Re: [PATCH] parisc: Fix syscall restarts (v2)

[John David Anglin <dave.anglin@bell.net>]

On 2015-12-24 11:07 AM, Mathieu Desnoyers wrote:
>>> Is it valid to have unaligned instructions ? Does the architecture
>>> >>allow it, or it's a fumble and we should pr_warn ?
>> >
>> >How can it be unaligned? It's about u32...
> That would be an instruction that is volountarily offset
> from 1, 2, 3 bytes from 4-bytes multiples by the application.
> The only situation where I have seen this is in cases where
> applications are trying to play games with the debugger or
> disassembler and hide what they are doing: they can offset
> the start of a function like this, and therefore all the
> instructions within that function.
>
This is not possible on PA-RISC.  Indeed, user instruction addresses are 
always
offset by three.  There is no way to branch to an instruction that is 
offset.

The least significant two bits of an instruction address contain a 
priority level.
User code on linux and hpux executes at level 3.  The only way a user 
can change
privilege level is with a "gate" instruction (a special branch). Whether 
this is
permitted or not depends on page table permissions that the user can't 
change.
In practice, a level change is only allowed on the gateway page.

Dave

-- 
John David Anglin  dave.anglin@bell.net