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Wed, 13 May 2020 09:45:33 -0700 (PDT) Subject: Re: [PATCH v3 19/23] arm64: mte: Add PTRACE_{PEEK,POKE}MTETAGS support From: Luis Machado To: Catalin Marinas References: <20200421142603.3894-1-catalin.marinas@arm.com> <20200421142603.3894-20-catalin.marinas@arm.com> <20200513104849.GC2719@gaia> <3d2621ac-9d08-53ea-6c22-c62532911377@linaro.org> <20200513141147.GD2719@gaia> Message-ID: Date: Wed, 13 May 2020 13:45:27 -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: Content-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200513_094536_207579_D918A255 X-CRM114-Status: GOOD ( 34.70 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, Richard Earnshaw , Omair Javaid , Szabolcs Nagy , Andrey Konovalov , Kevin Brodsky , Peter Collingbourne , linux-mm@kvack.org, Alan Hayward , Vincenzo Frascino , Will Deacon , linux-arm-kernel@lists.infradead.org Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 5/13/20 12:09 PM, Luis Machado wrote: > On 5/13/20 11:11 AM, Catalin Marinas wrote: >> On Wed, May 13, 2020 at 09:52:52AM -0300, Luis Machado wrote: >>> On 5/13/20 7:48 AM, Catalin Marinas wrote: >>>> 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 >>>>> >>>>> 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? >> >> Right, but "actively" is not well defined either. The only way to tell >> whether a process is using MTE is to look for any PROT_MTE mappings. You >> can access these via /proc//maps. In theory, one can use MTE >> without enabling the tagged address ABI or even tag checking (i.e. no >> prctl() call). >> > > I see the problem. I was hoping for a more immediate form of runtime > check. One debuggers would validate and enable all the tag checks and > register access at process attach/startup. > > With that said, checking for PROT_MTE in /proc//maps may still be > useful, but a process with no immediate PROT_MTE maps doesn't mean such > process won't attempt to use PROT_MTE later on. I'll have to factor that > in, but I think it'll work. > > I guess HWCAP2_MTE will be useful after all. We can just assume that > whenever we have HWCAP2_MTE, we can fetch MTE registers and check for > PROT_MTE. > >>> 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. >> >> I'm happy to expose GCR_EL1.Excl and the SCTLR_EL1.TCF0 bits via ptrace >> as a thread state. The tags, however, are a property of the memory range >> rather than a per-thread state. That's what makes it different from >> other register-based features like SVE. > > That's my understanding as well. I'm assuming, based on our previous > discussion, that we'll have those couple registers under a regset (maybe > NT_ARM_MTE). > >> >>> 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. >> >> We don't store this information in the kernel as a bool and I don't >> think it would be useful either. I think gdb, when displaying memory, >> should attempt to show tags as well if the corresponding range was >> mapped with PROT_MTE. Just probing whether a thread ever used MTE >> doesn't help since you need to be more precise on which address supports >> tags. > > Thanks for making this clear. Checking with ptrace won't work then. It > seems like /proc//maps is the way to go. > >> >>>> 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". >> >> And you could use this for probing whether the range has tags or not. >> With my current patches it returns -EFAULT but happy to change this to >> -EOPNOTSUPP or -EINVAL. Note that it only returns an error if no tags >> copied. If gdb asks for a range of two pages and only the first one has >> PROT_MTE, it will return 0 and set the number of tags copied equivalent >> to the first page. A subsequent call would return an error. >> >> In my discussion with Dave on the documentation patch, I thought retries >> wouldn't be needed but in the above case it may be useful to get an >> error code. That's unless we change the interface to return an error and >> also update the user iovec structure. >> > > Let me think about this for a bit. I'm trying to factor in the > /proc//maps contents. If debuggers know which pages have PROT_MTE > set, then we can teach the tools not to PEEK/POKE tags from/to those > memory ranges, which simplifies the error handling a bit. I was checking the output of /proc//maps and it doesn't seem to contain flags against which i can match PROT_MTE. It seems /proc//smaps is the one that contains the flags (mt) for MTE. Am i missing something? Is this the only place debuggers can check for PROT_MTE? If so, that's unfortunate. /proc//smaps doesn't seem to be convenient for parsing. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel