linux-kernel - Re: [PATCH v2] dma-buf: dma-heap: Add a size check for allocation

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Date:   Wed, 5 Jan 2022 14:36:43 +0800
From:   <guangming.cao@...iatek.com>
To:     <christian.koenig@....com>
CC:     <Brian.Starkey@....com>, <benjamin.gaignard@...aro.org>,
        <bo.song@...iatek.com>, <dri-devel@...ts.freedesktop.org>,
        <guangming.cao@...iatek.com>, <jianjiao.zeng@...iatek.com>,
        <john.stultz@...aro.org>, <labbott@...hat.com>,
        <libo.kang@...iatek.com>, <linaro-mm-sig@...ts.linaro.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-media@...r.kernel.org>,
        <linux-mediatek@...ts.infradead.org>, <lmark@...eaurora.org>,
        <matthias.bgg@...il.com>, <mingyuan.ma@...iatek.com>,
        <sumit.semwal@...aro.org>, <wsd_upstream@...iatek.com>,
        <yf.wang@...iatek.com>
Subject: Re: [PATCH v2] dma-buf: dma-heap: Add a size check for allocation

From: Guangming.Cao <guangming.cao@...iatek.com>

On Tue, 2022-01-04 at 08:47 +0100, Christian K鰊ig wrote:
> Am 03.01.22 um 19:57 schrieb John Stultz:
> > On Mon, Dec 27, 2021 at 1:52 AM <guangming.cao@...iatek.com> wrote:
> > > From: Guangming <Guangming.Cao@...iatek.com>
> > > 
> > 
> > Thanks for submitting this!
> > 
> > > Add a size check for allcation since the allocation size is
> > 
> > nit: "allocation" above.
> > 
> > > always less than the total DRAM size.
> > 
> > In general, it might be good to add more context to the commit
> > message
> > to better answer *why* this change is needed rather than what the
> > change is doing.  ie: What negative thing happens without this
> > change?
> > And so how does this change avoid or improve things?
> 
> Completely agree, just one little addition: Could you also add this
> why 
> as comment to the code?
> 
> When we stumble over this five years from now it is absolutely not 
> obvious why we do this.
> 
> Thanks,
> Christian.
> 
Thanks for your reply!
I will update the related reason in the patch later.

The reason for adding this check is that we met a case that the user
sent an invalid size(It seems it's a negative value, MSB is 0xff, it's
larger than DRAM size after convert it to size_t) to dma-heap to alloc
memory, and this allocation was running on a process(such as "gralloc"
on Android device) can't be killed by OOM flow, and we also couldn't
find the related dmabuf in "dma_buf_debug_show" because the related
dmabuf was not exported yet since the allocation is still on going.

Since this invalid argument case can be prevented at dma-heap side, so,
I added this size check, and moreover, to let debug it easily, I also
added logs when size is bigger than a threshold we set in mtk system
heap.
If you think that print logs in dma-heap framework is better, I will
update it in next version.

If you have better solution(such as dump the size under allocating
in "dma_buf_debug_show", which maybe need add global variable to record
it), please kindly let me know.
Thanks :)
Guangming

> > 
> > 
> > > Signed-off-by: Guangming <Guangming.Cao@...iatek.com>
> > > Signed-off-by: jianjiao zeng <jianjiao.zeng@...iatek.com>
> > > ---
> > > v2: 1. update size limitation as total_dram page size.
> > >      2. update commit message
> > > ---
> > >   drivers/dma-buf/dma-heap.c | 2 ++
> > >   1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/drivers/dma-buf/dma-heap.c b/drivers/dma-buf/dma-
> > > heap.c
> > > index 56bf5ad01ad5..e39d2be98d69 100644
> > > --- a/drivers/dma-buf/dma-heap.c
> > > +++ b/drivers/dma-buf/dma-heap.c
> > > @@ -55,6 +55,8 @@ static int dma_heap_buffer_alloc(struct
> > > dma_heap *heap, size_t len,
> > >          struct dma_buf *dmabuf;
> > >          int fd;
> > > 
> > > +       if (len / PAGE_SIZE > totalram_pages())
> > > +               return -EINVAL;
> > 
> > This seems sane. I know ION used to have some 1/2 of memory cap to
> > avoid unnecessary memory pressure on crazy allocations.
> > 
> > Could you send again with an improved commit message?
> > 
> > thanks
> > -john
> 
>