linux-kernel - Re: [RFC PATCH 1/2] lib, stackdepot: check stackdepot handle before accessing slabs.

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Date:   Thu, 2 Sep 2021 10:12:58 +1000
From:   imran.f.khan@...cle.com
To:     Vlastimil Babka <vbabka@...e.cz>, geert@...ux-m68k.org,
        akpm@...ux-foundation.org
Cc:     linux-kernel@...r.kernel.org,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [RFC PATCH 1/2] lib, stackdepot: check stackdepot handle before
 accessing slabs.



On 1/9/21 6:58 pm, Vlastimil Babka wrote:
> +CC linux-mm, Alex
> 
> On 9/1/21 07:19, Imran Khan wrote:
>> stack_depot_save allocates slabs that will be used for storing
>> objects in future.If this slab allocation fails we may get to
>> a situation where space allocation for a new stack_record fails,
>> causing stack_depot_save to return 0 as handle.
>> If user of this handle ends up invoking stack_depot_fetch with
>> this handle value, current implementation of stack_depot_fetch
>> will end up using slab from wrong index.
>> To avoid this check handle value at the beginning.
>> Also issue a warning for nil handle values and when slab allocation
>> for stackdepot fails for the first time.
>>
>> Signed-off-by: Imran Khan <imran.f.khan@...cle.com>
>> Suggested-by: Vlastimil Babka <vbabka@...e.cz>
> 
> Agree but without the warnings please, especially the "stack depot handle is
> absent" one. It's just something that can happen e.g. in GFP_NOWAIT contexts
> and no need to spam dmesg.
> 

Okay. I have removed warnings in v2 of patch set.

>> ---
>>   lib/stackdepot.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
>> index 0a2e417f83cb..1d42ef9ef766 100644
>> --- a/lib/stackdepot.c
>> +++ b/lib/stackdepot.c
>> @@ -232,6 +232,10 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
>>   	struct stack_record *stack;
>>   
>>   	*entries = NULL;
>> +	if (!handle) {
>> +		WARN(1, "stack depot handle is absent.\n");
>> +		return 0;
>> +	}
>>   	if (parts.slabindex > depot_index) {
>>   		WARN(1, "slab index %d out of bounds (%d) for stack id %08x\n",
>>   			parts.slabindex, depot_index, handle);
>> @@ -303,6 +307,8 @@ depot_stack_handle_t stack_depot_save(unsigned long *entries,
>>   		page = alloc_pages(alloc_flags, STACK_ALLOC_ORDER);
>>   		if (page)
>>   			prealloc = page_address(page);
>> +		else
>> +			WARN_ONCE(1, "slab allocation for stack depot failed.\n");
>>   	}
>>   
>>   	raw_spin_lock_irqsave(&depot_lock, flags);
>>
> 
Thanks for reviewing this.

--Imran