[BUG] arm64: an infinite loop in generic_perform_write()

[Xiaoming Ni]

On 2021/6/23 10:50, Al Viro wrote:
> On Wed, Jun 23, 2021 at 10:39:31AM +0800, Chen Huang wrote:
> 
>> Then when kernel handles the alignment_fault, it will not panic. As the
>> arm64 memory model spec said, when the address is not a multiple of the
>> element size, the access is unaligned. Unaligned accesses are allowed to
>> addresses marked as Normal, but not to Device regions. An unaligned access
>> to a Device region will trigger an exception (alignment fault).
>> 	
>> do_alignment_fault
>>      do_bad_area
>> 	__do_kernel_fault
>>             fixup_exception
>>
>> But that fixup cann't handle the unaligned copy, so the
>> copy_page_from_iter_atomic returns 0 and traps in loop.
> 
> Looks like you need to fix your raw_copy_from_user(), then...
> .
> 

Exit loop when iov_iter_copy_from_user_atomic() returns 0.
This should solve the problem, too, and it's easier.

Thanks.
Xiaoming Ni