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Re: [f-cpu] little feed-back from the libre softawre meeting



Seriously,

'x' : executable -> ICACHE, because it is the instruction fetcher which
needs to access bytes in code page : IT NEVER WRITES !!! so 'x' is in fact a
disguised 'r' and 'w' a non-sense.

'r','w' : readable, writable -> DCACHE, because it is the LSU which needs to
access bytes in data page : IT NEVER EXECUTES !!!

When you write code in page you are indeed using LSU for that purpose so you
handle its page like a data page not as a code page.

----- Original Message -----
From: "Cedric BAIL" <cedric.bail@free.fr>
To: <f-cpu@seul.org>
Sent: Sunday, July 14, 2002 8:09 PM
Subject: Re: [f-cpu] little feed-back from the libre softawre meeting


> > > 2) Supervisor/user bits :
> > >     - 's' : some instructions which are considered as priviledged
requires
> > > this bit set.
> >
> > Inside the TLB?
>
> Question: which instruction, normally the only one is get and put the
other
> didn't affect the CPU ?
>
> > > But I aggree with you : we should have like 'sr','sw','ur','uw' for
data
> > > memory and 'sx','ux' for code memory (that way we can protect some
user
> > > applications for accessing supervisor code pages with 'sx = 1' and
'ux' = 0
> > > in fact) if we want a more fined-grain protection.
>
> you must have rwx bits that must be differencied. I mean that on a x86,
you
> have difficult to sy that a page is only executable and not readable, in
fact
> it was the only big recommandation, we must have 3 bits to say rwx rights.
>
> > > 5) Ring bits : well instead of Supervisor/user bit we have several
bits to
> > > encode a level ring. A page must have a ring of lower priority to be
> > > accessed.
> >
> > Big can of worms. If privileged code always has at least the same
> > access rights as unprivileged code, you have a built-in security hole
> > (see Intel). Contrary to popular belief, one must be able to give
> > privileged code *less* access rights than unprivileged code.
>
> In fact ring a really not useful, I think that on the majority of the
> processor where it's implemented it is not used. (in hurd, and linux it's
> not used, i don't know for bsd).
>
> > > 6) Inheritance bits : mostly a software issue I think, so I wouldn't
detail
> > > them here.
> >
> > Who inherits what?
>
> I didn't understand the meaning of inheritance bits.
>
> Cedric
>
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