netdev - Re: [PATCH v2 06/10] mm/kmemleak: Replace strncpy() with __get_task

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Message-ID: <Zmqvu-1eUpdZ39PD@arm.com>
Date: Thu, 13 Jun 2024 09:37:15 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Yafang Shao <laoar.shao@...il.com>
Cc: torvalds@...ux-foundation.org, ebiederm@...ssion.com,
	alexei.starovoitov@...il.com, rostedt@...dmis.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, audit@...r.kernel.org,
	linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
	bpf@...r.kernel.org, netdev@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 06/10] mm/kmemleak: Replace strncpy() with
 __get_task_comm()

On Thu, Jun 13, 2024 at 10:30:40AM +0800, Yafang Shao wrote:
> Using __get_task_comm() to read the task comm ensures that the name is
> always NUL-terminated, regardless of the source string. This approach also
> facilitates future extensions to the task comm.
> 
> Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> ---
>  mm/kmemleak.c | 8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
> 
> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> index d5b6fba44fc9..ef29aaab88a0 100644
> --- a/mm/kmemleak.c
> +++ b/mm/kmemleak.c
> @@ -663,13 +663,7 @@ static struct kmemleak_object *__alloc_object(gfp_t gfp)
>  		strncpy(object->comm, "softirq", sizeof(object->comm));
>  	} else {
>  		object->pid = current->pid;
> -		/*
> -		 * There is a small chance of a race with set_task_comm(),
> -		 * however using get_task_comm() here may cause locking
> -		 * dependency issues with current->alloc_lock. In the worst
> -		 * case, the command line is not correct.
> -		 */
> -		strncpy(object->comm, current->comm, sizeof(object->comm));
> +		__get_task_comm(object->comm, sizeof(object->comm), current);
>  	}

You deleted the comment stating why it does not use get_task_comm()
without explaining why it would be safe now. I don't recall the details
but most likely lockdep warned of some potential deadlocks with this
function being called with the task_lock held.

So, you either show why this is safe or just use strscpy() directly here
(not sure we'd need strscpy_pad(); I think strscpy() would do, we just
need the NUL-termination).

-- 
Catalin