Re: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c"

Segher Boessenkool
2020-10-23T18:35:02Z

On Fri, Oct 23, 2020 at 06:58:57PM +0100, Al Viro wrote:
> On Fri, Oct 23, 2020 at 03:09:30PM +0200, David Hildenbrand wrote:
>
> > Now, I am not a compiler expert, but as I already cited, at least on
> > x86-64 clang expects that the high bits were cleared by the caller - in
> > contrast to gcc. I suspect it's the same on arm64, but again, I am no
> > compiler expert.
> >
> > If what I said and cites for x86-64 is correct, if the function expects
> > an "unsigned int", it will happily use 64bit operations without further
> > checks where valid when assuming high bits are zero. That's why even
> > converting everything to "unsigned int" as proposed by me won't work on
> > clang - it assumes high bits are zero (as indicated by Nick).
> >
> > As I am neither a compiler experts (did I mention that already? ;) ) nor
> > an arm64 experts, I can't tell if this is a compiler BUG or not.
>
> On arm64 when callee expects a 32bit argument, the caller is *not* responsible
> for clearing the upper half of 64bit register used to pass the value - it only
> needs to store the actual value into the lower half. The callee must consider
> the contents of the upper half of that register as undefined. See AAPCS64 (e.g.
> https://github.com/ARM-software/abi-aa/blob/master/aapcs64/aapcs64.rst#parameter-passing-rules
> ); AFAICS, the relevant bit is
> "Unlike in the 32-bit AAPCS, named integral values must be narrowed by
> the callee rather than the caller."

Or the formal rule:

C.9 If the argument is an Integral or Pointer Type, the size of the
argument is less than or equal to 8 bytes and the NGRN is less
than 8, the argument is copied to the least significant bits in
x[NGRN]. The NGRN is incremented by one. The argument has now
been allocated.

Segher