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

[John Baldwin]

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?

-- 
John Baldwin