On Tue, Feb 16, 2016 at 12:21:53PM +0000, Catalin Marinas wrote: > On Tue, Feb 16, 2016 at 10:57:49AM +0000, Robin Murphy wrote: > > On 16/02/16 10:31, Will Deacon wrote: > > >On Tue, Feb 16, 2016 at 04:44:35AM +0000, EunTaik Lee wrote: > > >>Userspace memory is mapped as below: > > >>F2A7F000--F2A7FFFF Normal Memory > > >>F2A80000--F2A80FFF Device nGnRnE > > >> > > >>And that userspace application makes a system call > > >>as below: > > >> > > >>-009 |do_strncpy_from_user(inline) > > >>-009 |strncpy_from_user() > > >>-010 |getname_flags() > > >>-011 |user_path_at_empty() > > >>-012 |user_path_at() > > >>-013 |SYSC_faccessat(inline) > > >>-013 |sys_faccessat() > > >>-014 |__sys_trace(asm) > > >> --> |exception > > >> > > >>The string spans from 0xF2A7FFC1 to 0xF2A7FFFB. > > >> > > >>When do_strncpy_from_user() reads the last (unsigned long) > > >>value, the alignement fault is triggered. The 8 byte > > >>from 0xF2A7FFC1 spans to the next page that is mapped as > > >>Device nGnRnE, which does not allow an unaligned access, > > >>causes the abort. > > >> > > >>The instruction which caused the alignment fault is registered > > >>in the fixup table but the exception handler does not reach there. > > >> > > >>This patch registers a alignment fault handler and fixes up the > > >>pc if appropriate. > > > > > >As discussed with Catalin previously, we should solve this by adding a > > >guard page rather than handling the fault. > > I don't think we can trivially add this without implementing an arm64 > specific arch_get_unmapped_area().
Even overriding arch_get_unmapped_area doesn't help as much as you might like since, in the case of something like /dev/mem, the memory is remapped using remap_pfn_range later on, so you can't necessarily tell what the final attributes are likely to be when you initially allocate the virtual space. Will