Re: [RFC, PATCHv1 16/28] x86/asm: remove __VIRTUAL_MASK_SHIFT==47 assert

[Kirill A. Shutemov]

On Thu, Dec 08, 2016 at 10:39:57AM -0800, Andy Lutomirski wrote:
> On Thu, Dec 8, 2016 at 8:21 AM, Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > We don't need it anymore. 17be0aec74fb ("x86/asm/entry/64: Implement
> > better check for canonical addresses") made canonical address check
> > generic wrt. address width.
> 
> This code existed in part to remind us that this needs very careful
> adjustment when the paging size becomes dynamic.  If you want to
> remove it, please add test cases to tools/testing/selftests/x86 that
> verify:
> 
> a. Either mmap(2^47-4096, ..., MAP_FIXED, ...) fails or that, if it
> succeeds and you put a syscall instruction at the very end, that
> invoking the syscall instruction there works.  The easiest way to do
> this may be to have the selftest literally have a page of text that
> has 4094 0xcc bytes and a syscall and to map that page or perhaps move
> it into place with mremap.  That will avoid annoying W^X userspace
> stuff from messing up the test.  You'll need to handle the signal when
> you fall off the end of the world after the syscall.
> 
> b. Ditto for the new highest possible userspace page.
> 
> c. Ditto for one page earlier to make sure that your test actually works.
> 
> d. For each possible maximum address, call raise(SIGUSR1) and, in the
> signal handler, change RIP to point to the first noncanonical address
> and RCX to match RIP.  Return and catch the resulting exception.  This
> may be easy to integrate into the sigreturn tests, and I can help with
> that.

Thanks, for hints.

I'll come back to you with testcases to verify that they are you wanted
to see.

-- 
 Kirill A. Shutemov