On Wed, Jun 23, 2021 at 04:27:37AM +0000, Al Viro wrote: > On Wed, Jun 23, 2021 at 11:24:54AM +0800, Xiaoming Ni wrote: > > 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. > > It might be easier, but it's not going to work correctly. > If the page gets evicted by memory pressure, you are going > to get spurious short write. > > Besides, it's simply wrong - write(2) does *NOT* require an > aligned source. It (and raw_copy_from_user()) should act the > same way memcpy(3) does. On arm64, neither memcpy() nor raw_copy_from_user() are expected to work on Device mappings, we have memcpy_fromio() for this but only for ioremap(). There's no (easy) way to distinguish in the write() syscall how the source buffer is mapped. generic_perform_write() does an iov_iter_fault_in_readable() check but that's not sufficient and it also breaks the cases where you can get intra-page faults (arm64 MTE or SPARC ADI). I think in the general case it's racy anyway (another thread doing an mprotect(PROT_NONE) after the readable check passed). So I think generic_perform_write() returning -EFAULT if copied == 0 would make sense (well, unless it breaks other cases I'm not aware of). -- Catalin
