Nouveau - Mar 2024 - [PATCH] nouveau/dmem: handle kcalloc() allocation failure

On Sun, 03 Mar 2024, Timur Tabi <timur at kernel.org>
wrote:
> On Sun, Mar 3, 2024 at 4:46?AM Duoming Zhou <duoming at zju.edu.cn>
wrote:
>>
>> The kcalloc() in nouveau_dmem_evict_chunk() will return null if
>> the physical memory has run out. As a result, if we dereference
>> src_pfns, dst_pfns or dma_addrs, the null pointer dereference bugs
>> will happen.
>>
>> This patch uses stack variables to replace the kcalloc().
>
> Won't this blow the stack?  And why not just test the return value of
> kcalloc?
VLAs should not be used in the kernel anymore. Building this results in
a warning due to -Wvla. See 0bb95f80a38f ("Makefile: Globally enable VLA
warning").

Error checking and propagation is the way to go.


BR,
Jani.

-- 
Jani Nikula, Intel

On Mon, 04 Mar 2024 14:14:52 +0200 Jani Nikula wrote:
> >> The kcalloc() in nouveau_dmem_evict_chunk() will return null if
> >> the physical memory has run out. As a result, if we dereference
> >> src_pfns, dst_pfns or dma_addrs, the null pointer dereference bugs
> >> will happen.
> >>
> >> This patch uses stack variables to replace the kcalloc().
> >
> > Won't this blow the stack?  And why not just test the return value
of
> > kcalloc?
> 
> VLAs should not be used in the kernel anymore. Building this results in
> a warning due to -Wvla. See 0bb95f80a38f ("Makefile: Globally enable
VLA
> warning").
> 
> Error checking and propagation is the way to go.
The GPU is going away. If the kcalloc() in nouveau_dmem_evict_chunk() fail,
we could not evict all pages mapping a chunk. Do you think we should add a
__GFP_NOFAIL flag in kcalloc()? I see the __GFP_NOFAIL flag is used in the
following code:

/*
* _GFP_NOFAIL because the GPU is going away and there
* is nothing sensible we can do if we can't copy the
* data back.
*/
dpage = alloc_page(GFP_HIGHUSER | __GFP_NOFAIL);

Best regards,
Duoming Zhou

On Tue, 05 Mar 2024, duoming at zju.edu.cn wrote:
> On Mon, 04 Mar 2024 14:14:52 +0200 Jani Nikula wrote:
>> >> The kcalloc() in nouveau_dmem_evict_chunk() will return null
if
>> >> the physical memory has run out. As a result, if we
dereference
>> >> src_pfns, dst_pfns or dma_addrs, the null pointer dereference
bugs
>> >> will happen.
>> >>
>> >> This patch uses stack variables to replace the kcalloc().
>> >
>> > Won't this blow the stack?  And why not just test the return
value of
>> > kcalloc?
>> 
>> VLAs should not be used in the kernel anymore. Building this results in
>> a warning due to -Wvla. See 0bb95f80a38f ("Makefile: Globally
enable VLA
>> warning").
>> 
>> Error checking and propagation is the way to go.
>
> The GPU is going away. If the kcalloc() in nouveau_dmem_evict_chunk() fail,
> we could not evict all pages mapping a chunk. Do you think we should add a
> __GFP_NOFAIL flag in kcalloc()? I see the __GFP_NOFAIL flag is used in the
> following code:
>
> /*
> * _GFP_NOFAIL because the GPU is going away and there
> * is nothing sensible we can do if we can't copy the
> * data back.
> */
> dpage = alloc_page(GFP_HIGHUSER | __GFP_NOFAIL);
This is all up to the nouveau maintainers, really. All I'm saying is
that VLA isn't the solution you're looking for.

BR,
Jani.


-- 
Jani Nikula, Intel