Re: [PATCH v2] async_pq: Remove VLA usage

[Kees Cook]

On Tue, May 29, 2018 at 3:40 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> [ add Vinod's korg address ]
>
> On Tue, May 29, 2018 at 3:37 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>> [ adding linux-raid ]
>>
>> Apologies for the delay, the recent ping from Kees made me take a closer look.
>>
>> On Sat, May 5, 2018 at 12:58 AM, Kyle Spiers <ksspiers@xxxxxxxxxx> wrote:
>>> In the quest to remove VLAs from the kernel[1], this moves the
>>> allocation of coefs and blocks from the stack to being kmalloc()ed.
>>>
>>> [1] https://lkml.org/lkml/2018/3/7/621
>>>
>>> Signed-off-by: Kyle Spiers <ksspiers@xxxxxxxxxx>
>>> ---
>>> Forgot to add slab.h
>>> ---
>>>  crypto/async_tx/async_pq.c  | 18 ++++++++++++++----
>>>  crypto/async_tx/raid6test.c |  9 ++++++++-
>>>  2 files changed, 22 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/crypto/async_tx/async_pq.c b/crypto/async_tx/async_pq.c
>>> index 56bd612927ab..af1912313a23 100644
>>> --- a/crypto/async_tx/async_pq.c
>>> +++ b/crypto/async_tx/async_pq.c
>>> @@ -194,9 +194,9 @@ async_gen_syndrome(struct page **blocks, unsigned int offset, int disks,
>>>             (src_cnt <= dma_maxpq(device, 0) ||
>>>              dma_maxpq(device, DMA_PREP_CONTINUE) > 0) &&
>>>             is_dma_pq_aligned(device, offset, 0, len)) {
>>> -               struct dma_async_tx_descriptor *tx;
>>> +               struct dma_async_tx_descriptor *tx = NULL;
>>>                 enum dma_ctrl_flags dma_flags = 0;
>>> -               unsigned char coefs[src_cnt];
>>> +               unsigned char *coefs;
>> At a minimum this needs to be GFP_NOIO since raid may be in the
>> page-reclaim path.
>>
>> However, I think it may be better to allocate this with the rest of
>> the stripe resources and pass it in as a scratch buffer. See the usage
>> / implementation of scribble_alloc() in drivers/md/raid5.c.

Given this:

        int src_cnt = disks - 2;
...
        BUG_ON(disks > 255 || ...)

what about just making something like:

#define MAX_DISKS 255
...
        int src_cnt = disks - 2;
...
        BUG_ON(disks > MAX_DISKS || ...)
...
                 unsigned char coefs[MAX_DISKS];

?

-Kees

-- 
Kees Cook
Pixel Security