Re: [RFC PATCH v2 2/2] target/ppc: make gdb able to translate priviledged addresses

[Richard Henderson]

On 6/15/21 4:32 AM, Bruno Piazera Larsen wrote:
> On 14/06/2021 19:37, Richard Henderson wrote:
> > On 6/14/21 12:16 PM, Bruno Larsen (billionai) wrote:
> > > This patch changes ppc_cpu_get_phys_page_debug so that it is now
> > > able to translate both, priviledged and real mode addresses
> > > independently of whether the CPU executing it has those permissions
> > >
> > > This was mentioned by Fabiano as something that would be very useful to
> > > help with debugging, but could possibly constitute a security issue if
> > > that debug function can be called in some way by prodution code. the
> > > solution was implemented such that it would be trivial to wrap it around
> > > ifdefs for building only with --enable-debug, for instance, but we are
> > > not sure this is the best approach, hence why it is an RFC.
> > >
> > > Suggested-by: Fabiano Rosas<farosas@linux.ibm.com>
> > > Signed-off-by: Bruno Larsen (billionai)<bruno.larsen@eldorado.org.br>
> > > ---
> > >   target/ppc/mmu_helper.c | 23 +++++++++++++++++++++++
> > >   1 file changed, 23 insertions(+)
> >
> > I think the first part is unnecessary.  Either the cpu is in supervisor mode or it 
> > isn't, and gdb should use the correct address space.  If you really want to force 
> > supervisor lookup from a guest that is paused in usermode, I suppose you could force 
> > MSR.PR=1 while you're performing the access and set it back afterward.
> I don't see why GDB should not be able to see supervisor level addresses just because the 
> CPU can't.

Because then when you are debugging, you then don't know whether the address is actually 
accessible in the current cpu context.

> > I think the second part is actively wrong -- real-mode address lookup will (for the most 
> > part) always succeed.  Moreover, the gdb user will have no idea that you've silently 
> > changed addressing methods.
>
> I disagree. Real-mode address will mostly fail, since during the boot process Linux 
> kernels set the MMU to use only virtual addresses, so real mode addresses only work when 
> debugging the firmware or the early setup of the kernel. After that, GDB can basically 
> only see virtual addresses.

Exactly.  But you changed that so that any unmapped address will re-try with real-mode, 
which (outside of hv) simply maps real->physical and returns the input.

One should have to perform some special action to see addresses in a different cpu 
context.  I don't think that gdb supports such a special action at the moment.  If you 
want that feature though, that's where you should start.


r~