Re: [rfa/amd64] Zero fill 32-bit registers

Andrew Cagney
2004-02-28T15:17:16Z

> Date: Thu, 26 Feb 2004 20:22:11 -0500
> From: Andrew Cagney <ac131313@redhat.com>
>
> Hello,
>
> For a 64-bit gregset, the code was only modifying the low 32-bits of the
> register field - leaving the upper 64-bits undefined.
>
> That's not completely unintentional. The idea is to leave any
> "reserved" bits untouched, and in a sense for 32-bit stuff the upper
> 32 bits are "reserved"; they are not necessarily zero, at least not
> for all registers.

Er, the upper 32-bits here aren't reserved. The request was for a
64-bit register, and this code is erreneously only supplying half that
value - that leaves the upper 32-bits undefined.

We've hit the same problem in the past with the MIPS. When only 32-bits
were available the value was expanded (in accordance with the ISA) to
the full 64-bits.

> I guess the thread code isn't doing the equivalent of the PT_GETREGS
> call. I think the correct way to fix this is to make sure the buffer
> is properly initialized before you pass it to
> amd64_collect_native_gregset.

Don't look at me, the buffer originated in libthread-db.

> Another problem with your patch is that I'd rather like avoid assuming
> that the register buffer is an array of 8-byte registers.

That code already assumems that, and that the values are little-endian.

Andrew