On 22.10.20 21:24, Al Viro wrote:

On Thu, Oct 22, 2020 at 12:04:52PM -0700, Nick Desaulniers wrote:

quoted

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_.

The interesting case is having g() in a separate compilation unit and
force-calling g() with 0x100000001 via inline ASM. So forcing garbage
into high bits.

I'll paly with it.

-- 
Thanks,

David / dhildenb