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Subject: Re: [PATCH 4/4] gpio: Remove VLA from stmpe driver
From:   Laura Abbott <labbott@redhat.com>
To:     Phil Reid <preid@electromag.com.au>,
        Linus Walleij <linus.walleij@linaro.org>,
        Kees Cook <keescook@chromium.org>,
        Patrice Chotard <patrice.chotard@st.com>
Cc:     linux-gpio@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-hardening@lists.openwall.com
References: <20180310001021.6437-1-labbott@redhat.com>
 <20180310001021.6437-5-labbott@redhat.com>
 <9175611d-049d-ec28-0716-e45fccfd579f@electromag.com.au>
 <4e64bbae-e91b-f0c5-ff79-3dde8fbe2119@redhat.com>
Message-ID: <d37dbce5-43ca-b70a-41da-5ccfc673d3ac@redhat.com>
Date:   Tue, 13 Mar 2018 18:16:51 -0700
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On 03/13/2018 05:18 PM, Laura Abbott wrote:
> On 03/13/2018 02:13 AM, Phil Reid wrote:
>> On 10/03/2018 08:10, Laura Abbott wrote:
>>> The new challenge is to remove VLAs from the kernel
>>> (see https://lkml.org/lkml/2018/3/7/621)
>>>
>>> This patch replaces a VLA with an appropriate call to kmalloc_array.
>>>
>>> Signed-off-by: Laura Abbott <labbott@redhat.com>
>>> ---
>>>   drivers/gpio/gpio-stmpe.c | 7 ++++++-
>>>   1 file changed, 6 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpio/gpio-stmpe.c b/drivers/gpio/gpio-stmpe.c
>>> index f8d7d1cd8488..b7854850bcdb 100644
>>> --- a/drivers/gpio/gpio-stmpe.c
>>> +++ b/drivers/gpio/gpio-stmpe.c
>>> @@ -369,10 +369,14 @@ static irqreturn_t stmpe_gpio_irq(int irq, void *dev)
>>>       struct stmpe *stmpe = stmpe_gpio->stmpe;
>>>       u8 statmsbreg;
>>>       int num_banks = DIV_ROUND_UP(stmpe->num_gpios, 8);
>>> -    u8 status[num_banks];
>>> +    u8 *status;
>>>       int ret;
>>>       int i;
>>> +    status = kmalloc_array(num_banks, sizeof(*status), GFP_ATOMIC);
>>> +    if (!status)
>>> +        return IRQ_NONE;
>>> +
>>>       /*
>>>        * the stmpe_block_read() call below, imposes to set statmsbreg
>>>        * with the register located at the lowest address. As STMPE1600
>>> @@ -424,6 +428,7 @@ static irqreturn_t stmpe_gpio_irq(int irq, void *dev)
>>>           }
>>>       }
>>> +    kfree(status);
>>>       return IRQ_HANDLED;
>>>   }
>>>
>>
>> Doing this in an irq handler seems wrong.
>> Perhaps better if a buffer is pre-allocated in stmpe_gpio
>>
>>
> 
> Sure, I can pre-allocate the buffer in the probe.
> 
> Thanks,
> Laura

Actually I wonder if there would be concurrency problems if we
tried to pre-allocate a global buffer. But the IRQ handler
calls stmpe_block_read which takes a mutex to sleep so I think
doing the (small) kmalloc should be fine and I can change it to
a GFP_KERNEL.

Thanks,
Laura