Re: [PATCH 2/5] kernel/fork: allocate task->comm dynamicly

[Kees Cook]

On Wed, Sep 29, 2021 at 11:50:33AM +0000, Yafang Shao wrote:
> task->comm is defined as an array embedded in struct task_struct before.
> This patch changes it to a char pointer. It will be allocated in the fork
> and freed when the task is freed.
> 
> Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> ---
>  include/linux/sched.h |  2 +-
>  kernel/fork.c         | 19 +++++++++++++++++++
>  2 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index e12b524426b0..b387b5943db4 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1051,7 +1051,7 @@ struct task_struct {
>  	 * - access it with [gs]et_task_comm()
>  	 * - lock it with task_lock()
>  	 */
> -	char				comm[TASK_COMM_LEN];
> +	char				*comm;

This, I think, is basically a non-starter. It adds another kmalloc to
the fork path without a well-justified reason. TASK_COMM_LEN is small,
yes, but why is growing it valuable enough to slow things down?

(Or, can you prove that this does NOT slow things down? It seems like
it would.)

-Kees

>  
>  	struct nameidata		*nameidata;
>  
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 38681ad44c76..227aec240501 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -721,6 +721,20 @@ static void mmdrop_async(struct mm_struct *mm)
>  	}
>  }
>  
> +static int task_comm_alloc(struct task_struct *p)
> +{
> +	p->comm = kzalloc(TASK_COMM_LEN, GFP_KERNEL);
> +	if (!p->comm)
> +		return -ENOMEM;
> +
> +	return 0;
> +}
> +
> +static void task_comm_free(struct task_struct *p)
> +{
> +	kfree(p->comm);
> +}
> +
>  static inline void free_signal_struct(struct signal_struct *sig)
>  {
>  	taskstats_tgid_free(sig);
> @@ -753,6 +767,7 @@ void __put_task_struct(struct task_struct *tsk)
>  	bpf_task_storage_free(tsk);
>  	exit_creds(tsk);
>  	delayacct_tsk_free(tsk);
> +	task_comm_free(tsk);
>  	put_signal_struct(tsk->signal);
>  	sched_core_free(tsk);
>  
> @@ -2076,6 +2091,10 @@ static __latent_entropy struct task_struct *copy_process(
>  	if (data_race(nr_threads >= max_threads))
>  		goto bad_fork_cleanup_count;
>  
> +	retval = task_comm_alloc(p);
> +	if (retval)
> +		goto bad_fork_cleanup_count;
> +
>  	delayacct_tsk_init(p);	/* Must remain after dup_task_struct() */
>  	p->flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER | PF_IDLE | PF_NO_SETAFFINITY);
>  	p->flags |= PF_FORKNOEXEC;
> -- 
> 2.17.1
> 

-- 
Kees Cook