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

[Luis Machado <luis.machado@arm.com>]

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".

> 
> But to answer the general question, if there is a case that the existing
> code doesn't handle, then we can for sure override the new method in
> specific *-nat.c targets in order to handle weird cases.> 
> Thanks,
> Andrew
>