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

[David Laight <David.Laight@ACULAB.COM>]

From: Al Viro
> Sent: 22 October 2020 20:25
> 
> On Thu, Oct 22, 2020 at 12:04:52PM -0700, Nick Desaulniers wrote:
> 
> > Passing an `unsigned long` as an `unsigned int` does no such
> > narrowing: https://godbolt.org/z/TvfMxe (same vice-versa, just tail
> > calls, no masking instructions).
> > So if rw_copy_check_uvector() is inlined into import_iovec() (looking
> > at the mainline@1028ae406999), then children calls of
> > `rw_copy_check_uvector()` will be interpreting the `nr_segs` register
> > unmodified, ie. garbage in the upper 32b.
> 
> FWIW,
> 
> void f(unsinged long v)
> {
> 	if (v != 1)
> 		printf("failed\n");
> }
> 
> void g(unsigned int v)
> {
> 	f(v);
> }
> 
> void h(unsigned long v)
> {
> 	g(v);
> }
> 
> main()
> {
> 	h(0x100000001);
> }
> 
> must not produce any output on a host with 32bit int and 64bit long, regardless of
> the inlining, having functions live in different compilation units, etc.
> 
> Depending upon the calling conventions, compiler might do truncation in caller or
> in a callee, but it must be done _somewhere_.

Put g() in a separate compilation unit and use the 'wrong' type
in the prototypes t() used to call g() and g() uses to call f().

Then you might see where and masking does (or does not) happen.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)