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

From: Segher Boessenkool <[email-masked]>
Date: Sat, 24 Oct 2020 12:29:03 -0500

On Fri, Oct 23, 2020 at 09:28:59PM +0000, David Laight wrote:
> From: Segher Boessenkool
> > Sent: 23 October 2020 19:27
> > 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:
> > > 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.
> 
> So, in essence, if the value is in a 64bit register the calling
> code is independent of the actual type of the formal parameter.
> Clearly a value might need explicit widening.

No, this says that if you pass a 32-bit integer in a 64-bit register,
then the top 32 bits of that register hold an undefined value.

> I've found a copy of the 64 bit arm instruction set.
> Unfortunately it is alpha sorted and repetitive so shows none
> of the symmetry and makes things difficult to find.

All of this is ABI, not ISA.  Look at the AAPCS64 pointed to above.

> But, contrary to what someone suggested most register writes
> (eg from arithmetic) seem to zero/extend the high bits.

Everything that writes a "w" does, yes.  But that has nothing to do with
the parameter passing rules, that is ABI.  It just means that very often
a 32-bit integer will be passed zero-extended in a 64-bit register, but
that is just luck (or not, it makes finding bugs harder ;-) )

Segher