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

[greg@kroah.com (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