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[109.152.215.36]) by smtp.gmail.com with ESMTPSA id i23-20020a1c5417000000b00394708a3d7dsm20881289wmb.15.2022.06.06.07.38.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 06 Jun 2022 07:38:09 -0700 (PDT) From: Andrew Burgess To: Luis Machado , John Baldwin , gdb-patches@sourceware.org Subject: Re: [PATCH 5/5] gdb: native target invalid architecture detection In-Reply-To: <09afe250-9573-45e1-993b-a2f911f03630@arm.com> References: <71a986a5-2cfa-543e-4034-70f3af7dfecf@FreeBSD.org> <87ee09d4rt.fsf@redhat.com> <09afe250-9573-45e1-993b-a2f911f03630@arm.com> Date: Mon, 06 Jun 2022 15:38:08 +0100 Message-ID: <87ilpdhn73.fsf@redhat.com> MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain X-Spam-Status: No, score=-4.9 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, RCVD_IN_BARRACUDACENTRAL, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_NONE, TXREP, T_SCC_BODY_TEXT_LINE autolearn=no autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org X-BeenThere: gdb-patches@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gdb-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 06 Jun 2022 14:38:18 -0000 Luis Machado via Gdb-patches writes: > On 5/31/22 17:51, Andrew Burgess via Gdb-patches wrote: >> John Baldwin 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 (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 (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. If I understand correctly a aarch64 binary will have bfd_arch_aarch64, and a aarch32 (armv7?) binary will be bfd_arch_arm. What I don't understand, is how this all hangs together when running on an aarch64 machine, using a native target. Unless I'm missing something (please let me know if I am) then there's just a single native linux target, the_aarch64_linux_nat_target, which is registered from _initialize_aarch64_linux_nat in aarch64-linux-nat.c. In aarch64_linux_nat_target::fetch_registers, we assume the gdbarch_tdep is of type aarch64_gdbarch_tdep, so this will only work for bfd_arch_aarch64 binaries. Then there's the_arm_linux_nat_target, from arm-linux-nat.c, which has arm_linux_nat_target::fetch_registers, which assumed gdbarch_tdep will be of type arm_gdbarch_tdep, so will only work for bfd_arch_arm targets. So, I guess, if we can debug bfd_arch_arm and bfd_arch_aarch64 binaries on a native aarch64 target, we must somehow be switching the current native target object between these two, right? But I can't figure out where that's happening... > >> >> 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... Does this mean you're on an aarch64 machine, start GDB with an aarch32 (armv7?) binary, and then "run" to start the process? > > The target does not support architecture "armv7". I'd be really interested to know if you only take the first 4 patches from this series, can you still run armv7 binaries using an aarch64 GDB? I'm wonderinging if the assert added in patch #4 will trigger. Thanks, Andrew > >> >> 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 >>