On Thu, Oct 22, 2020 at 12:25 PM Al Viro [off-list ref] 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);
}
A good/analogous example, but things get weird when the leaf node in
the call chain is inline asm: https://godbolt.org/z/s19TY5
(I'm not sure that's precisely what's going on here; I'll need to dive
more into the calls rw_copy_check_uvector() makes to see if there's
inline asm somewhere, pretty sure calls to get_user with `nr_regs`
exist).
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_.
--
Thanks,
~Nick Desaulniers