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([2804:7f0:8283:1510:1c7:af77:437a:ffd0]) by smtp.gmail.com with ESMTPSA id 88sm824883qth.9.2020.05.13.05.52.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 13 May 2020 05:52:59 -0700 (PDT) Subject: Re: [PATCH v3 19/23] arm64: mte: Add PTRACE_{PEEK,POKE}MTETAGS support To: Catalin Marinas Cc: linux-arm-kernel@lists.infradead.org, Will Deacon , Vincenzo Frascino , Szabolcs Nagy , Richard Earnshaw , Kevin Brodsky , Andrey Konovalov , Peter Collingbourne , linux-mm@kvack.org, linux-arch@vger.kernel.org, Alan Hayward , Omair Javaid References: <20200421142603.3894-1-catalin.marinas@arm.com> <20200421142603.3894-20-catalin.marinas@arm.com> <20200513104849.GC2719@gaia> From: Luis Machado Message-ID: <3d2621ac-9d08-53ea-6c22-c62532911377@linaro.org> Date: Wed, 13 May 2020 09:52:52 -0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <20200513104849.GC2719@gaia> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 5/13/20 7:48 AM, Catalin Marinas wrote: > Hi Luis, > > On Tue, May 12, 2020 at 04:05:15PM -0300, Luis Machado wrote: >> On 4/21/20 11:25 AM, Catalin Marinas wrote: >>> Add support for bulk setting/getting of the MTE tags in a tracee's >>> address space at 'addr' in the ptrace() syscall prototype. 'data' points >>> to a struct iovec in the tracer's address space with iov_base >>> representing the address of a tracer's buffer of length iov_len. The >>> tags to be copied to/from the tracer's buffer are stored as one tag per >>> byte. >>> >>> On successfully copying at least one tag, ptrace() returns 0 and updates >>> the tracer's iov_len with the number of tags copied. In case of error, >>> either -EIO or -EFAULT is returned, trying to follow the ptrace() man >>> page. >>> >>> Note that the tag copying functions are not performance critical, >>> therefore they lack optimisations found in typical memory copy routines. >>> >>> Signed-off-by: Catalin Marinas >>> Cc: Will Deacon >>> Cc: Alan Hayward >>> Cc: Luis Machado >>> Cc: Omair Javaid >>> --- >>> >>> Notes: >>> New in v3. >>> >>> arch/arm64/include/asm/mte.h | 17 ++++ >>> arch/arm64/include/uapi/asm/ptrace.h | 3 + >>> arch/arm64/kernel/mte.c | 127 +++++++++++++++++++++++++++ >>> arch/arm64/kernel/ptrace.c | 15 +++- >>> arch/arm64/lib/mte.S | 50 +++++++++++ >>> 5 files changed, 211 insertions(+), 1 deletion(-) >>> >> I started working on MTE support for GDB and I'm wondering if we've already >> defined a way to check for runtime MTE support (as opposed to a HWCAP2-based >> check) in a traced process. >> >> Originally we were going to do it via empty-parameter ptrace calls, but you >> had mentioned something about a proc-based method, if I'm not mistaken. > > We could expose more information via proc_pid_arch_status() but that > would be the tagged address ABI and tag check fault mode and intended > for human consumption mostly. We don't have any ptrace interface that > exposes HWCAPs. Since the gdbserver runs on the same machine as the > debugged process, it can check the HWCAPs itself, they are the same for > all processes. Sorry, I think i haven't made it clear. I already have access to HWCAP2 both from GDB's and gdbserver's side. But HWCAP2 only indicates the availability of a particular feature in a CPU, it doesn't necessarily means the traced process is actively using MTE, right? So GDB/gdbserver would need runtime checks to be able to tell if a process is using MTE, in which case the tools will pay attention to tags and additional MTE-related registers (sctlr and gcr) we plan to make available to userspace. This would be similar to SVE, where we have a HWCAP bit indicating the presence of the feature, but it may not be in use at runtime for a particular running process. The original proposal was to have GDB send PTRACE_PEEKMTETAGS with a NULL address and check the result. Then GDB would be able to decide if the process is using MTE or not. > > BTW, in my pre-v4 patches (hopefully I'll post v4 this week), I changed > the ptrace tag access slightly to return an error (and no tags copied) > if the page has not been mapped with PROT_MTE. The other option would > have been read-as-zero/write-ignored as per the hardware behaviour. > Either option is fine by me but I thought the write-ignored part would > be more confusing for the debugger. If you have any preference here, > please let me know. > I think erroring out is a better alternative, as long as the debugger can tell what the error means, like, for example, "this particular address doesn't make use of tags".