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

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Date:	Thu, 23 Aug 2012 18:38:21 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Michal Hocko <mhocko@...e.cz>
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 08/23/2012 06:33 PM, Michal Hocko wrote:
> 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?

They would return something meaningful here anyway, otherwise this would
already error out and exit.

But you actually have a point. Not all architectures (all but ia64) will
allocathe the task struct from the slab... sigh...

Sorry, I will come up with something for this.
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