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

From: Bruno Piazera Larsen
Subject: Re: [RFC PATCH v2 2/2] target/ppc: make gdb able to translate priviledged addresses
Date: Tue, 15 Jun 2021 08:32:32 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

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. when debugging, we wanna see exactly what QEMU sees, not what the guest sees, right? Now, if this is changing more than just privilege level, I agree there is a problem, but I wouldn't think it is the case...

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.

Maybe there is a better way to handle this by having GDB warn the user that the CPU can not decode the address in it's current state, but I do think it is a good tool to have, as it would've made debugging the first RFC on this topic a bit easier, and farosas was actively complaining that isn't a feature yet.

Bruno Piazera Larsen
Instituto de Pesquisas ELDORADO
Departamento Computação Embarcada
Analista de Software Trainee
Aviso Legal - Disclaimer

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