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

[John Baldwin <jhb@FreeBSD.org>]

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?

-- 
John Baldwin