linux-kernel - Re: [PATCH] fork: fix oops after fork failure

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Date:	Thu, 23 Aug 2012 16:33:12 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	torvalds@...ux-foundation.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] fork: fix oops after fork failure

On Thu 23-08-12 17:08:46, Glauber Costa wrote:
> When we want to duplicate a new process, dup_task_struct() will undergo
> a series of allocations. If alloc_thread_info_node() fails, we call
> free_task_struct() and return.
> 
> This seems right, but it is not. free_task_struct() will not only free
> the task struct from the kmem_cache, but will also call
> arch_release_task_struct(). The problem is that this function is
> supposed to undo whatever arch-specific work done by
> arch_dup_task_struct(), that is not yet called at this point.  The
> particular problem I ran accross was that in x86, we will arrive at
> fpu_free() without having ever allocated it.
> 
> This code is very ancient, and according to git, it is there since the
> pre-git era. But forks don't fail that often, so that made it well
> hidden.
> 
> Signed-off-by: Glauber Costa <glommer@...allels.com>
> Reported-by: Frederic Weisbecker <fweisbec@...hat.com>
> ---
>  kernel/fork.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 152d023..b397435 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -299,7 +299,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
>  
>  	ti = alloc_thread_info_node(tsk, node);
>  	if (!ti) {
> -		free_task_struct(tsk);
> +		kmem_cache_free(task_struct_cachep, tsk);

What about ia64 (or !CONFIG_ARCH_THREAD_INFO_ALLOCATOR in general) which
doesn't allocate thread_info at all?

>  		return NULL;
>  	}
>  
> -- 
> 1.7.11.4
> 
> --
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-- 
Michal Hocko
SUSE Labs
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