linux-kernel - Re: [PATCH bpf-next 1/4] bpf: enable task local storage for tracing programs

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Date:   Mon, 11 Jan 2021 10:56:50 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     Song Liu <songliubraving@...com>
CC:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <mingo@...hat.com>,
        <peterz@...radead.org>, <ast@...nel.org>, <daniel@...earbox.net>,
        <andrii@...nel.org>, <john.fastabend@...il.com>,
        <kpsingh@...omium.org>, <kernel-team@...com>, <haoluo@...gle.com>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH bpf-next 1/4] bpf: enable task local storage for tracing
 programs

On Fri, Jan 08, 2021 at 03:19:47PM -0800, Song Liu wrote:

[ ... ]

> diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
> index dd5aedee99e73..9bd47ad2b26f1 100644
> --- a/kernel/bpf/bpf_local_storage.c
> +++ b/kernel/bpf/bpf_local_storage.c
> @@ -140,17 +140,18 @@ static void __bpf_selem_unlink_storage(struct bpf_local_storage_elem *selem)
>  {
>  	struct bpf_local_storage *local_storage;
>  	bool free_local_storage = false;
> +	unsigned long flags;
>  
>  	if (unlikely(!selem_linked_to_storage(selem)))
>  		/* selem has already been unlinked from sk */
>  		return;
>  
>  	local_storage = rcu_dereference(selem->local_storage);
> -	raw_spin_lock_bh(&local_storage->lock);
> +	raw_spin_lock_irqsave(&local_storage->lock, flags);
It will be useful to have a few words in commit message on this change
for future reference purpose.

Please also remove the in_irq() check from bpf_sk_storage.c
to avoid confusion in the future.  It probably should
be in a separate patch.

[ ... ]

> diff --git a/kernel/bpf/bpf_task_storage.c b/kernel/bpf/bpf_task_storage.c
> index 4ef1959a78f27..f654b56907b69 100644
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 7425b3224891d..3d65c8ebfd594 100644
[ ... ]

> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -96,6 +96,7 @@
>  #include <linux/kasan.h>
>  #include <linux/scs.h>
>  #include <linux/io_uring.h>
> +#include <linux/bpf.h>
>  
>  #include <asm/pgalloc.h>
>  #include <linux/uaccess.h>
> @@ -734,6 +735,7 @@ void __put_task_struct(struct task_struct *tsk)
>  	cgroup_free(tsk);
>  	task_numa_free(tsk, true);
>  	security_task_free(tsk);
> +	bpf_task_storage_free(tsk);
>  	exit_creds(tsk);
If exit_creds() is traced by a bpf and this bpf is doing
bpf_task_storage_get(..., BPF_LOCAL_STORAGE_GET_F_CREATE),
new task storage will be created after bpf_task_storage_free().

I recalled there was an earlier discussion with KP and KP mentioned
BPF_LSM will not be called with a task that is going away.
It seems enabling bpf task storage in bpf tracing will break
this assumption and needs to be addressed?