Re: [PATCH 5/5] gdb: native target invalid architecture detection

[John Baldwin <jhb@FreeBSD.org>]

On 6/1/22 2:21 PM, Christophe Lyon wrote:
> 
> 
> On 6/1/22 23:06, John Baldwin wrote:
>> On 6/1/22 1:25 AM, Luis Machado wrote:
>>> On 5/31/22 17:51, Andrew Burgess via Gdb-patches wrote:
>>>> John Baldwin <jhb@FreeBSD.org> writes:
>>>>
>>>>> On 5/31/22 7:30 AM, Andrew Burgess via Gdb-patches wrote:
>>>>>> If GDB is asked to start a new inferior, or attach to an existing
>>>>>> process, using a binary file for an architecture that does not match
>>>>>> the current native target, then, currently, GDB will assert.  Here's
>>>>>> an example session using current HEAD of master with GDB built for an
>>>>>> x86-64 GNU/Linux native target, the binary being used is a RISC-V ELF:
>>>>>>
>>>>>>       $ ./gdb/gdb -q --data-directory ./gdb/data-directory/
>>>>>>       (gdb) file /tmp/hello.rv32imc.x
>>>>>>       Reading symbols from /tmp/hello.rv32imc.x...
>>>>>>       (gdb) start
>>>>>>       Temporary breakpoint 1 at 0x101b2: file hello.rv32.c, line 23.
>>>>>>       Starting program: /tmp/hello.rv32imc.x
>>>>>>       ../../src/gdb/gdbarch.h:166: internal-error: gdbarch_tdep:
>>>>>> Assertion `dynamic_cast<TDepType *> (tdep) != nullptr' failed.
>>>>>>       A problem internal to GDB has been detected,
>>>>>>       further debugging may prove unreliable.
>>>>>>
>>>>>> The same error is encountered if, instead of starting a new inferior,
>>>>>> the user tries to attach to an x86-64 process with a RISC-V binary set
>>>>>> as the current executable.
>>>>>>
>>>>>> These errors are not specific to the x86-64/RISC-V pairing I'm using
>>>>>> here, any attempt to use a binary for one architecture with a native
>>>>>> target of a different architecture will result in a similar error.
>>>>>>
>>>>>> Clearly, attempting to use this cross-architecture combination is a
>>>>>> user error, but I think GDB should do better than an assert; ideally a
>>>>>> nice error should be printed.
>>>>>>
>>>>>> The problem we run into is that, when the user starts a new inferior,
>>>>>> or attaches to an inferior, the inferior stops.  At this point GDB
>>>>>> attempts to handle the stop, and this involves reading registers from
>>>>>> the inferior.
>>>>>>
>>>>>> These register reads end up being done through the native target, so
>>>>>> in the example above, we end up in the amd64_supply_fxsave function.
>>>>>> However, these functions need a gdbarch.  The gdbarch is fetched from
>>>>>> the register set, which was constructed using the gdbarch from the
>>>>>> binary currently in use.  And so we end up in amd64_supply_fxsave
>>>>>> using a RISC-V gdbarch.
>>>>>>
>>>>>> When we call:
>>>>>>
>>>>>>       i386_gdbarch_tdep *tdep = gdbarch_tdep<i386_gdbarch_tdep>
>>>>>> (gdbarch);
>>>>>>
>>>>>> this will assert as the gdbarch_tdep data within the RISC-V gdbarch is
>>>>>> of the type riscv_gdbarch_tdep not i386_gdbarch_tdep.
>>>>>>
>>>>>> The solution I propose in this commit is to add a new target_ops
>>>>>> method supports_architecture_p.  This method will return true if a
>>>>>> target can safely be used with a specific architecture, otherwise, the
>>>>>> method returns false.
>>>>>>
>>>>>> I imagine that a result of true from this method doesn't guarantee
>>>>>> that GDB can start an inferior of a given architecture, it just means
>>>>>> that GDB will not crash if such an attempt is made.  A result of false
>>>>>> is a hard stop; attempting to use this target with this architecture
>>>>>> is not supported, and may cause GDB to crash.
>>>>>>
>>>>>> This distinction is important I think for things like remote targets,
>>>>>> and possibly simulator targets.  We might imagine that GDB can ask a
>>>>>> remote (or simulator) to start with a particular executable, and the
>>>>>> target might still refuse for some reason.  But my thinking is that
>>>>>> these refusals should be well handled (i.e. GDB should give a user
>>>>>> friendly error), rather than crashing, as is the case with the native
>>>>>> targets.
>>>>>>
>>>>>> For example, if I start gdbserver on an x86-64 machine like this:
>>>>>>
>>>>>>       gdbserver --multi :54321
>>>>>>
>>>>>> Then make use of this from a GDB session like this:
>>>>>>
>>>>>>       $ ./gdb/gdb -q --data-directory ./gdb/data-directory/
>>>>>>       (gdb) file /tmp/hello.rv32imc.x
>>>>>>       Reading symbols from /tmp/hello.rv32imc.x...
>>>>>>       (gdb) target extended-remote :54321
>>>>>>       Remote debugging using :54321
>>>>>>       (gdb) run
>>>>>>       Starting program: /tmp/hello.rv32imc.x
>>>>>>       Running the default executable on the remote target failed;
>>>>>> try "set remote exec-file"?
>>>>>>       (gdb)
>>>>>>
>>>>>> Though the error is not very helpful in diagnosing the problem, we can
>>>>>> see that GDB has not crashed, but has given the user an error.
>>>>>>
>>>>>> And so, the supports_architecture_p method is created to return true
>>>>>> by default, then I override this in inf_child_target, where I compare
>>>>>> the architecture in question with the default_bfd_arch.
>>>>>>
>>>>>> Finally, I've added calls to supports_architecture_p for the
>>>>>> run (which covers run, start, starti) and attach commands.
>>>>>>
>>>>>> You will notice a lack of tests for this change.  I'm not sure of a
>>>>>> good way that I can build a binary for a different architecture as
>>>>>> part of a test, but if anyone has any ideas then I'll be happy to add
>>>>>> a test here.
>>>>>
>>>>> Have you considered multi-arch cases such as running i386 binaries
>>>>> on an x86-64
>>>>> host or 32-bit arm binaries on an AArch64 host?  Will we need to
>>>>> override this
>>>>> method in certain targets (e.g. x86-linux-nat.c or x86-fbsd-nat.c)
>>>>> to support
>>>>> such cases?
>>>>
>>>> For the x86 examples you gave, I think these all have the bfd_arch_i386
>>>> bfd architecture, so should work just fine.
>>>>
>>>> But for the aarch64 case, I admit I don't know how this works.  A 32-bit
>>>> ARM binary is going to have bfd_arch_arm, while the AArch64 target will
>>>> be expecting a gdbarch with bfd_arch_aarch64.  But how GDB supports
>>>> running the former on the latter, I don't know.
>>>>
>>>> I notice there's aarch64-linux-nat.c and aarch32-linux-nat.c, I wonder
>>>> if this has something to do with it...
>>>
>>> aarch32 is the 32-bit state of aarch64, but the BFD architecture is
>>> different. So this won't work out-of-the-box.
>>>
>>>>
>>>> Maybe someone with more ARM/AArch64 knowledge will chip in to fill in
>>>> some of the blanks.
>>>
>>> When attempting to run a 32-bit binary in 64-bit state, I get...
>>>
>>> The target does not support architecture "armv7".
>>
>> Does Linux support running 32-bit binaries on a 64-bit aarch64 host?
>>
> Yes. For instance one can start a docker container in aarch32 (32-bit)
> mode on a aarch64 host (provided the CPU supports both modes, which is
> not always the case)

Ok.  FreeBSD supports this too, but I think it's fine to deal with this
by overriding the relevant method in the aarch64-*-nat.c classes.

-- 
John Baldwin