Is it illegal to have kernel virtual address == 0 ?

Is it illegal to have kernel virtual address == 0 ?

[Ran Shalit]

Hello,

I've seen some modules where it is tested if virtual address is 0, and
if yes, it is acted as if it is an error.
But can't virtual address be zero too ?

For example, in the following module:
https://github.com/khilman/omap-test-dmatest/blob/master/main.c
it is checked the returned value of dma_alloc_coherent as following:

dma_test[i].src_buf = (unsigned long) dma_alloc_coherent(NULL,
buf_size (dma_addr_t *)&dma_test[i].src_buf_phys, GFP_KERNEL |
GFP_DMA);
if (!dma_test[i].src_buf) {
   printk("dmatest_init(): get_zeroed_page() failed.\n");
   r = -ENOMEM;
   goto cleanup;
}

As you can see , when  dma_alloc_coherent returns 0 , it is acted as
if it is an error.

Thank you,
Ran

Is it illegal to have kernel virtual address == 0 ?

[Greg KH]

On Sun, Jan 15, 2017 at 12:17:40PM +0200, Ran Shalit wrote:
> Hello,
> 
> I've seen some modules where it is tested if virtual address is 0, and
> if yes, it is acted as if it is an error.
> But can't virtual address be zero too ?
> 
> For example, in the following module:
> https://github.com/khilman/omap-test-dmatest/blob/master/main.c
> it is checked the returned value of dma_alloc_coherent as following:
> 
> dma_test[i].src_buf = (unsigned long) dma_alloc_coherent(NULL,
> buf_size (dma_addr_t *)&dma_test[i].src_buf_phys, GFP_KERNEL |
> GFP_DMA);
> if (!dma_test[i].src_buf) {
>    printk("dmatest_init(): get_zeroed_page() failed.\n");
>    r = -ENOMEM;
>    goto cleanup;
> }
> 
> As you can see , when  dma_alloc_coherent returns 0 , it is acted as
> if it is an error.

Yes, if dma_alloc_coherent returns NULL, an error happened and the
memory could not be allocated.

In-kernel memory pointers will never be NULL if they are valid.

Hope this helps,

greg k-h

Is it illegal to have kernel virtual address == 0 ?

[Ran Shalit]

On Sun, Jan 15, 2017 at 12:54 PM, Greg KH <greg@kroah.com> wrote:
> On Sun, Jan 15, 2017 at 12:17:40PM +0200, Ran Shalit wrote:
>> Hello,
>>
>> I've seen some modules where it is tested if virtual address is 0, and
>> if yes, it is acted as if it is an error.
>> But can't virtual address be zero too ?
>>
>> For example, in the following module:
>> https://github.com/khilman/omap-test-dmatest/blob/master/main.c
>> it is checked the returned value of dma_alloc_coherent as following:
>>
>> dma_test[i].src_buf = (unsigned long) dma_alloc_coherent(NULL,
>> buf_size (dma_addr_t *)&dma_test[i].src_buf_phys, GFP_KERNEL |
>> GFP_DMA);
>> if (!dma_test[i].src_buf) {
>>    printk("dmatest_init(): get_zeroed_page() failed.\n");
>>    r = -ENOMEM;
>>    goto cleanup;
>> }
>>
>> As you can see , when  dma_alloc_coherent returns 0 , it is acted as
>> if it is an error.
>
> Yes, if dma_alloc_coherent returns NULL, an error happened and the
> memory could not be allocated.
>
> In-kernel memory pointers will never be NULL if they are valid.
>
> Hope this helps,

Thank you very much,
Ran

>
> greg k-h