644
> --- a/kernel/liveupdate/luo_file.c
> +++ b/kernel/liveupdate/luo_file.c
> @@ -302,6 +302,7 @@ int luo_preserve_file(struct luo_file_set *file_set, u64 token, int fd)
>  	mutex_init(&luo_file->mutex);
>  
>  	args.handler = fh;
> +	args.session = luo_session_from_file_set(file_set);
>  	args.file = file;
>  	err = fh->ops->preserve(&args);
>  	if (err)
> @@ -355,6 +356,7 @@ void luo_file_unpreserve_files(struct luo_file_set *file_set)
>  					   struct luo_file, list);
>  
>  		args.handler = luo_file->fh;
> +		args.session = luo_session_from_file_set(file_set);
>  		args.file = luo_file->file;
>  		args.serialized_data = luo_file->serialized_data;
>  		args.private_data = luo_file->private_data;
> @@ -383,6 +385,7 @@ static int luo_file_freeze_one(struct luo_file_set *file_set,
>  		struct liveupdate_file_op_args args = {0};
>  
>  		args.handler = luo_file->fh;
> +		args.session = luo_session_from_file_set(file_set);
>  		args.file = luo_file->file;
>  		args.serialized_data = luo_file->serialized_data;
>  		args.private_data = luo_file->private_data;
> @@ -404,6 +407,7 @@ static void luo_file_unfreeze_one(struct luo_file_set *file_set,
>  		struct liveupdate_file_op_args args = {0};
>  
>  		args.handler = luo_file->fh;
> +		args.session = luo_session_from_file_set(file_set);
>  		args.file = luo_file->file;
>  		args.serialized_data = luo_file->serialized_data;
>  		args.private_data = luo_file->private_data;
> @@ -590,6 +594,7 @@ int luo_retrieve_file(struct luo_file_set *file_set, u64 token,
>  	}
>  
>  	args.handler = luo_file->fh;
> +	args.session = luo_session_from_file_set(file_set);
>  	args.serialized_data = luo_file->serialized_data;
>  	err = luo_file->fh->ops->retrieve(&args);
>  	if (!err) {
> @@ -615,6 +620,7 @@ static int luo_file_can_finish_one(struct luo_file_set *file_set,
>  		struct liveupdate_file_op_args args = {0};
>  
>  		args.handler = luo_file->fh;
> +		args.session = luo_session_from_file_set(file_set);
>  		args.file = luo_file->file;
>  		args.serialized_data = luo_file->serialized_data;
>  		args.retrieved = luo_file->retrieved;
> @@ -632,6 +638,7 @@ static void luo_file_finish_one(struct luo_file_set *file_set,
>  	guard(mutex)(&luo_file->mutex);
>  
>  	args.handler = luo_file->fh;
> +	args.session = luo_session_from_file_set(file_set);
>  	args.file = luo_file->file;
>  	args.serialized_data = luo_file->serialized_data;
>  	args.retrieved = luo_file->retrieved;
> @@ -919,3 +926,67 @@ int liveupdate_unregister_file_handler(struct liveupdate_file_handler *fh)
>  	return err;
>  }
>  EXPORT_SYMBOL_GPL(liveupdate_unregister_file_handler);
> +
> +/**
> + * liveupdate_get_token_outgoing - Get the token for a preserved file.
> + * @s:      The outgoing liveupdate session.
> + * @file:   The file object to search for.
> + * @tokenp: Output parameter for the found token.
> + *
> + * Searches the list of preserved files in an outgoing session for a matching
> + * file object. If found, the corresponding user-provided token is returned.
> + *
> + * This function is intended for in-kernel callers that need to correlate a
> + * file with its liveupdate token.
> + *
> + * Context: Can be called from any context that can acquire the session mutex.
> + * Return: 0 on success, -ENOENT if the file is not preserved in this session.
> + */
> +int liveupdate_get_token_outgoing(struct liveupdate_session *s,
> +				  struct file *file, u64 *tokenp)
> +{
> +	struct luo_file_set *file_set = luo_file_set_from_session(s);
> +	struct luo_file *luo_file;
> +	int err = -ENOENT;
> +
> +	list_for_each_entry(luo_file, &file_set->files_list, list) {

Shouldn't we hold a lock while traversing the file_set? Couldn't this
race with an unpreserve? If a concurrent unpreserve (writer) calls 
list_del while this reader is mid-iteration, we'll likely follow a stale
pointer..

I noticed that luo_preserve_file / upreserve and retrieve also don't
take locks while manipulating / iterating  over file_set->files_list..

Shouldn't we protect these with a lock? Otherwise, how do we avoid
races in situations like:

1. CPU 0 (reader) is iterating the list A->B->C ..
2. CPU 1 (writer) is handling an unpreserve which remove file B

Am I missing something? If we're expecting the callers to hold a lock,
we should have a lockdep_assert in these functions..

> +		if (luo_file->file == file) {
> +			if (tokenp)
> +				*tokenp = luo_file->token;
> +			err = 0;
> +			break;
> +		}
> +	}
> +
> +	return err;
> +}
> +
> +/**
> + * liveupdate_get_file_incoming - Retrieves a preserved file for in-kernel use.
> + * @s:      The incoming liveupdate session (restored from the previous kernel).
> + * @token:  The unique token identifying the file to retrieve.
> + * @filep:  On success, this will be populated with a pointer to the retrieved
> + *          'struct file'.
> + *
> + * Provides a kernel-internal API for other subsystems to retrieve their
> + * preserved files after a live update. This function is a simple wrapper
> + * around luo_retrieve_file(), allowing callers to find a file by its token.
> + *
> + * The operation is idempotent; subsequent calls for the same token will return
> + * a pointer to the same 'struct file' object.
> + *
> + * The caller receives a new reference to the file and must call fput() when it
> + * is no longer needed. The file's lifetime is managed by LUO and any userspace
> + * file descriptors. If the caller needs to hold a reference to the file beyond
> + * the immediate scope, it must call get_file() itself.
> + *

I'm little confused here, we say the op is idempotent, but also mention
that the caller receives a new reference. I'm wondering of a situation 
where a driver calls this multiple times, incrementing the refcount with
each call. Do we rely on flb_file_finish to drop all the refcounts?

We should clarify the lifecycle requirements here: is the driver 
expected to call fput() for every single call to 
liveupdate_get_file_incoming(), or is the flb_finish callback intended
to be a 'catch-all' that reaps these? 

> + * Context: Can be called from any context in the new kernel that has a handle
> + *          to a restored session.
> + * Return: 0 on success. Returns -ENOENT if no file with the matching token is
> + *         found, or any other negative errno on failure.
> + */
> +int liveupdate_get_file_incoming(struct liveupdate_session *s, u64 token,
> +				 struct file **filep)
> +{
> +	return luo_retrieve_file(luo_file_set_from_session(s), token, filep);
> +}
> diff --git a/kernel/liveupdate/luo_internal.h b/kernel/liveupdate/luo_internal.h
> index 8083d8739b09..a24933d24fd9 100644
> --- a/kernel/liveupdate/luo_internal.h
> +++ b/kernel/liveupdate/luo_internal.h
> @@ -77,6 +77,22 @@ struct luo_session {
>  	struct mutex mutex;
>  };
>  
> +static inline struct liveupdate_session *luo_session_from_file_set(struct luo_file_set *file_set)
> +{
> +	struct luo_session *session;
> +
> +	session = container_of(file_set, struct luo_session, file_set);
> +
> +	return (struct liveupdate_session *)session;
> +}
> +
> +static inline struct luo_file_set *luo_file_set_from_session(struct liveupdate_session *s)
> +{
> +	struct luo_session *session = (struct luo_session *)s;
> +
> +	return &session->file_set;
> +}
> +
>  int luo_session_create(const char *name, struct file **filep);
>  int luo_session_retrieve(const char *name, struct file **filep);
>  int __init luo_session_setup_outgoing(void *fdt);
> -- 
>

Thanks,
Praan