Re: [PATCH] cred: add get_cred_many and put_cred_many

[Paul Moore <paul@paul-moore.com>]

On Aug 15, 2023 Mateusz Guzik <mjguzik@gmail.com> wrote:
> 
> Shaves back-to-back atomics in a few places.
> 
> Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
> ---
>  include/linux/cred.h | 27 +++++++++++++++++++++------
>  kernel/cred.c        | 29 +++++++++++++++++------------
>  2 files changed, 38 insertions(+), 18 deletions(-)

Since there doesn't appear to be a dedicated maintainer for the
credential code I can volunteer to pick this up via the LSM tree.
Perhaps I should volunteer to maintain the credential code.

Regardless, I think this patch needs some additional work, starting
with the commit description.  I understand this isn't a very large
patch, but a quick discussion of how you are adding two new functions
to reduce the number of atomic operations that happen in a select
number of code paths where the caller takes multiple references ...
or something along those lines.

More comments below.

> diff --git a/include/linux/cred.h b/include/linux/cred.h
> index 9ed9232af934..b2b570ba204a 100644
> --- a/include/linux/cred.h
> +++ b/include/linux/cred.h
> @@ -226,12 +226,17 @@ static inline bool cap_ambient_invariant_ok(const struct cred *cred)
>   * Get a reference on the specified set of new credentials.  The caller must
>   * release the reference.
>   */
> -static inline struct cred *get_new_cred(struct cred *cred)
> +static inline struct cred *get_new_cred_many(struct cred *cred, int nr)
>  {
> -	atomic_inc(&cred->usage);
> +	atomic_add(nr, &cred->usage);
>  	return cred;
>  }
>  
> +static inline struct cred *get_new_cred(struct cred *cred)
> +{
> +	return get_new_cred_many(cred, 1);
> +}

You need to add a kdoc comment header for get_new_cred_many() and
move the get_new_cred() comment down to match up with the function.

>  /**
>   * get_cred - Get a reference on a set of credentials
>   * @cred: The credentials to reference
> @@ -245,14 +250,19 @@ static inline struct cred *get_new_cred(struct cred *cred)
>   * accidental alteration of a set of credentials that should be considered
>   * immutable.
>   */
> -static inline const struct cred *get_cred(const struct cred *cred)
> +static inline const struct cred *get_cred_many(const struct cred *cred, int nr)
>  {
>  	struct cred *nonconst_cred = (struct cred *) cred;
>  	if (!cred)
>  		return cred;
>  	validate_creds(cred);
>  	nonconst_cred->non_rcu = 0;
> -	return get_new_cred(nonconst_cred);
> +	return get_new_cred_many(nonconst_cred, nr);
> +}
> +
> +static inline const struct cred *get_cred(const struct cred *cred)
> +{
> +	return get_cred_many(cred, 1);
>  }

Same issue with the kdoc comment headers.

>  static inline const struct cred *get_cred_rcu(const struct cred *cred)
> @@ -278,17 +288,22 @@ static inline const struct cred *get_cred_rcu(const struct cred *cred)
>   * on task_struct are attached by const pointers to prevent accidental
>   * alteration of otherwise immutable credential sets.
>   */
> -static inline void put_cred(const struct cred *_cred)
> +static inline void put_cred_many(const struct cred *_cred, int nr)
>  {
>  	struct cred *cred = (struct cred *) _cred;
>  
>  	if (cred) {
>  		validate_creds(cred);
> -		if (atomic_dec_and_test(&(cred)->usage))
> +		if (atomic_sub_and_test(nr, &cred->usage))
>  			__put_cred(cred);
>  	}
>  }
>  
> +static inline void put_cred(const struct cred *cred)
> +{
> +	put_cred_many(cred, 1);
> +}

Same issue with the kdoc comment headers.

>  /**
>   * current_cred - Access the current task's subjective credentials
>   *
> diff --git a/kernel/cred.c b/kernel/cred.c
> index 811ad654abd1..8a506bc7c1b8 100644
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -159,23 +159,30 @@ EXPORT_SYMBOL(__put_cred);
>   */
>  void exit_creds(struct task_struct *tsk)
>  {
> -	struct cred *cred;
> +	struct cred *real_cred, *cred;
>  
>  	kdebug("exit_creds(%u,%p,%p,{%d,%d})", tsk->pid, tsk->real_cred, tsk->cred,
>  	       atomic_read(&tsk->cred->usage),
>  	       read_cred_subscribers(tsk->cred));
>  
> -	cred = (struct cred *) tsk->real_cred;
> +	real_cred = (struct cred *) tsk->real_cred;
>  	tsk->real_cred = NULL;
> -	validate_creds(cred);
> -	alter_cred_subscribers(cred, -1);
> -	put_cred(cred);
>  
>  	cred = (struct cred *) tsk->cred;
>  	tsk->cred = NULL;
> -	validate_creds(cred);

Since both branches of the if-statement below end up validating @cred
you might as well still do the validation check here.

> -	alter_cred_subscribers(cred, -1);
> -	put_cred(cred);
> +
> +	if (real_cred == cred) {
> +		validate_creds(cred);
> +		alter_cred_subscribers(cred, -2);
> +		put_cred_many(cred, 2);
> +	} else {
> +		validate_creds(real_cred);
> +		validate_creds(cred);
> +		alter_cred_subscribers(real_cred, -1);
> +		put_cred(real_cred);
> +		alter_cred_subscribers(cred, -1);
> +		put_cred(cred);
> +	}
>  
>  #ifdef CONFIG_KEYS_REQUEST_CACHE
>  	key_put(tsk->cached_requested_key);
> @@ -352,8 +359,7 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
>  #endif
>  		clone_flags & CLONE_THREAD
>  	    ) {
> -		p->real_cred = get_cred(p->cred);
> -		get_cred(p->cred);
> +		p->real_cred = get_cred_many(p->cred, 2);
>  		alter_cred_subscribers(p->cred, 2);
>  		kdebug("share_creds(%p{%d,%d})",
>  		       p->cred, atomic_read(&p->cred->usage),
> @@ -517,8 +523,7 @@ int commit_creds(struct cred *new)
>  		proc_id_connector(task, PROC_EVENT_GID);
>  
>  	/* release the old obj and subj refs both */
> -	put_cred(old);
> -	put_cred(old);
> +	put_cred_many(old, 2);
>  	return 0;
>  }
>  EXPORT_SYMBOL(commit_creds);
> -- 
> 2.39.2

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