Re: [PATCH v2] async_pq: Remove VLA usage

[Dan Williams]

On Wed, May 30, 2018 at 2:33 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 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];
>
> ?

Yeah, we were already potentially suffering a value that large on the
stack in a max config case anyway with the VLA. Looks ok to me.