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From: "Robert N. M. Watson" <rwatson@freebsd.org>
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Date: Mon, 28 May 2012 13:16:27 -0400
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To: Konstantin Belousov <kostikbel@gmail.com>
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Cc: src-committers@freebsd.org, svn-src-all@freebsd.org,
	Ed Schouten <ed@freebsd.org>, Bruce Evans <brde@optusnet.com.au>,
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Subject: Re: svn commit: r236026 - in head/sys: amd64/linux32
	compat/freebsd32 kern
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On 28 May 2012, at 09:36, Konstantin Belousov wrote:

> On Sun, May 27, 2012 at 07:49:36AM +1000, Bruce Evans wrote:
>> On Sat, 26 May 2012, Konstantin Belousov wrote:
>>=20
>>> On Sat, May 26, 2012 at 10:21:25PM +1000, Bruce Evans wrote:
>>> The 'low level' AKA magic happens in several *_fetch_syscall_args()
>>> functions. For both linux32 and freebsd32, the magic code =
automatically
>>> zero-extends the arguments into 64bit entities. Linux passes args in
>>> registers, while FreeBSD uses words on stack.
>>=20
>> Actually, the amd64 linux_fetch32_fetch_syscall_args() just copies =
from
>> 64-bit registers frame->tf_r* to 64-bit sa->args[*].  I can't see how
>> this gives anything except garbage in the top bits.  Is there magic =
in
>> the switch to 64-bit mode that sets the top bits?  Anyway, sign =
extension
>> would give garbage for unsigned args, and zero-extension would give
>> garbage for negative signed args.
> Hardware zero-extends any register touched in the 32bit mode.
>=20
> In fact, please see r217991 for related bug.

This may well be true on Intel, but is not true of MIPS -- which we =
probably don't care about currently for the purposes of Linux emulation, =
but maybe someday we will. On MIPS, 32-bit values are sign-extended =
rather than zero-extended.

I see a somewhat complex thread here, but am not sure I quite understand =
the import for Capsicum. Is the 64-bit rights mask as part of system =
call arguments not working properly in compat32 scenarios? Or are there =
issues outside of the compat environment? Right now compat32 is not =
well-supported with Capsicum, but fixing that is quite important to =
productionising Capsicum.

Robert=